How to use this calculator
- Enter the room's length, width and ceiling height.
- Choose sun exposure, insulation quality and your climate.
- Enter the usual number of occupants and tick 'kitchen' if appliances add heat.
Formula
Cooling starts from about 20 BTU/h per square foot for an 8 ft ceiling (the ENERGY STAR room-size guideline) and is adjusted: −10% for heavy shade, +10% for strong sun, +600 BTU/h per extra occupant and +4,000 BTU/h for kitchens. Heating uses typical BTU/h per ft² by climate.
Cooling ≈ 20 × area(ft²) × (height/8) × adjustments
1 ton of cooling = 12,000 BTU/h = 3.517 kW
1 kW = 3,412 BTU/h
Example
- 15 × 12 ft room (180 ft²), 8 ft ceiling, average sun and insulation, 2 people.
- Cooling ≈ 20 × 180 = 3,600 BTU/h, so the smallest common unit (5,000 BTU/h) is enough.
- Heating in a moderate climate ≈ 40 × 180 = 7,200 BTU/h (2.1 kW).
Frequently asked questions
Is bigger always better for an air conditioner?
No. An oversized unit cools quickly but cycles on and off, removing less humidity and wasting energy. Choose the closest size at or slightly above the estimate.
When do I need a professional load calculation?
For whole-house systems, ductwork, heat pumps or unusual rooms, an ACCA Manual J (or equivalent) calculation by an HVAC professional is recommended.
What does SEER mean?
Seasonal energy efficiency ratio: cooling output per unit of electricity over a season. Higher SEER means lower running costs, not more capacity.