What wattage do I need for bathroom underfloor heating?

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.

Do I need insulation board under bathroom UFH?

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.

Do I need self-levelling compound over the heating mat?

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.

How much does it cost to run bathroom UFH?

A typical 3 m² bathroom at 150 W/m² (450W total) costs approximately £0.06 per hour at 34p/kWh (UK average 2026), assuming a 40% thermostat duty cycle. Running 6 hours per day during the heating season (October-March, ~180 days), the annual cost is approximately £65. A programmable thermostat significantly reduces running costs by only heating when needed.

Can I install UFH under a shower tray or bath?

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.

Heated Bathroom Floor Calculator — Electric UFH Mat Sizing

Calculate electric underfloor heating mat size, thermostat, insulation board and self-levelling compound for a heated bathroom floor.

Room Length

Internal room length

Room Width

Internal room width

Fixed Furniture Area

Bath, WC, vanity — do NOT heat under these

Heat Output

150 W/m² standard; 200 W/m² if sole heat source

Mat Width

0.5m standard; 0.3m for narrow areas

Include thermostat

Programmable thermostat with floor probe

Include insulation board

Strongly recommended for faster heat-up

Mat Price (optional)

Per m² of heating mat

Thermostat Price (optional)

Programmable thermostat with probe

Insulation Board Price (optional)

Per 600×1200mm board

SLC Price per Bag (optional)

Per 20kg bag

How We Calculate This

This calculator sizes the electric underfloor heating mat and associated materials for a bathroom installation, including insulation, self-levelling compound and thermostat requirements.

The formula

Total wattage = Heated area × W/m²

Mat length = Heated area ÷ Mat width

For a 4 m² bathroom with 1.5 m² fixed furniture: heated area = 2.5 m². At 150 W/m² = 375W. Mat = 2.5 ÷ 0.5 = 5.0m of 0.5m-wide mat. SLC at 3mm: 2.5 × 1.8 × 3 = 13.5 kg (1 × 20kg bag).

Floor build-up

Subfloor: Ply or existing screed (must be flat and clean)
Insulation board: 6mm tile backer with foil face
Heating mat: 3-4mm thick cable on mesh
Self-levelling compound: 3-5mm fibre-reinforced SLC
Tiles: Fixed with flexible adhesive (3-6mm bed)
Total build-up: Typically 20-25mm above subfloor

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.