Underfloor Heating Calculator

Written byOleh Soloninupdated 25 September 2026

Room area, m²

Free of furniture, %

Nothing may be heated under cupboards or appliances.

System

Purpose

Pipe spacing, cm

Hours a day

Price of a kilowatt-hour

Advanced options

Share of the time the floor heats, %

The calculator's assumption, not a property of every floor: the thermostat switches the heat on for only part of the hours set. 100 gives an upper bound.

Result

Heated area, m²12.6
Power, W1,512
Cable or pipe length, m88.9
How it is laidcable at 17 W/m drawing 6.6 A
Use a month, kWh145.2
Cost a month$26.13

Heat only the floor you can walk on

Calculates the power, the cable or pipe length and the running cost of an underfloor heating system for a given room.

The first number in this job is not the room area but the free area. Nothing goes under a wardrobe, a bath or a fridge: trapped heat has nowhere to go, and an electric cable under a cupboard is a cable that fails early.

From that free area the rest follows: watts per square metre by purpose, cable length from the wattage, pipe length from the spacing, and the running cost from the hours and the tariff.

How to use

Enter the room's floor area and the share free of furniture — floors are not heated under cupboards and appliances.
Pick the system — electric cable or a water circuit — and the purpose: comfort warming, primary heating, or a bathroom.
For a water circuit set the pipe spacing; for either system set the hours a day and the price per kilowatt-hour.

The purpose sets the power

Only the free part of the floor is heated: of 18 square metres at 70 per cent free, that is 12.6. Power per metre depends on the purpose — comfort warming on top of radiators needs less than a floor that heats the room on its own.

The gap between comfort warming and primary heating is a quarter, in power and in the electricity bill alike. A bathroom sits in between: the floor has to warm the tiles, but the room is small.

Every row comes from the calculator on this page for a 17 W/m heating cable. The current to size the circuit breaker for is in the layout line: 6.6 A for comfort warming.
Purpose, 12.6 m² heated	Power, W	Cable, m	A month at 8 h a day, kWh
comfort warming	1,512	88.9	145.2
bathroom and balcony	1,764	103.8	169.3
primary heating	1,890	111.2	181.4

Pipe spacing and the number of circuits

For a water floor the pipe length is set by the spacing: the closer the loops, the more pipe and the more even the heat. A 10-centimetre spacing needs 130 metres of pipe for 12.6 square metres of floor; a 20-centimetre spacing only 67.

The calculator caps a circuit at a hundred metres — its own assumption; a real design checks the allowed length with a hydraulic calculation for the pipe, flow and pump. So at a 10-centimetre spacing the calculator splits the pipe into two 65-metre circuits by itself.

Every row comes from the calculator on this page for a water circuit over 12.6 m² of heated floor. The hundred-metre limit per circuit is built into the calculation.
Pipe spacing	Pipe, m	Circuits
10 cm	130	2 of 65 m
15 cm	88	1
20 cm	67	1

What this calculator shows

The heated area and the total power
The cable or pipe length and how to lay it
Monthly consumption and cost

What to keep in mind

Comfort warmth is taken as 120 watts a square metre, primary heating as 150 and a bathroom or balcony as 140.
The monthly figure assumes the thermostat heats the floor for 40 percent of the hours set; the field can be changed, and 100 percent gives an upper bound.

FAQs

How many watts per square metre does underfloor heating need?

By this page's calculation, 120 for comfort warming, 140 for a bathroom and 150 for a floor that heats the room on its own. Only the furniture-free area counts.

Can I lay underfloor heating under furniture?

No. Heat with nowhere to go raises the cable temperature, shortens its life and damages the furniture. That is what the free-area figure is for.

What pipe spacing should I use?

The spacing comes from the heating design and the system maker's instructions; the table shows 10, 15 and 20 cm for comparison. Closer spacing means more pipe: 130 metres at 10 cm against 67 at 20 for 12.6 square metres of floor.

Why does the calculator cap a circuit at a hundred metres?

That is the calculator's rule: the longer the loop, the higher the pressure loss, and the pump may not overcome it. A hydraulic calculation gives the real limit; long runs are split into several circuits on a manifold — the calculator does it for you.

What does underfloor heating cost to run?

Comfort warming over 12.6 square metres at eight hours a day comes to 145 kWh a month; the cost is that times your tariff, which the calculator applies. That already assumes the thermostat heats for 40 percent of the hours set; it is an assumption, and the real use can be higher or lower.

What circuit breaker does the cable need?

One sized to the current the calculator shows in the layout line: 6.6 amps for 1,512 watts of comfort warming. Breaker and wiring are chosen with headroom above that current, and the connection is a job for an electrician.

Worked example

A living room of eighteen square metres

Input: 18 m², 70 percent free of furniture, electric, comfort warmth, 8 hours a day

Output: 1,512 W and about 89 metres of cable

Note: That works out at seven metres of cable to the square metre and about 145 kWh a month. Switch to primary heating and the power rises to 1,890 W for the same floor.

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Result

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