AC Tonnage Calculator accurately estimates the size in Tons & BTUs, as well as the cost to install Central Air in different climate zones (regions), based on your house size and AC SEER rating, house size, heat load and other parameters.
The ideal solution to reduce electric costs related to air conditioning is to install a higher efficiency central air system (16 SEER or higher). However, high SEER central AC systems are expensive, and are typically the more advanced Central Heat Pump type (18 SEER+ systems).
On the down side, if you install this advanced, higher efficiency AC, you may not see the payoff for many years.
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What Size AC Unit Do I Need?
Getting the right size AC unit for your house is very important if you want to have the desired level of comfort and good energy efficiency. The smallest central air conditioning unit is 18,000 BTUs (1.5 tons), while the largest is 60,000 BTUs (5 tons).
Today electricity is very expensive (anywhere from $0.14 / kWh in most southern states to $0.25-$0.45 / kWh in CA, MA, NJ, and most of New England) and you don’t want to waste it on running central air that is too big. On the other hand, installing a central AC that is too small will not keep your house cool! So while you will be saving money, you will not have the optimal temperature control.
PRO TIP: Use our new AC Energy Cost calculator, to get an idea of how much it costs you to operate your Central Air Conditioner, based on your location, AC size, SEER rating, cooling days and local electric costs.
The calculator is using live power cost feed from US Department of Energy, which is updated every month, to give you most accurate estimates of power costs.
With that calculator, you can also estimate your energy savings from replacing your old 9-13 SEER AC with something like a 16-20 SEER two-stage AC / Heat Pumps.
Your average energy saving from replacing a 9 SEER AC with a 16 SEER ac will be $450-780 / year, plus you can get Energy Efficiency Rebates in most states!
NOTE: When calculating AC efficiency over 16 SEER, you will get replacement cost for Central Heat Pump systems, instead of regular Central AC. This is because regular Central AC will max out at 16 SEER.
All residential central systems with efficiency of 17 SEER or higher, available on the market today are HEAT PUMPs, which can also heat your home in moderately cold temperatures (above 32°F).
Heat Pumps are generally more efficient than traditional ACs, and with addition of a reversing valve, they can change the refrigerant flow when needed, and change from Cooling to Heating mode!
As far as efficiency ratings go, a 16 SEER Heat Pump is just as efficient as 16 SEER traditional AC, and the size in TONs, which is required to cool your house is the same. Again, the primary difference that is “visible” to homeowners, is the ability to heat the house.
In general, you should have approximately 1 ton of ac capacity to cool 600 sq.ft. of area. (600 sq.ft./1 ton). This estimate is most accurate in new construction or newer homes that have proper insulation. In older homes, many factors will skew this estimate, and you may require a larger ac unit size to cool your house, depending on the factors we will discuss.
Don’t forget that the climate zone you live in as well as the temperature at which you would like to keep your home will also make a big difference in the size of the ac unit that would work best. For example, any given central air unit size will cool about 200-300 sq.ft. LESS in Florida (zone 1) vs Montana (zone 5)
For the most accurate heat load estimate and ac equipment recommendation, use our advanced Heat Load Calculator (BTUs)
DIY vs Professional AC Size Estimate
If you are looking for the most accurate size calculation for a central air system, the best course of action is to get free estimates from licensed HVAC installers.
A pro will have a comprehensive, professionally designed HVAC calculator that can do a complete cooling load calculation. He will come in for an in-house estimate and tell you exactly what size central AC you need.
Lets take a look at what this cooling load calculation entails and why its important to get it. We will also show you how to do a DIY estimate to figure out the central AC unit size you need.
Central AC Unit Cost
Before we get into sizing a residential central air unit, lets take a quick look at pricing. On average, homeowners report spending $3,900 – 5,000 to install a 2.5 ton central air system in a house that has functional ductwork.
Fixing or changing the ductwork to accommodate the AC system can add as much as $3,000-4,000 to the total price. If the house does not have a ductwork system, the cost of installing central air will skyrocket to as much as $8,000-10,000+
Prices for central air units vary primary based on size (tonnage).
The smallest AC unit 1 ton (12,000 BTUs) costs $1,300 – $1,700. The 1-ton units are very rarely installed as they are too small for a “CENTRAL AC” application.
If you need an AC that small, you are much better off going with a ductless mini-split system.
