BTU Calculator
Heating & Cooling · Updated July 2026
Enter your room area to estimate how many BTUs you need to cool or heat the space — so you can size an air conditioner or heater correctly.
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Estimates only. Add extra for waste and confirm with your supplier.
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How many BTUs do I need?
A common rule is about 20 BTU per square foot of living space for cooling. Adjust up for sunny rooms, high ceilings, or kitchens.
Sizing for the room
A common rule is about 20 BTU per square foot of living space for cooling. Adjust up for sunny rooms, high ceilings, and kitchens, and down for shaded or well-insulated spaces.
Why size matters
An oversized AC short-cycles and leaves the room humid; an undersized one runs constantly and never cools. Matching BTUs to the room keeps you comfortable and your energy bill lower.
How to use this calculator
- 1Measure your room area in square feet.
- 2Enter ceiling height.
- 3Read the estimated BTUs needed.
Frequently asked questions
What size air conditioner do I need?
Multiply room area by 20 BTU. A 300 sq ft room needs roughly 6,000 BTU — a typical small window unit.
How many BTUs do I need per square foot?
About 20 BTU per square foot for cooling — a 300 sq ft room needs roughly 6,000 BTU.
What size air conditioner do I need for a room?
Multiply the room's square footage by 20 BTU, then adjust for sun, ceiling height, and occupancy.
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