About this calculator
Quick sizing for a room air conditioner or heater in BTU per hour and tons.
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)
- Mixed (zone 4)
- 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 Manual J load calculation is the professional standard for whole-house sizing.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and local building codes before ordering.