How this calculator works
Electric floor heating is sold by the watt per square foot: about 12 watts for a normal heated floor and 15 to 20 when it must supply the main heat. Multiply the heated area (the floor you can see, not the area under fixtures) by the watts per square foot to get the power. The current is the power divided by the voltage, the breaker is sized at 125% of it because the heater is a continuous load, and the cost is the kilowatt-hours used times your price.
Worked example: a 10 × 8 ft bathroom with 75% of the floor heated (60 ft²) at 12 W per ft² needs 720 W, which is 6.0 A at 120 V (3.0 A at 240 V), on a 15 A breaker. Used 6 hours a day at 16¢ per kWh it costs $0.69 a day, or $20.74 a month. At 15 W per ft² it needs 900 W, heating the whole floor takes 960 W, and a 12 × 16 ft room needs 1,728 W.
Floor heating for common bathrooms (75% heated, 12 W per ft², 120 V)
Watts, amps, breaker and running cost at 6 hours a day, 16¢ per kWh.
| Bathroom | Heated area | Watts | Amps | Breaker | Cost per month |
|---|---|---|---|---|---|
| 5 × 8 ft | 30 ft² | 360 | 3.0 | 15 A | $10.37 |
| 6 × 10 ft | 45 ft² | 540 | 4.5 | 15 A | $15.55 |
| 8 × 10 ft | 60 ft² | 720 | 6.0 | 15 A | $20.74 |
| 10 × 12 ft | 90 ft² | 1,080 | 9.0 | 15 A | $31.10 |
| 12 × 12 ft | 108 ft² | 1,296 | 10.8 | 15 A | $37.32 |
| 12 × 16 ft | 144 ft² | 1,728 | 14.4 | 20 A | $49.77 |
Tips before you buy
- Heat only the area people stand on. Leave the floor under the tub, vanity and toilet unheated, since the heat would be wasted.
- Insulation board under the heating mat sends the heat up and cuts the running cost and warm-up time.
- Use a programmable thermostat with a floor sensor, and set it to warm the floor just before you use the room.
- Test the mat or cable with a multimeter (for resistance) before, during and after you lay it, since a damaged heating wire cannot be repaired under tile.
- Use a dedicated GFCI circuit, and check that the thermostat is rated for the total watts.
Tools & materials for this project
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Frequently asked questions
How many watts per square foot for floor heating?
About 12 W per ft² for a standard heated floor, 8 to take the chill off, and 15 to 20 for a cold floor or main heat. A 10 × 8 ft bathroom (60 ft² heated) needs 720 W at 12.
How much does electric floor heating cost to run?
For 720 W used 6 hours a day at 16¢ per kWh, about $0.69 a day or $20.74 a month. A thermostat with a floor sensor and a timer cuts it.
What breaker do I need for floor heating?
15 amps for 720 W on 120 V, since it is a continuous load rated at 125%. Larger floors use 240 V, which halves the current (3.0 A here).
Can floor heating heat a whole room?
Only in a well-insulated room, with 15 to 20 W per ft² and the whole floor heated. Usually it is for comfort, to warm the floor and take the chill off, with another heat source for the air.
Should I heat the floor under the vanity and tub?
No: heat only the visible floor. Heating 100% of this 80 ft² bathroom would take 960 W, compared with 720 W for the usable 75%.
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