Heater size calculator
Enter the room size and ceiling height. You’ll get the wattage to look for and how many standard 1,500 watt heaters that takes.
Heater size
1,440 watts
- Heating output
- 4,913 BTU per hour
- 1,500 watt heaters
- 1
- Room size
- 144 sq ft
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How it works
The common rule for electric heat is 10 watts per square foot of floor in a room with an 8-foot ceiling. The calculator scales that up or down for taller or lower ceilings, since there’s more or less air to heat, and by about 20–25% for insulation that’s better or worse than average.
One watt of electric heat is 3.412 BTU per hour, which lets you compare with gas and propane heaters rated in BTU.
Example
With room length 12 ft, room width 12 ft, ceiling height 8 ft, insulation: average, the answer is 1,440 watts.
Tips before you buy
- A standard plug-in space heater tops out at 1,500 watts, the most a typical household circuit can safely supply. Bigger rooms need more than one heater, on separate circuits, or a hardwired heater.
- If the heater is only taking the chill off a room that already gets some central heat, you can go smaller.
- Rooms with lots of windows, outside walls on several sides, or over an unheated garage need the higher end.
Common questions
What size heater do I need for a 12×12 room?
About 1,440 watts with an 8-foot ceiling and average insulation, so one standard 1,500 watt heater.
How many square feet will a 1,500 watt heater heat?
About 150 square feet as the main source of heat, by the 10 watts per square foot rule. As a supplement to central heat it can handle more.
How many BTU is 1,500 watts?
About 5,118 BTU per hour. Each watt of electric heat is 3.412 BTU per hour.