BTU Calculator
Estimate the BTU cooling capacity a room needs, as a rough guide.
Runs in your browser — nothing is sent or saved.
How to use the BTU calculator
- Enter the room's length and width.
- Choose the sun exposure.
- Set the number of occupants and whether it is a kitchen.
- Read the rough BTU estimate.
A rule of thumb, not a load calculation
A BTU, or British thermal unit, measures cooling capacity, and this BTU calculator estimates the BTU for room size so you can match the AC BTU to the space. It starts from a common rule of thumb, about 20 BTU per hour for each square foot of floor, then nudges the figure for sun, extra people, and heat from a kitchen. That is enough to land in the right range when shopping, but it is only a rule of thumb. A professional load calculation, such as an ACCA Manual J, is what decides the real size, because it accounts for insulation, windows, climate, and ceiling height that a floor-area rule cannot see.
What changes the number
Several things push the requirement up or down. A very sunny room gains heat and needs more cooling, while a heavily shaded one needs a little less. Each person beyond two adds warmth, and a kitchen adds a lot because of the appliances. Ceiling height, insulation quality, window size and direction, and your local climate all matter too, which is exactly why a proper load calculation beats any single rule.
Do not oversize
Bigger is not better with air conditioning. An oversized unit cools the air fast but switches off before it removes enough humidity, leaving the room cold and clammy and cycling on and off in a way that wastes energy and wears the unit. An undersized unit runs constantly and never quite copes. Getting reasonably close matters, which is another reason to confirm with a load calculation rather than rounding up on a hunch.
A worked example
A 15 by 12 foot room is 180 square feet. At 20 BTU per square foot that is 3,600 BTU. In average sun with two people and no kitchen, the estimate stays near 3,600 BTU per hour, roughly a small window unit.
FAQ
How many BTU do I need for my room?
As a rough guide, about 20 BTU per hour per square foot of floor, adjusted for sun, occupants, and kitchens. For a 180 square foot room that is around 3,600 BTU. Confirm with a load calculation.
Is this accurate enough to buy a unit?
It gets you in the right range, but no floor-area rule is exact. A professional load calculation, such as Manual J, decides the real size by accounting for insulation, windows, and climate.
Why should I not just buy a bigger unit?
An oversized air conditioner short-cycles, so it cools the air but leaves it humid, wastes energy, and wears out faster. Right-sizing matters more than going big.
What is a ton of cooling?
One ton equals 12,000 BTU per hour, a unit used for larger air conditioners. The tool shows your estimate in tons as well as BTU.
Is my information private?
Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.