A British Thermal Unit (BTU) measures heat energy. One BTU equals the energy needed to raise the temperature of one pound of water by 1°F.
Common Uses
- HVAC: AC units rated by BTU/hour (e.g., 12,000 BTU = 1 ton of cooling)
- Heating: Furnace output measured in BTUs
- Energy content: Natural gas ≈ 1,030 BTU per cubic foot
Rough Sizing Guide (AC)
- 150–250 sq ft: 6,000 BTU
- 250–350 sq ft: 8,000 BTU
- 350–550 sq ft: 12,000 BTU
- 550–700 sq ft: 14,000 BTU
Conversions
1 BTU = 1,055 joules. 1 kWh = 3,412 BTU. 1 therm = 100,000 BTU.
Why Oversizing an AC Unit Is a Mistake
Bigger isn't better: an oversized AC cools the air quickly but shuts off before adequately removing humidity, leaving rooms feeling clammy despite reaching the target temperature. Proper BTU sizing (via a Manual J load calculation, not just square footage) also reduces short-cycling, which wastes energy and shortens equipment lifespan.
BTU in Cooking
Gas stove burners are also rated in BTU/hour — a typical home burner outputs 5,000–12,000 BTU/hr, while professional restaurant ranges can exceed 20,000 BTU/hr per burner, which is why restaurant woks can sear food far faster than a home stovetop.