Here are unit prices for different tonnage (prices do not include installation):
2 ton AC unit costs $1,700 – $2,200
3 ton AC unit costs $2,200 – $2,500
4 ton AC unit costs $2,500 – $2,900
5 ton AC unit costs $2,900 – $3,500 ( a 5 ton unit is extremely powerful and is not required for most homes)
In general, you can roughly add $400-$700 for each additional ton, as the units go up in size. Residential Central AC systems come in a maximum size of 5-ton. Anything bigger, requires a commercial roof-top AC, which are in most situations not suitable for residential use.
Final pricing for central AC also varies based on brand name, as well as other features. Its best to purchase a unit from a well known, reputable manufacturer that offers a very good warranty.
Top AC brands include: Amana, Day and Night, Goodman Air, Carrier, Bryant, American Standard, York, Daikin, Mitsubishi, Maytag, and Heil. Ask your HVAC installer to a recommend a couple of brands he trusts.
AC Sizing Chart
The term “size” of a central air unit does NOT refer to the physical dimensions. Rather, size is a measure of its capacity to generate cool air.
Air conditioning unit sizes are measured in tonnage as well as BTU (British Thermal Unit) per hour.
1 BTU = the amount of energy needed to raise 1 pound of water by 1 degree F.
1 Ton = 12,000 BTUs per hour, or the amount of energy it takes to melt 1 ton of ice in a day.
Consequently, AC sizing involves determining the home’s cooling load: the exact capacity in BTUs per hour and cubic feet per minute of airflow required to maintain a 78 degree temperature in the house.
Residential central air conditioners range in size from 1.5 to 5 tons. Unit sizes go up in half ton increments:
12,000 BTUs – 1 ton
18,000 BTUs – 1.5 tons
24,000 BTUs – 2 tons
30,000 BTUs – 2.5 tons
36,000 BTUs – 3 tons
48,000 BTUs – 4 tons
60,000 BTUs – 5 tons
A unit that is above 5 tons is considered commercial size and these large units are actually not available for home installation. If your house actually requires a unit that is over 5 tons, your HVAC contractor will install two units to match your home’s cooling needs.
The most popular central AC size that works for the majority of residential homes is either 2 or 2.5 ton unit.
Air Conditioner Size Based On Heat Zone
If you want to get a sense of how much AC tonnage your home needs, take a look at this heat zone map and ac sizing chart.
First locate the zone that your state is in, based on color:
Next, look at the chart below, and find the appropriate zone as well as the square footage of your house. Then, look at the corresponding unit size.
Keep in mind that this is the least accurate way to determine the proper central air system size for your home. It does not take into consideration any of the specific factors that can really impact AC size needed for a particular house. We will get into these below, and show you a more accurate way to do the estimate.
AC Size Estimate Based On Room Size
Here is another very quick and basic way to figure out the size of an AC unit based on the number of square feet in your house.
Take the total amount of square feet in your home. Make sure to EXCLUDE the square footage of your attic/basement/garage, unless you are using any of them as living spaces. Take this number of square feet and divide it by 600.
For example: 2,400 sq.ft. : 600 = 4,tons
If your house is well insulated and energy efficient, you can estimate down. For example, a 2,200 sq.ft. house divided by 600 = 3.6666. So you would want to install a 3.5 ton unit.
If there is poor insulation/efficiency in your home, its best to estimate higher tonnage. In the example above, you would estimate a 4 ton unit for a poorly insulated 2,200 sq.ft. house.
House square footage/ AC unit size estimate
1,500 sq. ft./2.5 tons
1,800 sq. ft./3 tons
2,100 sq. ft./3.5 tons
2,400 sq. ft./4 tons
Greater than 3,000 sq. ft./5 tons
Manual J: Professional AC Unit Size Calculation
To get a truly accurate size for central air that will work most efficiently in your house, you need to do a professional load calculation, known as “the Manual J”.
Most HVAC installers include it for free as part of their estimate, when they give you the total cost to install central air in your home.
Pro Tip: its best for you to ask him to actually perform a complete Manual J assessment (they have the software program that runs it). Don’t settle for a ball park estimate based on a quick visual assessment, as it will not be accurate.
You should not trust an HVAC contractor who does a quick walk through your home, and gives you the AC tonnage your home needs just off the top of his head.
There are just too many factors at play here. The guy is not a magician and has no way of making a real estimate without using professional software.
Also, be ware of contractors who suggest that you should simply install the same size unit that you currently have. This is a huge mistake, as the current unit that is being replaced may not be the right efficiency for your house.
