How the floor heating calculator works
Electric underfloor heating is sized by the floor it can actually warm. Mats and cables must never run under a bath, shower tray, kitchen cabinet or toilet, because trapped heat can damage both the element and the unit above. So the calculator starts from the room, subtracts the floor hidden by fixed units, and multiplies what is left by the output printed on the product.
Power (W) = (room area − fixed units) × output per m² or ft² · Cost = W × hours ÷ 1,000 × 30 × priceCurrent is power divided by voltage. It tells you and your electrician whether the heater can share an existing circuit or needs its own. Running cost uses full-power hours: a thermostat switches the element on and off, so a floor that feels warm for twelve hours may only draw power for five or six of them.
Worked example
A 4 m × 3 m bathroom has 12 m² of floor. The bath, vanity and WC cover 2 m², leaving 10 m² to heat. A 150 W/m² mat therefore totals 1,500 W, drawing 1,500 ÷ 230 = 6.5 A. Running six full-power hours a day uses 9 kWh; at 0.30 per kWh that is 2.70 a day, or 81 over a 30-day winter month. In North America, an 11.5 ft × 8 ft bathroom with 16 ft² of fixtures leaves 76 ft². At 15 W/ft² the mat is 1,140 W, pulling 9.5 A at 120 V and costing about 35 dollars a month at 17 cents per kWh.
Choosing the output
| Use | Typical output | Notes |
|---|---|---|
| Floor warming under tile or stone | 100–150 W/m² (10–14 W/ft²) | Comfort only; the room keeps its radiators |
| Bathrooms and cold ground floors | 150–200 W/m² (14–18 W/ft²) | Faster warm-up over uninsulated slabs |
| Foil heaters under laminate or engineered wood | 60–140 W/m² (6–13 W/ft²) | Check the flooring's maximum surface temperature |
| Primary heating of a room | From a heat-loss calculation | Output must match the room's losses in the coldest weather |
Mats are usually 50 cm (20 in) wide and sold by area, from about 1 m² to 12 m² (10 to 150 ft²); pick the largest size that fits the free floor rather than cutting the heating element, which must never be cut. Loose cable kits suit awkward shapes.
Practical points
- Insulate underneath. On a concrete slab, 6–10 mm insulation boards under the mat cut warm-up time dramatically and send the heat up instead of down.
- Test the element three times. Measure its resistance out of the box, after laying and after tiling, and compare it with the label.
- Circuit and protection. Heaters need residual-current protection (a 30 mA RCD in Europe, GFCI in North America). A continuous load should stay within about 80% of the breaker rating: roughly 12 A on a 15 A, 120 V circuit. Connection to the mains is a job for a licensed electrician under local rules.
- Use a floor sensor. Wood and laminate makers often cap the surface near 27 °C (80 °F).
A reproducible calculation with complete inputs
For a reproducible electric floor heating calculator example, use the metric input system and enter Room length: 4 m; Room width: 3 m; Area under fixed units: 2 m²; Heater output: 150 W/m²; Supply voltage: 230 V; Full-power hours per day: 6 h; Electricity price: 0.3 per kWh. These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Heater power | 1500 | 1500 | W |
| Heated area | 10 | 10.0 | m² |
| Current draw | 6.52173913043 | 6.5 | A |
| Energy per day | 9 | 9.0 | kWh |
| Running cost per month | 81 | 81.00 |
How should I prepare these entries?
Room length: enter the value in m. Room width: enter the value in m. Area under fixed units: Bath, shower tray, vanity, kitchen cabinets, toilet: heating is never laid under them. Heater output: On the mat label. Common: 100, 150 or 200 W/m²; 12 or 15 W/ft². Supply voltage: choose among 230 V, 240 V, 120 V. Full-power hours per day: The thermostat cycles the heater, so this is time at full output, typically 3–8 h in winter. Electricity price: enter the value in per kWh. Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
Frequently asked questions
How many watts per square metre for underfloor heating?
About 100–150 W/m² for floor warming under tile, 150–200 W/m² in bathrooms or over cold slabs, and 60–140 W/m² for foil systems under laminate or wood.
How much does electric floor heating cost to run?
Multiply watts by full-power hours and your price. A 1,500 W mat running six hours a day uses 9 kWh, so 81 a month at 0.30 per kWh or about 46 dollars at 17 cents.
How many amps does a heated floor mat draw?
Divide watts by volts: a 1,200 W mat made for 120 V draws 10 A, while a 1,200 W mat made for 240 V is a different model drawing 5 A. Never connect a mat to a voltage other than its label: a 120 V mat on 240 V would draw 20 A and 4,800 W.
Can you put underfloor heating under a bath or cabinets?
No. Leave the area under baths, shower trays, cabinets and toilets unheated, which is why the calculator subtracts fixed units.
Can electric floor heating be the only heat source?
Sometimes, in well insulated rooms, but only if its output covers the room's heat loss. Otherwise treat it as comfort heating alongside radiators.