Square-foot coverage is only a starting point
Published coverage estimates assume particular insulation, ceiling height and temperature rise. A drafty garage in winter needs more heat than a small interior room of the same floor area. Use the manufacturer's assumptions when available rather than comparing square-foot numbers alone.
Heat output uses watts or BTU per hour
Electric resistance heaters are commonly rated in watts; fuel heaters in BTU/h. One watt is about 3.41 BTU/h, so a 1,500W heater produces roughly 5,100 BTU/h. That conversion compares heat rate, not operating cost or approved location.
Air volume and heat loss matter
Include ceiling height, window area, door openings, air leakage, insulation and outdoor temperature. Workshops with frequent door openings may need focused radiant heat near occupants rather than attempting to heat all the air.
Do not oversize past safety limits
A larger combustion heater may consume oxygen and produce moisture and exhaust faster, while a larger electrical load may exceed the circuit. Thermostats improve comfort but do not make an improperly installed heater safe. Select only equipment listed for the space and fuel system.
Sizing checklist
- Measure floor area and ceiling height.
- Assess insulation, drafts and door openings.
- Define the desired temperature rise.
- Compare watts or BTU/h under stated conditions.
- Check electrical or ventilation capacity.
- Prefer permanent heating evaluation for routine whole-room use.
Sources and further reading
A sizing calculation never overrides the heater's listing, clearances, ventilation instructions or electrical limits.