How To Calculate The Size Of A Central AC Unit For Your Home

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.



Central Air HVAC Costs
Central AC only
$3900 - $5600
AC + Hot Air Furnace
$7900 - $10400
Central Air + Ducts
$12100 - $17900

See costs in your area Start Here - Enter Your Zip Code



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 size AC 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 and figure out the central air size you need.

air conditioner size calculator

What does central air conditioner sizing mean?

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. It is measured in BTU (British Thermal Unit) per hour and in tons.

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, 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.

Brand name AC units come in the following sizes:

18,000 – 1.5 tons
24,000 – 2 tons
30,000 – 2.5 tons
36,000 – 3 tons
48,000 – 4 tons
60,000 – 5 tons

The most popular size that works for the majority of residential homes is either 2 or 2.5 ton unit.

Unit size reference chart based on heat zones

If you just want to get a quick sense about the air conditioning tonnage size 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:

heat zone map

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.

hvac unit size estimator

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.



Manual J: professional AC unit size calculation

AC calculator - Manual J Residential Load 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 of installing 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.

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.

Whole house manual J load calculation

HVAC Manual J

This 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
– 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 Manual J 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 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.

Factors that impact how big of an AC unit you need

air conditioner calculator

As you may have already guessed there are a number of important variables that will effect the size of the central air unit 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

DIY AC Size Calculator

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.



Why it’s a BAD idea to install the wrong size central air

central air unit that is too smallIt is important to understand exactly why it is not advisable to install the wrong size central air, whether too big or too small.

Downsides of a 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
  • Cycles on and off many times throughout the day, thereby creating more wear and tear on the unit
  • Does not cool your home evenly

Disadvantages of a 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

 



Central Air HVAC Costs
Central AC only
$3900 - $5600
AC + Hot Air Furnace
$7900 - $10400
Central Air + Ducts
$12100 - $17900

See costs in your area Start Here - Enter Your Zip Code



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4 thoughts on “How To Calculate The Size Of A Central AC Unit For Your Home

    1. Leo B

      Miranda,

      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?

      Leo

      Reply
  1. Bob Ysais

    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 .

    Reply
    1. Bob Ysais

      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

      Reply