How this calculator works
A common rule is 10 watts of electric heat per square foot of floor, for a room with an 8 foot ceiling and average insulation. Use fewer watts for a well-insulated room (8) and more for a cold or drafty one (12 to 15), and scale for a higher ceiling. Divide the watts by the heat each foot of baseboard gives (250 watts a foot is standard) to get the length, and by the voltage to get the current.
Worked example: a 14 × 12 ft room (168 ft²) with an 8 ft ceiling and average insulation needs 1,680 W (5,732 BTU per hour), which is 6.7 ft of baseboard (2 units of 4 ft), 7.0 A at 240 V and a 15 A breaker. With a 10 ft ceiling it is 2,100 W, in a cold room at 15 W per ft² it is 2,520 W, on 120 V the current is 14.0 A, and a 20 × 20 ft room needs 4,000 W.
Baseboard needed by room size (average insulation, 8 ft ceiling, 240 V)
Watts, feet of 250 W/ft baseboard, amps and breaker.
| Room | Watts | Baseboard | Amps | Breaker |
|---|---|---|---|---|
| 10 × 10 ft | 1,000 | 4.0 ft | 4.2 | 15 A |
| 12 × 12 ft | 1,440 | 5.8 ft | 6.0 | 15 A |
| 14 × 12 ft | 1,680 | 6.7 ft | 7.0 | 15 A |
| 15 × 20 ft | 3,000 | 12.0 ft | 12.5 | 20 A |
| 20 × 20 ft | 4,000 | 16.0 ft | 16.7 | 25 A |
| 20 × 30 ft | 6,000 | 24.0 ft | 25.0 | 35 A |
Tips before you buy
- Put baseboard heaters under windows, where the cold air falls, and along the outside walls.
- Use one thermostat for each room. Line-voltage thermostats are made for the load: check the rating.
- A 240 V heater uses half the current of a 120 V heater of the same power, so it needs a smaller wire and a smaller breaker.
- Allow for the whole circuit: the sum of the heaters on one breaker must not exceed 80% of the rating (see the circuit capacity calculator).
- If your electricity is expensive, ask about a heat pump (mini-split): it gives 2 to 4 times the heat for each kWh.
Tools & materials for this project
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Frequently asked questions
How many watts of baseboard heat do I need per square foot?
About 10 watts per square foot for a room with an 8 foot ceiling and average insulation. A 14 × 12 ft room needs 1,680 watts.
How many feet of baseboard heater do I need?
At 250 watts per foot, 6.7 ft for a 14 × 12 ft room. A 20 × 20 ft room needs 16 ft.
What breaker do I need for a baseboard heater?
15 amps at 240 V for 1,680 W, because a heater is a continuous load, rated at 125% of its current. On 120 V the same load draws 14.0 A.
Is electric baseboard heat expensive?
It turns every kWh into a kWh of heat. The 1.7 kW for this room costs 1.7 times your price per kWh for each hour at full output. A heat pump costs a third to half as much to run.
Where do I put baseboard heaters?
Under windows and along outside walls, where the heat opposes the cold air coming off the glass, with a thermostat in each room and nothing blocking the heater.
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