Heated Bathroom Floor Calculator — Electric UFH Mat Sizing

For a bathroom where the UFH supplements a radiator or towel rail, 150 W/m² is standard. If the UFH is the sole heat source, use 200 W/m². Bathroom UFH operates at higher wattages than living areas (typically 100-150 W/m²) because bathrooms are smaller and need to feel warm underfoot quickly. Always subtract fixed furniture (bath, WC, vanity) from the heated area.

Yes, insulation board is strongly recommended. A 6mm tile backer/insulation board (such as Prowarm or Ditra-Heat-Duo) placed under the heating mat prevents heat loss downward and improves warm-up time by up to 50%. Without insulation, a significant proportion of the heat goes into the subfloor rather than warming the tiles above. This is especially important on concrete ground floors.

A thin layer (3-5mm) of flexible self-levelling compound (SLC) over the heating mat is recommended before tiling. It encapsulates the heating cable, provides an even surface for tiling, and protects the cable from damage during tile fixing. Some systems allow direct tiling into flexible adhesive, but SLC gives a more reliable result. Use a fibre-reinforced SLC suitable for use with UFH.

A typical 3 m² bathroom at 150 W/m² (450W total) costs roughly £0.047 per hour at 26.11p/kWh (current Ofgem price cap for 1 July-30 September 2026 — check the latest quarter, as the cap changes every three months), assuming an illustrative 40% thermostat duty cycle. Running 6 hours per day during the heating season (October-March, ~180 days), the annual cost is approximately £51. These are indicative figures: real running cost depends on your setpoint, floor temperature and how well the room is insulated. A programmable thermostat significantly reduces running costs by only heating when needed.

No. Never install electric underfloor heating under fixed fixtures such as baths, shower trays, WC pedestals or vanity units. The heat cannot escape and will cause the cable to overheat and fail. Only heat the open floor area where people walk. Most manufacturers' warranties are voided if mats are installed under fixed items.