FREE ONLINE BTU CALCULATOR FOR UK HOMES

The Easy Way to Find the Right Radiator Size

Get instant, precise radiator sizing tailored to UK room specs and insulation standards. Use the free calculator below to figure out exactly how much heat output you need before you buy.

Advanced Radiator Size Calculator

Room length (m)

Room width (m)

Room height (m)

Room type

Target room temp (°C)

Design outdoor temp (°C)

Building type

Insulation level

Glazing

Window area (m²)

External walls

External wall length, optional

Floor position

Exposure

Heat source

Add 10% safety margin

Estimate only. For final radiator sizing, especially for heat pumps, a full heat loss survey or BS EN 12831 calculation is recommended.