Works in feet or metres. Room units that are too big cool quickly but leave the air clammy; too small and they run all day without keeping up.
ENERGY STAR room AC sizing chart
| Room area | Cooling capacity |
|---|---|
| up to 150 sq ft | 5,000 BTU/h |
| 150 to 250 sq ft | 6,000 BTU/h |
| 250 to 300 sq ft | 7,000 BTU/h |
| 300 to 350 sq ft | 8,000 BTU/h |
| 350 to 400 sq ft | 9,000 BTU/h |
| 400 to 450 sq ft | 10,000 BTU/h |
| 450 to 550 sq ft | 12,000 BTU/h |
| 550 to 700 sq ft | 14,000 BTU/h |
| 700 to 1,000 sq ft | 18,000 BTU/h |
| 1,000 to 1,200 sq ft | 21,000 BTU/h |
| 1,200 to 1,400 sq ft | 23,000 BTU/h |
| 1,400 to 1,500 sq ft | 24,000 BTU/h |
| 1,500 to 2,000 sq ft | 30,000 BTU/h |
| 2,000 to 2,500 sq ft | 34,000 BTU/h |
How the BTU calculator works
For cooling, BtuSize multiplies length by width to get the floor area, finds the matching row in the ENERGY STAR table, then applies the published adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU for every regular occupant beyond two, and 4,000 BTU if the unit will cool a kitchen. Rooms over 2,500 square feet are extrapolated, but at that size a professional load calculation is the right tool.
For heating, it multiplies the area by a rule-of-thumb figure for your climate, scales it by ceiling height relative to 8 feet, and adjusts for insulation: 15% less for a well-insulated modern room and 25% more for an old, drafty one. The result is the heat output you need on a typical cold day, shown in BTU per hour and in watts.
Units
BTU ratings on air conditioners and heaters mean BTU per hour. One watt is 3.412 BTU/h, one kilowatt is 3,412 BTU/h and one ton of cooling is 12,000 BTU/h. Cooling capacity is not the same as power draw: an efficient AC moves three or more watts of heat for every watt of electricity it uses. Everything is calculated in your browser.
Frequently asked questions
How many BTU do I need for a 12 × 15 room?
180 square feet needs about 6,000 BTU of cooling under the ENERGY STAR table.
How many square feet does 12,000 BTU cool?
About 450 to 550 square feet.
What does BTU stand for?
British thermal unit; AC and heater ratings are BTU per hour.
Is BtuSize free?
Yes, with no sign-up or ads.
Are my room sizes saved?
Yes, in this browser only.