What size underfloor heating mat do I need?

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. For a bathroom with 6 m² total area and 20% fixed furniture, you'd need a mat for approximately 4.8 m² — select a 5 m² mat.

How many watts per m² for underfloor heating?

For most rooms with tiles or stone floors, 150W/m² is the standard. Conservatories and rooms with high heat loss (large windows, poor insulation) need 200W/m². At 150W/m², electric UFH can be the primary heat source for well-insulated rooms. At lower wattages, it provides comfort heating only.

Can you tile directly over underfloor heating mats?

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.

Do I need a special thermostat for UFH under tiles?

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 wood or LVT flooring, the thermostat must have a maximum temperature limiter (typically 27°C for wood, 28°C for LVT) to prevent damage.

Does electric underfloor heating need its own circuit?

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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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.

Room Length

Room Width

Room Type

Determines wattage requirement

Mat Type

Floor Covering

Affects max temperature settings

Fixed Furniture (%)

Bath, vanity, WC — don't heat under these

Include thermostat

Floor probe thermostat (essential)

Include primer

Flexible primer for subfloor preparation

Include levelling compound

Self-levelling compound to cover mat before tiling

Mat Price per m² (optional)

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 to standard mat sizes and working out the electrical requirements.

The formulas

Heated area = Room area × (1 - Fixed furniture %)
Mat size = Next standard size up from heated area
Total wattage = Mat size × W/m²
Amps = Wattage ÷ 230V

Wattage guidelines

150W/m²: Bathroom, kitchen, hallway, living room — primary heating in insulated rooms
200W/m²: Conservatory, garden room — high heat loss areas

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

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.