What Size AC for a 1,200 Square Foot House?
Cooling size
2.5 tons
1,200 square feet, 8 foot ceilings, climate zone 4, average sun, three people, kitchen inside.
This is a rule of thumb and it is not a load calculation. The real answer is a Manual J, which counts your windows, your insulation, your infiltration and which way the house faces, and a contractor should run one before anybody orders equipment. Use this number to sanity check a quote, not to place an order.
Oversizing is the classic mistake and it is worse than being slightly small. A unit with too much capacity satisfies the thermostat in a few minutes and shuts off, so it never runs long enough to pull water out of the air, and the house ends up cold and clammy with the compressor short cycling itself to an early grave.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Conditioned floor area1200 sq ft
- Ceiling height8 ft
- Climate zoneZone 4, mixed, mid Atlantic and the lower Midwest
- Sun exposureAverage
- People living there3 each
- Kitchen inside the conditioned spaceYes
Cooling size2.5 tons
This is a rule of thumb and it is not a load calculation. The real answer is a Manual J, which counts your windows, your insulation, your infiltration and which way the house faces, and a contractor should run one before anybody orders equipment. Use this number to sanity check a quote, not to place an order.
Oversizing is the classic mistake and it is worse than being slightly small. A unit with too much capacity satisfies the thermostat in a few minutes and shuts off, so it never runs long enough to pull water out of the air, and the house ends up cold and clammy with the compressor short cycling itself to an early grave.
| Cooling load | 28,600 BTU/hr |
|---|---|
| That is | 2.38 tons12,000 BTU makes a ton |
| Equipment size to shop for | 2.5 tonnearest half ton step |
| Cooling per square foot | 24 BTU |
| Heating output needed | 54,000 BTU/hr |
| Furnace input, 80% AFUE | 67,500 BTU/hr |
| Furnace input, 95% AFUE | 56,842 BTU/hr |
The material list this produces
| Condenser, tons | 3 |
|---|---|
| Matched air handler or coil, tons | 3 |
| Line set, ft | 25 |
| Equipment pad, each | 1 |
| Disconnect and whip, each | 1 |
| Programmable thermostat, each | 1 |
Climate zone compared
The same house sizes differently by climate, and not in the direction people expect. Cooling load falls as you go north while heating load climbs, so a Minnesota house needs a smaller air conditioner and a much bigger furnace than a Florida one.
| Climate zone | Cooling size | Heating output needed |
|---|---|---|
| Zone 1 | 2.5 tons | 36,000 BTU/hr |
| Zone 2 | 2.5 tons | 42,000 BTU/hr |
| Zone 3 | 2.5 tons | 48,000 BTU/hr |
| Zone 4 | 2.5 tons | 54,000 BTU/hr |
| Zone 5 | 2.5 tons | 60,000 BTU/hr |
| Zone 6 | 2 tons | 66,000 BTU/hr |
| Zone 7 | 2 tons | 72,000 BTU/hr |
What else matters here
Twenty eight thousand six hundred BTU is 2.38 tons, and the sizes on the shelf are 2 and 2.5. It rounds to 2.5, which is 30,000 BTU, so you are buying about five percent more capacity than the estimate asked for.
That is the normal situation and it is fine. What is not fine is treating the gap as an argument for going to 3 tons, which would be 26 percent over and firmly into short cycling territory.
A 1,200 square foot house with a 2.5 ton system is one of the most common configurations in American housing stock, which means it is also one of the easiest to get a second quote on.
What this calculator assumes
- This is the square foot method, the same rule of thumb behind every free sizing chart: 20 BTU per square foot of cooling as the base, scaled for ceiling height, climate zone and sun, plus 600 BTU for each person over two and 4,000 for a kitchen.
- Heating is figured at 30 to 60 BTU per square foot depending on zone, which is output at the register. Furnace input is higher, because an 80 percent furnace throws a fifth of the gas up the flue.
- Equipment is sold in half ton steps up to 5 tons, so the number to shop with is the rounded one, not the raw one.
Questions people ask
How many BTU for 1,200 square feet?
About 28,600 for cooling in a mixed climate, which rounds up to a 2.5 ton, 30,000 BTU system.
Is it better to go one size up?
No, and this is the most common mistake in residential HVAC. An oversized unit cools the air fast, satisfies the thermostat and shuts off before the coil has been cold long enough to pull moisture out. You get a house that is cold and clammy, and a compressor that short cycles its way to an early death. A slightly small system just runs longer on the hottest afternoon of the year.
Is this the same as a Manual J?
No. A Manual J counts the actual windows and which way they face, the insulation in the walls and the attic, how tight the envelope is, the duct losses and the local design temperature. Two houses of identical size can be a ton and a half apart on that calculation. Use this figure to check whether a quote is in the right range, then have the load calculation done before anything is ordered.
The btu and ac size calculator takes any dimensions you like and hands back the same list.