Radiator Sizing Calculator — BTU & Watt Calculator for Any Room

Our radiator sizing calculator uses the volumetric W/m³ estimating method — a quick rule-of-thumb that estimates the heat output required to maintain a comfortable room temperature from the room's size and characteristics. It is a fast first-pass guide, not a substitute for the full room-by-room fabric heat-loss calculation to BS EN 12831-1 that UK heating engineers use for system design (see the caveat below).

The formula

Heat required (Watts) = Room volume (m³) x Adjusted W/m³

The base rate of 50 W/m³ sits at the older, more poorly-insulated end of the published 30–60 W/m³ rule-of-thumb range (a new build is closer to ~35 W/m³ and a well-insulated home ~40 W/m³). For reference, the common floor-area convention of ~80 / 100 / 130 W/m² for new-build / average / poor properties divided by a 2.4 m ceiling gives ~33 / 42 / 54 W/m³. You can lower the base rate in Advanced Options for a modern, well-insulated room. The base rate is then adjusted by multiplying the following factors:

• Room type: Bathrooms +15%, hallways +5%, kitchens -10%, conservatories +60%
• Insulation: Poor +40%, average +10%, good baseline, excellent -15%
• Glazing: Single +20%, double baseline, triple -10%
• External walls: +10% per external wall

This volumetric W/m³ method is a conservative estimating rule-of-thumb that tends to over-size, and the conservatory uplift in particular is an unverified heuristic — a largely-glazed conservatory should have its own dedicated heat-loss assessment. For a heat pump or low-temperature system, commission a full room-by-room fabric heat-loss calculation to BS EN 12831-1 (CIBSE Domestic Heating Design Guide / MCS MIS 3005).

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).