If the contractor is not willing to do the initial assessment thoroughly, he is likely to cut corners on the installation itself.
There are two types of Manual J calculations ( whole house and room by room) and your contractor will decide which one is best to do, depending on your particular situation.
Getting the right size AC unit for your house is very important if you want to have the desired level of comfort and good energy efficiency. The smallest unit is 18,000 BTUs (1.5 tons), while the largest is 60,000 BTUs (5 tons).
Today electricity is very expensive and you don’t want to waste it on running a unit that is too big. On the other hand, getting a system that is too small will not keep your house cool, so while you will save money, you will not have the optimal temperature control.
The best course of action is to hire a licensed installer who has a comprehensive, professionally designed HVAC calculator.
He will come in for an in-house estimate and tell you exactly what BTU capacity central air system you need.
Lets take a look at what this calculation entails and why its important to get it. We will also show you how to do a DIY estimate to figure out the right size for your central AC unit.
Whole House Heat Load Calculation
This estimate is used to determine the amount of cooling needed for your house as a whole. There are multiple factors included in this assessment, such as:
– heat transfer between walls
– number of windows
– insulation type and R-value
– ceiling height
– number of people in your home
– presence of sunlight or natural shade
– number of heat generating appliances
– many other factors are also used in the software program
It is recommended to do a whole house Manual J if you are only planning to replace/install the AC unit itself and will not touch the ductwork.
Pro Tip: we strongly recommend replacing your old ductwork, unless you know for sure that it is working properly. This is important because you will be loosing a lot of air through leaking ducts. This translates into wasting a lot of energy and money every month.
Even if you get the most efficient ac unit, these savings will be nullified if you have a poorly functioning duct system. If you are not sure about whether or not your ducts are in good working order, ask the installer to inspect them.
Room By Room Load Calculation
It has the same inputs as the whole house one, except it is done for each room. Then they are all added together to get the final result.
Overall, a room by room cooling load calculation is more accurate than a whole house one. It is recommended to use this method if you are planning to replace the ductwork in addition to the AC unit.
How To Estimate AC Unit Size DIY
If you want to get a sense of how big of a central air unit your house will need, it is possible to get a rough estimate on your own.
However, it is important to realize that because there are so many factors that may impact this calculation, you will most likely not be able to take them all into account properly. Thus, your final assessment may not be very accurate.
Here is a simple formula to do the estimate:
1. Use the following equation: ((House sq.ft. x 25) / 12,000) – 0.5) = required tons
For example: your house is 2,350 sq.ft. Plug it into the formula: ((2,350 x 25) / 12,000) – 0.5) = 4.39
This means that your house requires a central unit of 4 to 4.5 tons.
However, if you live in a hot climate, where temperatures are high most months out of the year, you will need to change the formula slightly, because otherwise your unit will be undersized.
Instead of subtracting 0.5, you will need to ADD it.
So the new formula looks like this: ((2,350 x 25) / 12,000) + 0.5) = 5.39
This means that your house requires a central air conditioning unit of 5 to 5.5 tons.
Pro Tip: as a rule of thumb, the maximum AC unit size you need to install should not be more than 15% more than the BTU’s you need to cool your house.
This means that if your house requires a 24,000 BTU unit (2 tons), you should not install one that is larger than 30,000 BTU’s (3 tons) to maintain energy efficiency.
Factors That Impact How Big Of An AC Unit You Need
As you may have already guessed there are a number of important variables that will effect the size of the central air system your house requires.
Total number of square feet is just one of these factors.
In fact, you can have homes that are identical in size, but still call for completely different size units.
Here are the additional factors that need to be taken into consideration:
- Number of people living in the house: (each person gives off around 250 BTUs per hour)
- Number, age and orientation of windows (south or north facing) and how well they are insulated
- Amount of sunshine your home gets: you may have very bright, hot rooms that get a lot of sun, as well as darker, naturally coolers rooms where the sun does not come in a lot.
- Insulation: how much of it and what type. For example, its very important to have proper roof and attic insulation, as that is where a lot of the air escapes.
- Number of skylights
- Heat transfer between walls and through the concrete slab
- Type and location of ducting
- Amount of shade from trees and bushes
- The area of sun-exposed exterior walls
Why It’s a BAD Idea To Install The Wrong Size AC
Many homeowners hold on to the misconception that it can’t hurt to get a bigger central air conditioner than needed, just in case.
