Radiator Sizing Calculator — BTU & Watt Calculator for Any Room
Calculate the radiator output you need for any room in your UK home. Uses the heat-loss method with adjustments for insulation, glazing, and exposure.
Longest dimension of the room
Shortest dimension of the room
Standard UK ceiling height is 2.4m
How We Calculate This
Our radiator sizing calculator uses the volumetric heat-loss method, which is the standard approach used by UK heating engineers. It calculates the heat output required to maintain a comfortable room temperature based on the room's size and characteristics.
The formula
Heat required (Watts) = Room volume (m³) x Adjusted W/m³
The base rate of 50 W/m³ (for an average room with double glazing and moderate insulation) is adjusted by multiplying the following factors:
- Room type: Bathrooms +15%, conservatories +30%, kitchens -10%
- Insulation: Poor +40%, average +10%, good baseline, excellent -15%
- Glazing: Single +20%, double baseline, triple -10%
- External walls: +10% per external wall
Converting to BTU
The result in Watts is converted to BTU by multiplying by 3.412. When shopping for radiators, match the BTU/Watt output to your calculated requirement — ensuring you compare values at the same Delta T rating (DT50 is the current UK standard).
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates. Verify with your supplier.