Air Conditioner BTU Calculator
Size an air conditioner for a room. Enter the dimensions, how sunny the room is, and how many people use it to get a recommended BTU rating.
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❄️ Shop air conditioners by BTU
Check it outSizing rules of thumb
A common guideline is about 20 BTU per square foot, adjusted for conditions: sunny rooms need more, heavily shaded rooms less, and each person beyond two adds roughly 600 BTU. Kitchens need extra for appliance heat. Oversizing isn't better — an oversized unit cools fast but won't dehumidify well, leaving the room clammy.
How it’s calculated
This calculator returns its result directly from the values you enter using standard arithmetic for this calculation; all figures are estimates for general information.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Worked example
A 15x12 room with average sun and 2 people needs about 3,600 BTU of cooling (~0.3 tons).
Common mistakes
- Oversizing the unit, which cools fast but leaves the room clammy.
- Ignoring sun exposure, ceiling height, and occupancy.
Where it is used
- Sizing a window or mini-split AC for a room.
- Sanity-checking an HVAC recommendation.
Frequently asked questions
Why not just buy the biggest unit?
An oversized AC short-cycles and removes less humidity, so the room feels cold and damp. Right-sizing is more comfortable and efficient.
What's a 'ton' of cooling?
One ton equals 12,000 BTU per hour. Central systems are usually rated in tons; window units in BTU.
Do ceilings and climate matter?
Yes — very high ceilings, hot climates, and poor insulation all call for more capacity than the baseline.
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