If the HVAC contractor recommends a 2.5 ton unit, many are tempted to install a 3 ton or even 3.5, just to make sure that the house will be cool enough.
This would have been a good idea 20 years ago, when electricity rates were cheap, and AC units used A LOT of energy to quickly cool off the space. In those times, bigger really was better.
So what changed?
Today, new air conditioner units are energy efficient. This means that they save energy by running in cycles, which results in your home being cooled at a slow and steady, rather than a rapid pace. Because of this change in how the AC unit operates, its essential to get one that is just the right size for your house.
Here is a quick summary of the main reasons why it is not advisable to install the wrong size central air, whether too big or too small.
AC unit that is too big
- Costs more upfront – you can overspend by about $800+
- Higher monthly electricity bills
- Doesn’t get rid of humidity as well as a properly sized unit, especially in very hot climates
- Cycles on and off many times throughout the day, thereby creating more wear and tear on the unit
- Does not cool your home evenly
AC unit that is too small
- Will not keep your house at a comfortably cool temperature on the hottest days
- Uses a lot of energy
- Has to run all the time
- Running all the time instead of cycling will drastically increase your electric bill
In addition to sizing, its important to consider other features of a central air unit, such as the SEER rating, 1 vs 2 cycle system, fancy extras that high-end brands like to include, etc.
AC units are not all created equal. There are significant differences in pricing, quality, durability and warranties offered by various HVAC manufacturers.
The importance of a quality install cannot be overstated enough. Over 40% of all HVAC systems, including central air conditioners, fail due to incompetent, hack job installation.
Especially if you are willing to spend extra money on a premium brand name AC unit, such as Carrier, don’t try to squeeze a few hundred dollars of savings by hiring a sub-par installer.
Mini Split vs Central Air
$2950 - $4150
If you need to cool just a single room then a central AC may not be the best or most cost efficient system for you.
Ductless mini split systems provide targeting cooling (and heating) to a single room or a large area. If you get a multi-zone mini split system, then you can heat and cool multiple rooms.
Besides flexibility, mini split systems offer a far superior energy efficiency compared to central AC systems. Whereas modern central air units have an average rating of 16 SEER (seasonal energy efficiency rating), a typical ductless mini split has a SEER rating of 22, while high – end models are rated at 33-40 SEER, in single zone installations. Multi-zone mini split systems go as high as 24 SEER.
Another major benefit is that a mini split AC system can be installed DIY, which is not possible with central air. Doing a DIY install saves you at least a few thousand dollars on professional labor.
While your mini split equipment may not look as attractive as invisible central air, you need to consider that you have a significant loss energy in the ducts of a central air system.
Mini splits do not have this issue because the cold or hot air is produced right inside the room. Therefore, actual efficiency of a mini split is considerably higher compared to central AC.
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I’ll take this one Leo.
Kathleen your AC running for under 30 minutes with a cover on it is no fiasco. However two elderly people with serious medical issues running around in the middle of the night turning off thermostats and untying covers could accurately be described as such. Keep in mind if it is 80° outdoors the inside of your home will generally be several degrees warmer due to heat generated by people, appliances, etc. that could affect the level of comfort enjoyed by your guests. Don’t worry this is all easily fixed in a few simple steps. 1) apologize sincerely to your cold-blooded family member for your overreaction. 2) Since you heat with gas and that does not require your outdoor unit to function I would suggest turning off the power supplied to your outdoor unit whenever the cover is installed to prevent it from happening again. It almost certainly did not cause any lasting harm to your equipment. Compressors are tough but more importantly they are all protected by an overload that automatically shuts them down if they get too hot then allowing them to restart after they’ve cooled down. 3) whenever they come to perform maintenance on your equipment ask them to record on the invoice the system operating pressures, the amp draw measured with a meter that each of your fan motors and compressor are pulling as well as what they are rated at. Along with the visual inspection this will give you a general idea of their health. They should be checking and recording on your invoice the condition of electrical connections, contactor points, the microfarad rating on your capacitors compared to the actual reading on a meter. Ask them the condition of your condenser coils and your evaporator coils. If they find anything unsatisfactory have them show you and explain it to you. This is how you will come to know your system and what condition it’s in and it also keeps your maintenance man on his game. It’s nearly impossible to predict when your unit will fail but these things will give you an idea as will summer electric bills rising year over year that could indicate longer run times to get the same job done. Lastly, it probably wouldn’t hurt to run a camera through that duct work and clean out all the Natural Light cans you’re installer no doubt left in there.
