An air conditioner that is too small cannot keep up, but one that is too big cools the room quickly without removing enough humidity, leaving it clammy. This calculator sizes a room or window unit using the ENERGY STAR guidelines for room area, sun, occupancy and kitchens.
For central air or whole-house systems, ask an HVAC contractor for a Manual J load calculation, which accounts for insulation, windows and climate.
Formula
BTU = Base (from room area) × Sun factor + 600 × (People − 2) + 4,000 if kitchen
Base sizes range from 5,000 BTU/h for 100–150 ft² up to 34,000 BTU/h for 2,000–2,500 ft². One ton of cooling is 12,000 BTU/h.
Worked example
A very sunny 300 ft² living room used by 3 people: the base is 8,000 BTU/h, plus 10% for sun is 8,800, plus 600 for the third person = 9,400 BTU/h — so choose a 10,000 BTU unit.
Frequently asked questions
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for typical rooms, but the ENERGY STAR table used here is more accurate for small and large rooms.
What size AC for a 12 × 12 room?
A 144 ft² room needs about 5,000 BTU/h, before adjustments for sun, extra people or a kitchen.
Last updated · Calculations checked by automated tests· Editorial policy