How this calculator works
Electric underfloor heating mats are sold by the watt per square metre: about 100 W/m² for a comfort heat on an insulated floor, 150 W/m² as the standard, and 200 W/m² 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 metre to get the power. The current is the power divided by 230 volts, and the cost is the kilowatt-hours used times your price per kWh.
Worked example: a 4 × 3 m room with 80% of the floor heated (9.6 m²) at 150 W/m² needs 1,440 W, which is 6.3 A. Used 6 hours a day at 27p per kWh it costs £2.33 a day, or £69.98 a month. At 100 W/m² it needs 960 W, and at 200 W/m² 1,920 W. Heating the whole floor takes 1,800 W, and a 6 × 4 m room needs 2,880 W (12.5 A).
Electric underfloor heating for common rooms (80% heated, 150 W/m², 27p per kWh, 6 hours a day)
Watts, amps and the cost per month.
| Room | Heated area | Watts | Amps | Cost per month |
|---|---|---|---|---|
| 2 × 2 m | 3.2 m² | 480 | 2.1 | £23 |
| 3 × 2 m | 4.8 m² | 720 | 3.1 | £35 |
| 3 × 3 m | 7.2 m² | 1,080 | 4.7 | £52 |
| 4 × 3 m | 9.6 m² | 1,440 | 6.3 | £70 |
| 5 × 4 m | 16.0 m² | 2,400 | 10.4 | £117 |
| 6 × 4 m | 19.2 m² | 2,880 | 12.5 | £140 |
Tips before you buy
- Heat only the part of the floor that people stand on. Keep the mat clear of the bath, vanity, toilet and fitted units.
- Lay the mat on insulation boards: they send the heat up, and cut the warm-up time and the running cost.
- Test the resistance of the mat with a multimeter before, during and after laying it, since a damaged heating wire cannot be repaired under tile.
- A programmable thermostat with a floor probe heats the floor only when you need it.
- Have a qualified electrician fit the circuit and the RCD, and keep the certificate.
Tools & materials for this project
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Frequently asked questions
How many watts per square metre for underfloor heating?
Usually 150 W/m² under tiles, 100 W/m² for comfort heat on an insulated floor, and 200 W/m² if it is the main heat. A 4 × 3 m room with 80% heated at 150 W/m² needs 1,440 W.
How much does electric underfloor heating cost to run?
For 1,440 W used 6 hours a day at 27p per kWh, about £2.33 a day or £69.98 a month. A thermostat with a floor probe and a timer cuts it.
What size cable or circuit for underfloor heating?
6.3 amps for 1,440 W on 230 V. The mat is wired to a dedicated circuit with a 30 mA RCD, sized by the installer for the load; larger floors may need more than one thermostat.
Can underfloor heating be the main heat?
Only in a well-insulated room, at 200 W/m² or more with the whole floor heated. Usually it is for comfort, to warm the floor and take the chill off.
Should I heat the floor under the bath and vanity?
No: heat only the visible floor. Heating 100% of this room would take 1,800 W, against 1,440 W for the usable 80%.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.