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I know there are a lot of possible factors that go into determining the size of the air-conditioning system a house needs. Is there a standard way of calculating what is needed, or do A/C companies each devise their own method? Is there a guide somewhere to aid a home-owner in figuring it out?

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up vote 11 down vote accepted

You'll want to check out the ENERGY STAR Air Conditioning Sizing Calculator.

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Note that your climate and insulation R-value will play a big role as well. – msemack Jun 27 '11 at 19:44
@msemack: Absolutely! – Brian Jun 27 '11 at 19:46

To roughly calculate the required BTU output of an air conditioner you will need to total up the results of six calculations.

  • Square footage of area to be cooled. (Length x Width) x 337
  • Area of south facing windows. (Height x Width) x 871
  • Area of north facing windows. (Height x Width) x 166
  • Heat produced by people (# of people x 400)
  • Heat produced by lights (Total wattage of all lights x 4.25)
  • Total heat produced by appliances (Total wattage of all appliances x 3.15)

Add all of these numbers up to determine the BTU output required of the air conditioner (Source).

You could also use a site like this Air Conditioner Sizing Worksheet to make the calculations.

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This does not appear to account for climate; or am I missing something? – retracile Aug 16 '10 at 20:23
This doesn't appear to account for east/west facing windows, is there a specific reason for that? – Stephen Jul 21 '11 at 14:12
It also does not account for shade trees that may be planted near the house, altitude, R value of insulation in the house, number of showers taken per day, hair dryer usage, heat produced by pets, etc. This is a rough calculation of AC unit size, if you follow the link near the bottom of the answer it will give you a more accurate calculation – Tester101 Jul 21 '11 at 14:34

It probably depends on climate as much as it does the square footage of your house but the general rule is one ton for every 400-500 square feet of house. There are a ton of other factors like vaulted ceilings and if you have dual pane windows and if you have 2x6 construction and therefor more insulation in the walls but for the most part none of that is going to factor into the unit. It goes more like this:

  • Contractor: How big is your house?
  • You: about 1900 square feet
  • Contractor: I see the house is older
  • You: Yeah, it was built in the 50s
  • Contractor: 5 ton it is!


  • Contractor: How big is your house?
  • You: about 1900 square feet
  • Contractor: I see the house is newer
  • You: Yeah, it was built in the 90s
  • Contractor: 5 ton it is!
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I think you need to look at the Discover Ways for Air Conditioner Sizing in order to get an idea of the size of the AC unit.

Two out of three suggested methods include Manual J which is exact science - but not many contractors want to do it for free.

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There's a ton of variables:

  • square footage
  • insulation amount
  • room flow
  • type of ducts
  • duct insulation
  • duct paths
  • climate
  • site setting
  • number of windows/doors

A good HVAC pro will be able to do the proper calculations for you.

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