BTU calculator
Reviewed 2026-07-04 · For window, portable and room AC sizing
Start at 25 BTU per square foot: a 150 sq ft room ≈ 5,000–6,000 BTU, a 300 sq ft room ≈ 7,000–8,000, a 500 sq ft space ≈ 11,000–12,000. Then adjust: hot climate or strong sun +10–15%, tall ceilings +12–25%, kitchen in the space +4,000 BTU. Buying portable? Size by the SACC rating, not the marketing BTU.
Size your room
Window units: buy the nearest size at or just above the number. Portables: match the SACC rating to the number; the marketing BTU runs high.
Common window unit sizes
| Unit | Typical room | Coverage |
|---|---|---|
| 5,000 BTU | Small bedroom | 100–150 sq ft |
| 6,000 BTU | Bedroom | 150–250 sq ft |
| 8,000 BTU | Large bedroom / office | 250–350 sq ft |
| 10,000 BTU | Living room | 350–450 sq ft |
| 12,000 BTU | Large living room | 450–550 sq ft |
| 15,000 BTU | Open plan | 550–800 sq ft |
Frequently asked
How many BTU do I need for a 12x12 room?
A 12x12 room is 144 sq ft, which lands around 5,000 to 6,000 BTU in a moderate climate with normal ceilings. Sunny room in a hot climate: step up to 7,000 to 8,000. Oversizing a window unit cools fast but leaves the room humid.
Is it better to get a bigger AC just in case?
No. An oversized unit hits the setpoint before it dehumidifies, so the room feels cold and clammy, and the constant on-off cycling wears the compressor. Size to the load; when between sizes, take the smaller one for a bedroom and the larger for a kitchen-connected space.
How many BTU per square foot?
The standard planning base is 20 to 25 BTU per square foot, before adjustments. This calculator uses 25 and then adjusts for climate, ceiling height, insulation, sun and kitchen load, which is where charts usually stop and get it wrong.
Window unit vs portable AC for the same BTU?
Window units deliver more real cooling per rated BTU. Portables lose capacity through hose heat gain and by exhausting room air. Many portables now show a SACC rating (their honest number), which can be 40 percent below the marketing BTU. If you must go portable, size by SACC.
Related tools
Sources
- Base BTU-per-square-foot planning range per Energy Star room AC guidance.
- SACC (Seasonally Adjusted Cooling Capacity) per DOE portable AC test standard.
- Coverage bands cross-checked against window unit manufacturer sizing charts (LG, GE, Frigidaire).