Says the guy whose house is a brisk 68°, in July but he has to run two dehumidifiers because the air conditioner doesn’t run enough to remove the moisture from the air. Blaming the “faulty” equipment they sold him for the third compressor failure because it can’t compress the liquid that is flooding back to it due to such a small heat load on your oversized system. Too bad nobody can hear him complain over the noise coming from the rattling grilles and high velocity air jamming through ductwork that was designed for a 3 ton system but is now trying to handle 5 tons of air. But hey, bigger is better! Right?
Thanks for your comments.
I’m not sure who you are replying to, or refer to as “the guy whose house is a brisk 68°” … I assume you meant Tom from Tampa. BTW, where are you located?
We actually have a function where you can reply to the comment of a particular person … Under each comment, there is a “Reply” button/link.
BTW … the issue with AC not running long enough is strictly for single speed compressors. Variable speed / inverter compressors can increase and decrease output, based on load. Like my heat pumps – they run at 1800-2400W in heating mode in Jan/Feb when it 10°F outside, and at 300W when it 45° outside, keeping indoor temps at 76° … Variable speed at work 🙂
Same with cooling, but I don’t really like AC, so only turn them on, if its above 95° outside
We live in North Tulsa County OK. Our home is 12 yrs old. This last Christmas 2021 while hosting family from Minnesota he became too hot and turned the central A/C unit on at night without telling us. We had offered to turn on the AC for him but didn’t anticipate he would do it without telling us. He didn’t know my husband covers our compressor every fall to keep out debris, leaves, wet, etc. Our service company has always complimented him on the cleanliness of the compressor unit, saying it help it work efficiently if not full of leaves, debris.
Christmas Fiasco! I estimate the AC ran for not more than 30 minutes before I realized what he did, told my husband who ran outside and whipped off the tied down covering while I shut off the AC. My husband said the compressor felt hot when the cover came off. We allowed the compressor to cool completely before turning it on to cool off our too hot native Minnesotan. We had an unexpected hot holiday with temperatures around 80 degrees during the day and not cooling down a lot at night during their visit. We were comfortable in shorts outside during the day for 2 days.
Our concern is possible damage to our unit from the “Christmas fiasco “, shortening the life of our system. Being on a fixed income with inflation and high medical bills means stretching our budget and doing without services we need. Our home was custom built one story for me with handicap features due to Parkinson’s Disease and Chronic Neurological Lyme Disease. 2100 sq. ft. 9-10 ft ceilings, with one 12 ft ceiling in relatively small master Bedroom. Better than builder grade insulation batts in walls and blown in attic insulation, not skimpy. Slab. No basement. Better than builder grade, double pane with gas between panes windows and no patio doors but two large windows (72” square) one on East one on west side of house. Wood blinds but no drapes on all windows. Our brick on west side of house gets very hot in afternoon, but the inside wall feels normal. Our AC unit with gas heater is a 2 ton unit builder grade installed spring 2011.
At first we had trouble balancing the rooms. 2 returns in house with anti allergy filters changed regularly according to recommendation. The original installer was an alcoholic who did a poor job of installation. We’ve improved the balance of the home by following recommendations of HVAC company we trust to check our unit for any problems in fall and spring every year. They modified the gradient flow of ducts to flow properly from tower by raising them off the attic floor and changing two duct sizes. Works fine since then. Never had a problem with either the gas heater or air conditioning. I can’t see the brand or SEER rating due to that famous cover on the outside compressor. You probably can guess what was normal during the time period 10-11 years ago when our home was built in this area.
In your opinion was our compressor permanently damaged by the Christmas fiasco? When should we replace our heating cooling system? We know winter is when the prices are a bit cheaper. We dread the cost since inflation has struck due to the pandemic. Do you think it’s a good risk to take to wait to replace our unit and get a “free” 80% efficient heater?
Will we be ok to make it through another summer where August, September can be 105 F+ for days on end? 2 elders in home. Both significant health problems. Cannot tolerate heat. Ceiling fans used constantly.Fine with 78-80 F. inside house due to not being very active except husband who still mows, edges grass, maintains flowers and yard and rides bike outside for exercise. Did you say there is a tax credit for replacing our HVAC? Is it still in place now 2022?
