How many watts per m² for underfloor heating?

For standard rooms (living rooms, bedrooms, kitchens), use 150 W/m². For bathrooms, conservatories and rooms with high heat loss, use 200 W/m². For supplementary heating (where radiators are also present), 100 W/m² is sufficient. These are standard UK recommendations for electric underfloor heating (UFH) systems and are built into this UFH coverage calculator.

Do I subtract fixed units from the floor area?

Yes, always subtract the floor area covered by permanent fixtures — kitchen units, bath, WC, vanity units, and built-in wardrobes. Heating mats and cables must not be installed under fixed objects as this can cause overheating and damage. Only the walkable floor area should be heated. This underfloor heating sizing tool handles the subtraction for you.

What size UFH mat do I need?

Electric heating mats come in fixed m² sizes (1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20 m² etc.). Choose the mat that is closest to but does not exceed your heating area. Never cut or overlap heating mats. If your room is between sizes, choose the smaller mat — it is safer to slightly undersize than to fold or overlap.

How much does electric underfloor heating cost to run?

At 150 W/m², a 10 m² room uses 1.5 kW per hour when on. With a thermostat, UFH typically runs 6-8 hours per day in winter. At current UK electricity rates (approximately 24.5p/kWh), that is roughly £2.20-£2.95 per day for a 10 m² room. Actual costs vary with insulation, thermostat settings and energy tariff.

Can I install underfloor heating under laminate and carpet?

Electric UFH mats work well under tile, stone, vinyl and laminate. For carpet, use electric cable or a low-profile mat — the carpet and underlay tog rating must not exceed 2.5 tog. Use our Carpet Calculator (/flooring/carpet-calculator) or Laminate Flooring Calculator (/flooring/laminate-flooring-calculator) to work out your floor covering quantities. UFH under tiles typically needs screed — see our Screed Calculator (/flooring/screed-calculator) for quantities. Water/wet UFH systems work with any floor covering but are more expensive to install.

Underfloor Heating Calculator — Mat Size, Cable & Pipe Coverage UK

How We Calculate This

Our underfloor heating calculator works by determining the effective heating area (room area minus permanent fixtures) and then recommending the correct mat size, cable length or pipe quantity based on your chosen system type.

The formula

Heating area = Room area - Fixed units area

Total wattage = Heating area × W/m² rating

System-specific calculations

Electric mats: Available in set m² sizes — we recommend the closest size that fits your heating area without exceeding it
Electric cable: Approximately 11m of cable per m² at 90mm spacing
Water/wet system: Approximately 6m of pipe per m² at 170mm spacing

UK wattage guidelines

100 W/m²: Supplementary heating alongside radiators
150 W/m²: Standard rooms — primary heating source
200 W/m²: Bathrooms, conservatories, high heat-loss rooms

Popular UK UFH brands include Warmup, Prowarm, ThermoSphere and Schluter DITRA-HEAT. Always check with your electrician that the circuit can handle the total wattage load. Electric UFH should be on a dedicated circuit with RCD protection per Part P.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Underfloor Heating Coverage Calculator — Mat, Cable & Pipe Sizing