In short: measure the free floor area, leaving out units, baths, shower trays and the toilet. Choose a mat that is the same size or slightly smaller, never bigger, because the heating cable must not be cut. For loose cable, multiply the free area by the output you want (W/m²) and pick the cable whose wattage is closest. The calculator does both and adds the boards, primer and thermostat.
Your heating
Parts list
Mat or loose cable?
Mats are quickest in square and rectangular rooms. The cable is fixed to a 0.5m wide mesh. You cut the mesh and turn the mat at the walls, but never cut the cable. Loose cable suits L-shaped and awkward rooms, small cloakrooms and floors with lots of obstacles. It presses into the studs of an uncoupling membrane at the spacing you choose.
Which output do I need?
- 100W/m²: well insulated rooms, en-suites and new builds where you want comfort warmth.
- 150W/m²: most kitchens, bathrooms and living spaces on an insulated subfloor.
- 200W/m²: conservatories and rooms with high heat loss.
Loose cable spacing
Spacing (mm) = free area (m²) / cable length (m) x 1,000. With 10W/m cable, 100mm spacing gives 100W/m², 75mm gives about 130W/m² and 65mm about 150W/m². The 20W/m cable gives the same outputs at twice the spacing.
Load and wiring
Current (A) = total watts / 230. Floor thermostats such as the Heatmiser neoStat-e and Slimline-E switch up to 16A, which is about 3.6kW. Bigger floors need a contactor or more than one zone. A qualified electrician must make the final connection.
Why insulate under an electric mat?
Without insulation, a lot of the heat goes down into a cold slab, so the floor takes longer to warm up and costs more to run. 6mm or 10mm XPS backer board is enough for most rooms and adds very little height. See the running cost calculator and the floor height calculator.
Frequently asked questions
What size electric underfloor heating mat do I need?
Measure the free floor area, leaving out units, the bath, shower tray and toilet. Choose the mat closest to that area without going over it. If no single mat fits well, two smaller mats often get closer.
Can I cut an electric underfloor heating mat?
You can cut the mesh and turn the mat to fit the room, but never cut the heating cable.
How far apart should loose heating cable be?
Divide the free area by the cable length. 4m² with a 60m cable gives about 67mm spacing, which is about 150W/m² with 10W/m cable.
Does electric underfloor heating need its own circuit?
It needs RCD protection and must be connected by a qualified electrician to BS 7671. Loads above 16A, about 3.6kW, need a contactor or splitting into zones.