BTU calculator
The air conditioner size, in BTU, a room needs for cooling.
About this calculator
What size air conditioner do I need? This BTU calculator estimates the cooling capacity a room needs from its size.
It adjusts for sunlight, the number of people and kitchens, and shows the result in BTU per hour, kilowatts and tons.
How it works
Starts from a standard sizing chart by floor area, then takes off 10% for a shaded room or adds 10% for a sunny one, adds 600 BTU for each person beyond two and 4,000 BTU for a kitchen.
Last reviewed:
Worked example
- Measure in: Feet
- Room length: 15
- Room width: 14
- Sunlight: Average
- People who usually use the room: 2
- Room is a kitchen: No
Cooling capacity needed: 6,000 BTU/h
Common questions
What is a BTU?
A British thermal unit is the energy needed to heat one pound of water by one degree Fahrenheit. Air conditioners are rated in BTU per hour of cooling.
What happens if my air conditioner is too big?
It cools the room quickly but switches off before removing enough humidity, leaving the room cold and clammy. A unit that is too small runs constantly.
How many BTU per square foot?
Roughly 20 BTU per square foot for a typical room, but small rooms need relatively more and large rooms relatively less, which the sizing chart allows for.
Sources and further reading
The story behind it
Where the idea came from: Willis Carrier designed the first modern air conditioning system in 1902, to control humidity in a printing plant in Brooklyn, New York.
Carrier's machine was built to stop paper from swelling, but people soon noticed how pleasant the cool air felt. Air conditioning spread to cinemas, offices and homes, and sizing it to the room is still the key to comfort.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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