Home Improvement
Radiator Size Calculator
Enter room dimensions and room type to see a planning heat-output estimate.
What size radiator do I need?
A quick UK-style planning estimate multiplies room volume (length × width × height) by a watts-per-cubic-metre factor that depends on room type — living spaces often around 45 W/m³, bedrooms nearer 35 W/m³, and bathrooms higher (around 55 W/m³) because of comfort temperatures and ventilation losses. The result is a suggested radiator output in Watts, with an approximate BTU conversion (W × 3.412) for packs that still quote British Thermal Units. Large windows, poor insulation, north-facing rooms and open-plan voids can push real heat loss well above this sketch. Use it to shortlist radiator sizes before a proper heat-loss calculation. For running-cost context on electric heating habits see the LED replacement savings tool when lighting upgrades are part of the same energy review.
Worked example
A 4.5 × 3.5 × 2.4 m living room is 37.8 m³. At 45 W/m³ → about 1,701 W ≈ 5,800 BTU. Choose a radiator (or combination) at or above that output at your system ΔT.
Limits and assumptions
Not a BS EN heat-loss survey. Ignores fabric U-values, air changes, outside design temperature and radiator ΔT rating. Always confirm with a heating engineer for boiler and pipework design.
Frequently asked questions
What is a BTU?
British Thermal Unit — an older heat-output unit. Many UK packs still list BTU alongside Watts. Roughly, Watts × 3.412 ≈ BTU/hour.
What is ΔT and why does it matter?
ΔT (delta T) is the temperature difference between the radiator and the room. Outputs are often quoted at ΔT50. Lower flow temperatures mean less output — check the manufacturer table.
Can I use two smaller radiators?
Yes if the combined output at your ΔT meets the estimate and pipework can supply both.
Why is bathroom factor higher?
Bathrooms are often kept warmer and lose heat through extraction and external walls — the higher factor is a blunt comfort buffer.
Does double glazing change the number?
Improved fabric reduces real heat loss. This simple volume method does not adjust automatically — reduce the factor only with experience or a proper calculation.