We also participate in our electric company’s energy rebate program which connects our smart thermostat to them so on highest usage days so they turn off our unit for brief amounts of time as necessary to manage power grid efficiency in exchange for percentage off bill. Coal fired plant. No huge shade tree looming over house, but red bud is big enough to shade that large western window some by filtering light. Covered back porch prevents triple windows from so much direct sunlight. East side of house is cool until noon. I’ve noticed our unit doesn’t come on but once or twice before noon. As afternoon progresses it is on more frequently. Thank you for taking your time to advise us. We appreciate professionals who give back to their community.
We prefer you to answer via our email address as well as on your website. I’m afraid I may miss your post.
Good info, however It didn’t really answer my question. I’m building a three story house in Tampa Florida. First floor in the garage. 2nd and 3rd is 2000 sq ft per. Would one 5 ton be enough to keep the 3rd floor comfortable during the summer 95+ heat. Or do I need 2-2.5 tons unit.
You will likely need two 3.5 ton systems. However my recommendation would then be to get two 4-zone Fujitsu ductless mini split systems (36K each). Much more efficient, powerful, and no need for ductwork.
Keep in mind that you lose about 25% of your “heat” in the ducts, even if they are well insulated. So a 5 ton really delivers only 4 tons of cooling (1 ton is lost in ducts). Mini Splits actually deliver their rated capacity right to your room, as indoor unit is in the room. The only loss is a little bit in the line set, but it’s the same for central systems. They all use line sets to carry “freon” (refrigerant).
If you still want central, like I said, 5 ton is not enough, and you will likely need 3.5 ton per floor.
For central, I recommend central heat pumps, which also provides efficient heating in the winter.
You can estimate your ductless system here: https://www.remodelingcalculator.org/mini-split-ac-cost-calculator/
Thanks for pointing out that having the wrong size for my AC unit will cost me money whether it’s too big or too small. I plan to get an AC system installation for my home soon because noon times can still get very hot in my area despite it being December. I hope that an AC will make things a lot more comfortable in the long run.
Very helpful for those who want to be a expert ac technician
Using your calculation I come up with needed 2.2 tons for the AC Unit. 1300 sq.ft. I installer recommended a 2 ton unit. Is he accurate?
The only reasons you don’t want a larger size unit is because it costs more upfront. That’s it! It is like buying a 5 Amp power supply to power a 0.5 Amp device. You can just buy a cheaper 1 Amp power supply for that. All this nonsense about costing more to run and other bs is not true. Costing more to run will depend on your thermostat setting. That’s it!
I hope you can help me. I’m planning on replacing my 28 year old 4 Ton 5 Seer air conditioning unit and ducting. Using your formula I figured a total square footage in my home at 1595.25 sq ft. I felt a 3 ton unit would be enough but in the square footage I measured itt includes a den with a high open beam celling. its like a rectangle measuring 17.5 x 19.5. the celling is on the longer side of the four walls. the outside edge is 10 ft high then it goes up a little over 12 ft. Because of the high celling and the open beam it was suggested we have two registers and a return at a nearby hall way .this still remains the warmest room in the house. I bumped up to a 31/2 Ton mainly because of the den. Two installers went into the attic and suggested 4 Ton units like I already had. I thought it was to easy to just t copy what I had,, one installer thought a 31/2 Ton would be perfect. Please let t me know what you think should I purchase a 4 Ton, m31?2 or a 3 Ton unit .
Here is a little more information for the above question asking about what size or Ton I should purchase. Our house has dual glazed windows with some type of gas that is supposed to keep out the heat. Two sliding doors in the den are just like the windows. When we remodeled 28 yr’s ago we added a den, bed room and bath. All was insulated with roll in insolation and later blown in type over the roll in. I don’t remember the R-Rating but it was quite a bit higher then required. Over the open beam there is 2 inch Styrofoam 1 inch plywood roofing felt and medium brown asphalt shingles. As far as I can tell 908 sg ft is the original house and only has the blown in insolation. I live in Southern California.. The formula I used to come up with the size I thought we needed was like this: ((2,350 x 25) / 12,000) + 0.5) = 5.39 my square footage was 1595.25
What if my rough calculation is 4.67. Would you round up to a 5 ton unit?
While I don’t know how you did the calculations and what values you used, I generally would recommend with 4 ton instead, because most specifiers / contractors and even homeowners overestimate what they really need for capacity, and in the end spend much more than they need, in operating costs.
Also keep in mind, that it’s much more “efficient” to fix air leaks, insulate, etc., than to oversize your HVAC system.
If you don’t mind – what is the square footage that you used in calculations?