BTU Calculator
BTU (British Thermal Unit) measures the amount of energy needed to raise one pound of water by 1°F. For air conditioning and heating, BTU/hr ratings indicate how much thermal energy a system can move per hour. The right BTU capacity prevents under-cooling (too weak) or short-cycling (too powerful).
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Tip: Oversizing is as bad as undersizing. A too-large AC cools fast but cycles off before removing humidity, leaving the room cold and clammy.
- 1Start with 20 BTU per square foot of living space
- 2Adjust for ceiling height: multiply by height/8 (standard 8ft ceiling)
- 3Factor in climate zone, sun exposure, and number of occupants
- 4Add 600 BTU per person beyond two regular occupants
300 sq ft bedroom, 8ft ceiling, moderate climate=6,000 BTU (0.5 ton)300 × 20 = 6,000 BTU
500 sq ft open plan, 10ft ceilings, sunny=~13,750 BTU (1.1 ton)Height and sun adjustments add ~37%
| Room Size (sq ft) | BTU Needed | AC Tons |
|---|---|---|
| 100–150 | 5,000 | 0.4 |
| 150–250 | 6,000 | 0.5 |
| 250–350 | 7,000 | 0.6 |
| 350–450 | 10,000 | 0.8 |
| 450–550 | 12,000 | 1.0 |
| 550–700 | 14,000 | 1.2 |
| 700–1,000 | 18,000 | 1.5 |
| 1,000–1,200 | 21,000 | 1.75 |
| 1,200–1,400 | 23,000 | 1.9 |
| 1,400–1,500 | 24,000 | 2.0 |
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Fun Fact
1 ton of air conditioning equals 12,000 BTU/hr — originally derived from the amount of cooling produced by melting one ton of ice over 24 hours.
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