BTU Calculator
Estimate the heat output a room needs in BTU and watts.
Estimated heat output
5,732BTU/hr
- In watts
- 1,680 W
- Room volume
- 33.6 m³
Rough estimate using 50 W per cubic metre. Poorly insulated rooms need more.
v1.0 · Reviewed September 2026
How this is worked out
Room volume × 50 W per cubic metre gives the watts needed, which is converted at 3.412142 BTU/h per watt.
Worked example
- 4 m × 3.5 m × 2.4 m
33.6 m³ × 50 = 1,680 W, × 3.412142
5,732 BTU/hr
Assumptions and limits
- Fixed at 50 W per cubic metre, a middle-of-the-road figure for a reasonably insulated room. Poor insulation, large glazed areas or a conservatory need considerably more.
- A sizing rule of thumb, not a heat loss calculation, and it makes no allowance for the room's exposure or orientation.