About this calculator
Quick sizing for a room air conditioner, mini-split or heater in BTU per hour, tons and kW.
How it’s calculated
Cooling: 20 BTU per sq ft, ±10% for sun, +600 per extra person over two, +4,000 for a kitchen, scaled for ceiling height. 12,000 BTU/h = 1 ton.
Worked example
With these inputs:
- Size for
- Cooling (air conditioning)
- Room length
- 20 ft
- Room width
- 15 ft
- Ceiling height
- 8 ft
- Sun exposure
- Average
- People usually in the room
- 2
- Room is a kitchen
- No
- Climate (heating)
- Southern Ontario, southern Quebec, Maritimes
- Insulation
- Average
the result is:
- Cooling capacity
- 6,000 BTU/h
- Room area
- 300 sq ft
- Base (20 BTU/sq ft)
- 6,000 BTU/h
Tips from the trade
- Oversized AC short-cycles and leaves rooms clammy, so bigger is not better.
- A CSA F280 heat loss and gain calculation is the standard for whole-house sizing in Canada, and Part 9 of the building code requires it for heating and cooling equipment.
Common questions
What size AC for a 400 sq ft room?
About 8,000 BTU/h (2.3 kW) for an average room with average sun. Heating the same room through a Canadian winter takes much more, often 20,000 BTU/h or more, so size heat pumps for the heating load.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.