Underfloor Heating Calculator

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

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$798.34

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.

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 thermostat is assumed to run the floor about forty percent of the time once the screed is warm, which is where the monthly figure comes from.

FAQs

How many watts a square metre does underfloor heating need?

Around 120 for comfort warmth over an existing heating system, 140 in a bathroom, and 150 or more where the floor is the only source of heat.

Can heating go under furniture?

No. Heat that cannot escape raises the cable temperature, shortens its life and can damage the furniture. That is what the free-area figure is for.

What pipe spacing should a wet floor use?

Fifteen centimetres is the standard; ten near external walls where more heat is needed and twenty in the middle of a well-insulated room.

Why is one loop limited to a hundred metres?

Beyond that the pressure drop is more than the pump can push, and the far end of the loop stays cold. Longer runs are split into several loops on a manifold.

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.

Useful links

All tools in Home & Utilities

Underfloor Heating Calculator

Result

Related tools

Heating Power Calculator

Electricity Cost Calculator

Screed Calculator

Useful links