What size AC 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 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.
The best course of action is to get free estimates from licensed HVAC installers who have a comprehensive, professionally designed HVAC calculator. The pro will come in for an in-house estimate and tell you exactly what size central air 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
Central AC only
$3850 - $5630
AC + Hot Air Furnace
$7170 - $9650
Central Air + Ducts
$12100 - $17900
See costs in your area Start Here - Enter Your Zip Code
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 $1,800-2,000.
2 ton AC unit costs $2,400-2,800
3 ton AC unit costs $3,200-3,800
4 ton AC unit costs $4,000-4,200
5 ton AC unit costs $4,800-5,500 ( a 5 ton unit is extremely powerful and is not required for most homes)
In general, you can roughly add $800-1,000 for each additional ton, as the units go up in size.
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 unit size 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 calculator based on heat zones
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.
Air conditioner room size calculator
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.
AC Size Calculator – 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 central air
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
$2900 - $3800
$6000 - $7800
See costs in your area Start Here - Enter Your Zip Code
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 cenral 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.
See costs in your area Start Here - Enter Your Zip Code