Space heater watts = room square feet × BTU per sq ft for your climate zone ÷ 3.412, so a 12 × 14 ft bedroom with 8 ft ceilings in climate zone 4 needs about 2,350 watts, or 7,980 BTU per hour, which is two 1,500 W heaters. Enter your room above for watts, BTU and the number of heaters.
How the space heater calculator works
The calculator uses the same BTU engine as the BTU calculator: a heating load per square foot for the IECC climate zone, placed low for a well insulated room and high for a leaky one, then scaled for ceilings taller than 8 ft. Electric heat turns each watt into 3.412 BTU per hour (EIA), so the watts follow directly.
Watts = floor area (sq ft) × BTU/h per sq ft × (ceiling ÷ 8 ft) ÷ 3.412
The BTU per square foot bands are from InspectAPedia's zone table: zone 1 30 to 35, zone 2 35 to 40, zone 3 40 to 45, zone 4 45 to 50, zone 5 50 to 60, and Bryant's cold band of 50 to 60 for zones 6 and 7. Those figures size a whole house. For a single room they are a cautious choice, which is why the flat rule of one manufacturer, 10 W per square foot (Stelpro), is shown next to the answer as a cross-check.
Worked example: 12 × 14 ft bedroom
- Area: 12 ft × 14 ft = 168 sq ft.
- Zone 4, average insulation: the middle of the 45 to 50 band is 47.5 BTU/h per sq ft.
- Heat: 168 × 47.5 = 7,980 BTU/h. Watts: 7,980 ÷ 3.412 = 2,339 W.
- Plug-in heaters go up to 1,500 W, so the calculator plans two heaters of about 1,170 W each. The flat 10 W rule gives 1,680 W for the same room.
Space heater size for a bedroom, garage or basement
The room chips fill the size, ceiling and insulation for common cases.
| Room | Size | Ceiling | Insulation | Zone 4 result |
|---|---|---|---|---|
| Bedroom | 12 × 14 ft | 8 ft | Average | 7,980 BTU/h, 2,339 W |
| Living room | 15 × 20 ft | 8 ft | Average | 14,250 BTU/h, 4,177 W |
| Garage | 20 × 20 ft | 9 ft | Poor or none | 22,500 BTU/h, 6,594 W |
| Basement | 20 × 15 ft | 7.5 ft | Average | 14,250 BTU/h, 4,177 W |
A garage is the classic underestimate: a bare uninsulated garage sits at the high end of the band and its tall ceiling adds volume, so a 1,500 W heater only takes the chill off. Basements have a lower ceiling, but the calculator never reduces the load below the 8 ft base, since below-grade walls stay cold and an oversized answer is safer than a cold room.
Watts, BTU and amps
- A 1,500 W heater delivers 5,118 BTU/h.
- On a 120 V circuit 1,500 W draws 12.5 A, so a 15 A circuit has almost no room left for anything else. Work out the breaker in the breaker size calculator.
- A kerosene, propane or gas heater is rated in BTU per hour, so use the BTU/h figure and a vented model for enclosed spaces.
- To see what a heater costs to run, multiply its kilowatts by hours and your rate in the electricity cost calculator.
When a space heater is the wrong answer
If the result needs more than two plug-in heaters for a room you use every day, a permanent heater is the better plan. Baseboard heaters are sized in the baseboard heater calculator, and a ductless heat pump for one room comes from the mini split size calculator. A heater in an uninsulated garage works hardest of all: insulate before you buy more watts.
Safety
The US Fire Administration says to keep anything that can burn at least 3 feet from all heat sources and to shut off space heaters when leaving the room or sleeping. Portable heaters account for 3% of annual heating fires and 41% of the fatal ones. Use a model with tip-over shutoff, plug it straight into the wall, and never run it on a long extension cord. Find the other heating tools in the heating and plumbing calculators hub.