BTU Calculator
Enter your room size to get the cooling capacity you need, in BTU and tons, with adjustments for sun, ceiling height, people, and kitchens. Sizes an AC, window unit, or mini split for a single room or zone.
- Cooling capacity needed
- 9,000 BTU/hr
- In tons
- 0.8
Heating estimate
Heating depends heavily on climate. Pick your zone for a rough BTU estimate.
This uses the ENERGY STAR room sizing chart (about 20 BTU per square foot) with its standard adjustments for sun, occupancy, and kitchens. It sizes a single room or zone. For a whole-house central system, a professional Manual J load calculation is the accurate method. 12,000 BTU equals 1 ton. Bigger is not better: an oversized unit short-cycles, cools unevenly, and leaves the air humid.
Right-sizing matters more than you think
The most common HVAC mistake is buying too big. An oversized unit cools the air fast, then shuts off before it pulls out the humidity, leaving the room cold and clammy and the compressor wearing out from short-cycling. Match the capacity to the space. Choosing the unit itself? See best mini splits and best portable air conditioners, and whether a heat pump fits in heat pump vs furnace.
Frequently asked questions
What size air conditioner or mini split do I need?
Size by cooling capacity in BTU per hour, which depends mostly on your room's square footage. As a rule of thumb you need about 20 BTU per square foot, so a 400 square foot room needs roughly 9,000 BTU, adjusted up for a sunny room or kitchen and down for a shaded one. This calculator applies the full ENERGY STAR chart and adjustments.
How many square feet will a 12,000 BTU mini split cool?
About 450 to 550 square feet under normal conditions. A 12,000 BTU unit is 1 ton of cooling. A very sunny room, high ceilings, a kitchen, or a hot climate lower that coverage; a shaded room raises it. Enter your exact room to see the size you need rather than guessing from the BTU.
What does BTU mean for an air conditioner?
BTU per hour is how much heat the unit can remove from a room in an hour. More BTU means more cooling power. Air conditioners and mini splits are rated in BTU; central systems are often rated in tons, where 12,000 BTU equals 1 ton.
Is it better to oversize or undersize a mini split?
Neither. An oversized unit short-cycles, turning on and off too quickly, which cools unevenly and leaves the air humid and clammy, and it wastes energy. An undersized unit runs constantly and never quite cools the space. Sizing it correctly to the room is the goal, which is what this calculator is for.
How many BTU do I need per square foot?
About 20 BTU per square foot for cooling under typical conditions, which is the basis of the ENERGY STAR chart. Adjust up about 10 percent for a very sunny room, add 4,000 BTU for a kitchen, and add 600 BTU for each regular occupant beyond two. Heating needs more, roughly 30 to 60 BTU per square foot depending on climate.
Cooling sizing from the ENERGY STAR room air conditioner chart (about 20 BTU per square foot) and its standard adjustments. Heating is a climate-zone rule of thumb. Both size a single room or zone; a whole-house system should get a professional Manual J load calculation.