BTU Calculator (Air Conditioner Size)
What size air conditioner do you need? Get the cooling capacity in BTU per hour and kW from the room size, ceiling height, sun, people and use.
How it is calculated
The starting point is the ENERGY STAR room-size guide: about 20 BTU per square foot for an 8 ft ceiling, in steps – 150–250 ft² needs 6,000 BTU, 250–300 ft² 7,000 BTU, 450–550 ft² 12,000 BTU. Then the calculator adjusts:
- higher ceilings: in proportion to the height above 8 ft
- heavily shaded room: −10%; very sunny room: +10%
- more than two people regularly in the room: +600 BTU per person
- kitchen: +4,000 BTU
Worked example
A 270 ft² bedroom with an 8 ft ceiling and normal sun needs about 7,000 BTU/h (2.05 kW). Make it a very sunny 270 ft² living room where four people watch TV and you need 7,000 × 1.1 + 2 × 600 = 8,900 BTU – a 9,000 BTU unit.
Why size matters
An undersized unit runs nonstop and never cools the room. An oversized one cools the air so quickly that it switches off before it has removed the humidity, leaving the room cold and clammy. Buy the size closest to the result, not the biggest.
BTU, kW and tons
1,000 BTU/h = 0.293 kW, and 12,000 BTU/h = 1 ton of refrigeration = 3.52 kW. Note that the cooling capacity is not the electricity the unit uses: a unit with an efficiency (EER) of 12 uses about 1 kW of electricity to deliver 12,000 BTU/h of cooling.
Frequently asked questions
How many BTU do I need for a 300 ft² room?
About 7,000 BTU, or 8,000 BTU if the room is very sunny.
How many kW is 12,000 BTU?
3.52 kW of cooling.
Does the calculator work for heating?
It is made for cooling. Heat loss in winter depends mostly on insulation and the outdoor temperature.