Space Heater Size Calculator
Measure the room and say whether the heater adds to your main heat or is the only heat. You'll get the wattage to buy, the BTUs, how many plug-in heaters it takes and what it costs per hour.
- Manufacturer sizing rule
- Feet or metric
- Running cost per hour
2 What size to buy
Updates as you type| Watts per sq ft used | 10 (extra heat for a room that is already heated) |
|---|---|
| Heat output | 1,500 W = 5,118 BTU per hour |
| 1,500 W plug-in heaters | One |
| Running cost at full power | $0.27 per hour |
A 1,500 W heater draws 12.5 amps at 120 volts. If yours has a thermostat, it switches off once the room is warm, so the hourly cost above is the most it can use.
Quick answer
Size a space heater at about 10 watts per square foot for extra heat, or 15 watts where it is the only heat or the room is poorly insulated. A 15 × 10 ft room (150 sq ft) needs about 1,500 watts, which one standard heater supplies (5,118 BTU per hour).
Space heater size by room
| Room | Extra heat | Only heat | 1,500 W heaters (only heat) |
|---|---|---|---|
| 10 × 10 ft | 1,000 W | 1,500 W | 1 |
| 12 × 10 ft | 1,200 W | 1,800 W | 2 |
| 12 × 12 ft | 1,440 W | 2,160 W | 2 |
| 15 × 10 ft | 1,500 W | 2,250 W | 2 |
| 15 × 12 ft | 1,800 W | 2,700 W | 2 |
| 20 × 12 ft | 2,400 W | 3,600 W | 3 |
| 20 × 15 ft | 3,000 W | 4,500 W | 3 |
Broan-NuTone sizing: 10 W per sq ft for extra heat, 15 W per sq ft as the only heat or in a poorly insulated room. Put each plug-in heater on a different circuit; where a room needs more than two, a hard-wired heater installed by an electrician is the simpler choice. Use the calculator above for your exact size.
How to size a space heater
Heater maker Broan-NuTone gives a simple rule: 10 watts for every square foot of room supplies enough extra heat in most climates. Where the heater is the only heat in the room, or the room is poorly insulated, it says 15 watts per square foot may be needed.
For a 15 × 10 ft room: 150 sq ft × 10 W = 1,500 watts. As the only heat it would take 150 × 15 = 2,250 watts, more than one plug-in heater gives.
| Room | Extra heat (10 W/sq ft) | Only heat (15 W/sq ft) |
|---|---|---|
| 100 sq ft | 1,000 W | 1,500 W |
| 150 sq ft | 1,500 W | 2,250 W |
| 200 sq ft | 2,000 W | 3,000 W |
| 250 sq ft | 2,500 W | 3,750 W |
| 300 sq ft | 3,000 W | 4,500 W |
The 1,500 watt limit
Many whole-room plug-in heaters are 1,500 watts. EIA lists electric heaters from 500 to 2,500 watts, with 1,500 the most typical. Ontario's Save on Energy program says 1,500 watts is about as much as you can safely plug into a home wall outlet, so plan bigger heaters as hard-wired units. At 10 watts per square foot, one 1,500 W heater suits a room of up to 150 sq ft. For a bigger room, use two heaters on different circuits (switch off one breaker to see which outlets go dead) or have an electrician install a hard-wired wall or baseboard heater.
Watts to BTU
One watt of electric heat is 3.412 BTU per hour, so a 1,500 W heater gives 5,118 BTU per hour. Every electric resistance heater turns all its power into heat, so a 1,500 W heater of any type puts out the same heat. A fan or oil-filled fins change how quickly the heat reaches you, not the total.
What it costs to run
Cost per hour = kilowatts × price per kWh. A 1,500 W heater at the US average of 18.16¢ per kWh costs 1.5 × $0.1816 = $0.27 per hour while it runs at full power. To compare the cost of heating with other fuels, try the heating cost calculator.
Safety
Save on Energy's buying guide gives these tips:
- Buy a heater certified by a recognized testing lab, such as UL or CSA, with tip-over and overheat shut-off.
- Plug it straight into a wall outlet, with nothing else on that outlet. Never use a power strip or an extension cord, and don't run the cord under a rug.
- Keep it away from furniture, curtains and water, and never drape clothing over it.
Frequently asked questions
What size space heater do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 sq ft, so about 1,440 watts for extra heat (a 1,500 W heater) or 2,160 watts if it is the only heat, which takes two heaters on different circuits. That uses Broan-NuTone's 10 and 15 watts per square foot.
How many square feet will a 1,500 watt heater heat?
About 150 sq ft as extra heat, at 10 watts per square foot, or about 100 sq ft as the only heat in the room, at 15 watts per square foot.
How many BTU is a 1,500 watt heater?
About 5,118 BTU per hour. Each watt of electric heat is 3.412 BTU per hour.
How much does it cost to run a space heater?
Multiply its kilowatts by your electricity price. A 1,500 W heater at the US average of 18.16¢ per kWh (August 2026) costs about 27¢ an hour at full power.
Is a bigger space heater more efficient?
No. Every electric resistance heater turns all the electricity it uses into heat, so watts are the only thing that sets the heat output. A heat pump is the electric option that gives more heat per kWh.
Last reviewed 2026-10-28. See how we calculate for our sources and assumptions.