BTU Calculator
Last updated: June 2026 · Free · No sign-up required
Enter values above and click Calculate to see your result instantly.
Quick reference
Recommended cooling BTU by room size:
| Room size | BTU/hr |
|---|---|
| 150–250 sq ft | 5,000–6,000 |
| 250–350 sq ft | 7,000–8,000 |
| 350–550 sq ft | 10,000–12,000 |
| 550–1,000 sq ft | 14,000–18,000 |
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Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.
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Interpretation guide
Adjustments to base BTU:
| Condition | Adjust BTU |
|---|---|
| Very sunny room | +10% |
| Heavily shaded | -10% |
| Kitchen | +4,000 |
| Each extra occupant (>2) | +600 |
Formula & methodology
Formula: Base cooling BTU = area (sq ft) x 20; then adjust for sun, occupants, and kitchens
BTU (British Thermal Unit) per hour measures heating or cooling capacity. A common rule of thumb is about 20 BTU per square foot for cooling, then adjust up for sunny rooms, kitchens, and extra occupants, or down for shade. Undersizing leaves a room uncomfortable; oversizing cools too fast without removing humidity. The calculator sizes an AC unit or heater to your room and conditions.