Question 1

How many watts per m² for underfloor heating?

Accepted Answer

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.

Question 2

Do I subtract fixed units from the floor area?

Accepted Answer

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.

Question 3

What size UFH mat do I need?

Accepted Answer

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.

Question 4

How much does electric underfloor heating cost to run?

Accepted Answer

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 the current Ofgem price cap (1 July - 30 September 2026, electricity 26.11p/kWh), that is roughly £2.35-£3.13 per day for a 10 m² room. Actual costs vary with insulation, thermostat settings and energy tariff.

Question 5

Does the cable length match the wattage rating I selected?

Accepted Answer

Not automatically. For a loose-cable system the wattage per m² is fixed by the cable's linear output (~10 W per linear metre) and the spacing you lay it at — length per m² = 1000 ÷ spacing in mm (so 100mm spacing ≈ 10 m/m² ≈ 100 W/m², 90mm ≈ 11 m/m² ≈ 110 W/m², 75mm ≈ 13 m/m² ≈ 130 W/m²). This calculator lets you choose the wattage rating and the cable density independently, so the 'approx. cable length' figure is a coverage estimate only — it does not recompute the W/m² from the spacing. Always buy a loose-cable kit rated for your heating area and follow the manufacturer's spacing guide so the installed output matches the wattage you want.

Question 6

Can I install underfloor heating under laminate and carpet?

Accepted Answer

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 or Laminate Flooring Calculator to work out your floor covering quantities. UFH under tiles typically needs screed. Water/wet UFH systems work with any floor covering but are more expensive to install.

Underfloor Heating Coverage Calculator — Mat, Cable & Pipe Sizing

How We Calculate This

The formula

System-specific calculations

UK wattage guidelines

Frequently Asked Questions

Related Calculators

Floor DPM Calculator

Screed Calculator

Floor Tile Calculator

Carpet Calculator

Floor Insulation Calculator

UFH for Heat Pump Calculator

Was this calculator helpful?