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Underfloor Heating Mat Calculator — What Size Mat Do I Need?
Calculate electric underfloor heating mat size, wattage and thermostat requirements for tiled floors. UK electrical standards.
Longest dimension of the room
Shortest dimension of the room
Bath, vanity, WC — don't heat under these
Flexible primer for subfloor preparation
Self-levelling compound to cover mat before tiling
Enter your supplier price for a cost estimate
Primer litres per m² (default 0.2)
SLC kg/m² to encapsulate the mat (~1.6 kg/m²/mm; ~10mm over a UFH mat ⇒ ~16; default 16)
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How We Calculate This
This calculator determines the correct electric underfloor heating mat size by calculating the heated floor area (total area minus fixed furniture), then matching it to the nearest standard mat that still fits, and working out the electrical requirements.
The formulas
Heated area = Room area × (1 − Fixed furniture %)
Mat size = Largest standard size ≤ heated area (round DOWN — the cable can’t be cut, so the mat must fit)
Total wattage = Mat size × W/m²
Amps = Wattage ÷ 230V
Wattage guidelines
- 200W/m²: Most bathrooms and high heat-loss rooms (conservatory, garden room, older/poorly insulated property, large glazing, or where >20% of the floor is excluded by fixtures) — faster warm-up, sole-heat-source capable
- 150W/m²: Well-insulated living rooms, kitchens and hallways — comfort or supplementary heating
Electrical requirements
- Up to 3.6kW (16A): Standard 2.5mm² circuit with 16A MCB + 30mA RCD
- Over 3.6kW: Requires contactor or multiple circuits — consult electrician
- All UFH installations require 30mA RCD protection
- Floor probe sensor thermostat is essential
Frequently Asked Questions
Measure your room, then deduct the area taken by fixed furniture (bath, vanity unit, kitchen units, WC). Heating mats should NOT go under fixed furniture as this can cause overheating. Crucially, the heating cable inside an electric mat CANNOT be cut to length — only the mesh is trimmed and the run turned back on itself — so you must choose a mat equal to or slightly SMALLER than the free heated area, never larger. A mat bought too big simply will not fit. For a bathroom with 6 m² total area and 20% fixed furniture, the free heated area is about 4.8 m², so the right standard mat is 4.5 m² (the next size down that still fits).
200W/m² is recommended for most bathrooms and any high heat-loss room (older or poorly insulated property, large glazing, conservatory, or where more than 20% of the floor is excluded by fixtures), giving faster warm-up and the ability to act as the sole heat source. 150W/m² suits well-insulated rooms where the heating is comfort or supplementary. Choose the output to match the room's heat loss rather than the room name — both options are offered above.
Yes, but the mat must be covered first. Apply flexible tile adhesive directly over the mat (at least 3mm above the cable), or cover with self-levelling compound first then tile on top. Use flexible adhesive and grout, not rigid — the floor expands and contracts with heating cycles.
Yes, you need a thermostat with a floor probe sensor (not just air temperature). The probe goes into the floor between heating cables to measure actual floor temperature. For engineered wood or LVT/vinyl flooring, the thermostat must have a maximum floor-temperature limiter — manufacturers typically cap both at about 27°C — to protect the surface and adhesive from heat damage.
Yes. Electric UFH should be on a dedicated circuit with RCD protection (30mA). For mats under 3.6kW (approximately 24 m² at 150W/m²), a 16A MCB on a standard 2.5mm² cable is sufficient. Above this, a larger circuit or contactor may be needed. Installation must comply with BS 7671 and Part P — use a qualified electrician.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.