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Heated floor calculator (electric underfloor heating)

Work out the electric underfloor heating for a room: the area to heat, the watts and amps of the mat or cable, the circuit it needs, its output in BTU/h, and what it costs to run.

1The room
Common room sizes

Under cabinets, the bath or shower tray, the toilet and fixed furniture. Mats and cables must not go there.

2The heating mat or cable

From the mat or cable data: US mats are usually 12 or 15 W/ft², European mats 100–200 W/m².

3Running cost (optional)

The hours the heating runs at full output. A thermostat switches it off once the floor is warm, so the real figure is lower.

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How much electric floor heating do I need? Using the calculator

  1. 1Measure the room, or enter its area, and the part that won't be heated (under the vanity, the bath, the toilet).
  2. 2Choose the output of your mat or cable, and the supply voltage.
  3. 3For the running cost, enter the hours on per day and your electricity price per kWh.
  4. 4Read the watts, amps and circuit size, and the cost per day and per month.

Heated floor formulas: watts, amps and running cost

  1. 01Heated area = floor area − area under cabinets, bath, shower and toilet
  2. 02Watts = heated area × output (W/m² or W/ft²)
  3. 03Amps = watts ÷ volts
  4. 04US circuit rating = amps × 1.25 (NEC 424.3(B))
  5. 05BTU/h = watts × 3.412
  6. 06Cost per day = watts ÷ 1,000 × hours on × price per kWh

An electric floor heating mat turns all its power into heat, so its size is set by the floor area it covers and its rating per square metre or square foot. Only the floor people walk on is heated: the cable must not go under cabinets, baths or the toilet, where the heat would be trapped.

Current is power divided by voltage, so the same 1,200 W floor draws 10 A at 120 V but only 5 A at 240 V — a 240 V system can heat about twice the area on one thermostat. In the US the circuit must be rated for 125 % of that current because the heating can run for hours.

Heated bathroom floor example

A 10 ft × 8 ft bathroom with 15 ft² under the vanity and tub has 65 ft² to heat. A 12 W/ft² mat on 120 V is 780 W (2,661 BTU/h) and draws 6.5 A, so the circuit must be rated for at least 8.1 A — a 15 A circuit is enough.

Running 4 hours a day at full output it uses 3.12 kWh a day.

Tips before you buy

  • Put insulation board under the mat on a concrete slab: the floor warms faster and the heating runs less.
  • Test the cable's resistance before, during and after tiling, and write the values on the warranty card.
  • Set a floor sensor limit for wood and vinyl floors: check the flooring maker's maximum surface temperature, often about 27 °C (80 °F).
  • A programmable thermostat that warms the floor only before you use the room cuts the running cost the most.

Frequently asked questions

How many watts per square foot for a heated floor?

Most US mats and cables give 12 W/ft²; faster ones 15 W/ft². In Europe mats are sold at 150 W/m² for bathrooms and kitchens, 200 W/m² for conservatories and rooms that lose heat quickly, and around 100 W/m² under wood and vinyl.

How many amps does a heated floor draw?

Watts ÷ volts. At 12 W/ft² and 120 V that is 0.1 A per square foot, so a 15 A thermostat can run about 150 ft². At 240 V the current halves, and the same thermostat can run about 300 ft².

What size circuit does a heated floor need?

In the US, 125 % of the floor's current (NEC 424.3(B)): a 780 W floor at 120 V draws 6.5 A, so the circuit needs at least 8.1 A — a 15 A circuit. Larger floors often get their own 20 A circuit. Floor heating also needs GFCI protection.

How much does a heated floor cost to run?

Watts ÷ 1,000 × hours × price per kWh. A 780 W bathroom floor on for 4 hours uses 3.12 kWh; at 17 cents per kWh that is about 53 cents a day at full output. A thermostat cycles it, so the real cost is lower.

Can heating go under the toilet or cabinets?

No. Heat under a toilet can soften the wax ring, and under cabinets or a bath it is trapped and can overheat the cable. Leave those areas out — that is what the "won't be heated" box is for.

Also searched as: radiant floor heating calculatorfloor heating calculatorunderfloor heating calculatorelectric floor heating calculatorheated floor mat calculatorradiant floor btu calculatorheated floor running cost

Sources

Formulas last reviewed: September 30, 2026 ·How we check our formulas

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