What Size AC for a 1,500 Square Foot House?
Cooling size
3 tons
1,500 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 area1500 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 size3 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 | 34,600 BTU/hr |
|---|---|
| That is | 2.88 tons12,000 BTU makes a ton |
| Equipment size to shop for | 3 tonnearest half ton step |
| Cooling per square foot | 23 BTU |
| Heating output needed | 67,500 BTU/hr |
| Furnace input, 80% AFUE | 84,375 BTU/hr |
| Furnace input, 95% AFUE | 71,053 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 | 3.5 tons | 45,000 BTU/hr |
| Zone 2 | 3 tons | 52,500 BTU/hr |
| Zone 3 | 3 tons | 60,000 BTU/hr |
| Zone 4 | 3 tons | 67,500 BTU/hr |
| Zone 5 | 3 tons | 75,000 BTU/hr |
| Zone 6 | 2.5 tons | 82,500 BTU/hr |
| Zone 7 | 2.5 tons | 90,000 BTU/hr |
What else matters here
Thirty four thousand six hundred BTU is 2.88 tons and rounds to 3, which is 36,000. This is the single most searched house size in American HVAC and 3 tons is the answer it almost always lands on in a mixed climate.
Twenty three BTU a square foot. Move the same house to the Gulf coast and it climbs, move it to Minnesota and it falls to around 32,000 while the furnace nearly doubles. The sweep below shows exactly that, and it is the part of HVAC sizing that catches people who move states and expect the same equipment.
The heating side is 67,500 BTU of output, which is an 84,375 input furnace at 80 percent AFUE or 71,053 at 95. Furnaces are sold in 20,000 BTU steps around here, so the shop will quote you an 80,000 or a 90,000 and both are defensible.
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 1,500 square feet?
About 34,600 for cooling in a mixed climate, which is a 3 ton system. In zone 2 it is nearer 38,000 and in zone 6 nearer 32,000.
Is 3 tons enough for 1,500 square feet?
In a mixed climate with ordinary insulation, yes. The cases where it is not are a lot of west facing glass, a cathedral ceiling, or an older house with little wall insulation, and those are exactly the cases a load calculation catches and a square foot rule does not.
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.