How Many BTU for a 12x12 Room?
Cooling load
2,880 BTU/hr
144 square feet, 8 foot ceilings, climate zone 4, average sun, one person, no kitchen.
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 area144 sq ft
- Ceiling height8 ft
- Climate zoneZone 4, mixed, mid Atlantic and the lower Midwest
- Sun exposureAverage
- People living there1 each
- Kitchen inside the conditioned spaceNo
Cooling size1 ton
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 | 2,880 BTU/hr |
|---|---|
| That is | 0.24 tons12,000 BTU makes a ton |
| Equipment size to shop for | 1 tonnearest half ton step |
| Cooling per square foot | 20 BTU |
| Heating output needed | 6,480 BTU/hr |
| Furnace input, 80% AFUE | 8,100 BTU/hr |
| Furnace input, 95% AFUE | 6,821 BTU/hr |
The material list this produces
| Condenser, tons | 1 |
|---|---|
| Matched air handler or coil, tons | 1 |
| 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 | 1 ton | 4,320 BTU/hr |
| Zone 2 | 1 ton | 5,040 BTU/hr |
| Zone 3 | 1 ton | 5,760 BTU/hr |
| Zone 4 | 1 ton | 6,480 BTU/hr |
| Zone 5 | 1 ton | 7,200 BTU/hr |
| Zone 6 | 1 ton | 7,920 BTU/hr |
| Zone 7 | 1 ton | 8,640 BTU/hr |
What else matters here
Two thousand eight hundred and eighty BTU, which is 20 a square foot with nothing added because a bedroom has no kitchen in it and one occupant sits below the two the base figure already assumes.
Nothing is sold that small. The smallest window units are 5,000 BTU and the smallest mini splits are 6,000 to 9,000, so any of them will cool a 12 by 12 room easily. What matters at this size is not capacity, it is whether the unit modulates: an inverter mini split will throttle down and run continuously, and a fixed speed window unit will cycle on and off all night.
If the room is a west facing bedroom with a lot of glass, or a converted attic space, the figure climbs fast. Switch the sun setting above to sunny and watch what happens.
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 do I need for a 12x12 bedroom?
About 2,880 by the square foot method, so any unit on the market has enough capacity. The smallest window air conditioner is 5,000 BTU and the smallest mini split is around 6,000.
Can an air conditioner be too big for one room?
Yes, and in a single room it is very noticeable. An oversized unit cools the air in a few minutes, shuts off before it has dehumidified anything, and leaves the room cold and damp. An inverter driven mini split avoids this by running slowly rather than cycling.
What is a ton of cooling?
12,000 BTU per hour. It comes from the ice trade, the heat needed to melt one ton of ice over twenty four hours. Residential split systems are sold in half ton steps from one and a half up to five, so the only sizes you can actually buy are 18,000, 24,000, 30,000 and so on.
The btu and ac size calculator takes any dimensions you like and hands back the same list.