Free tool
Radiator Size & Output Calculator
Select a radiator type and size to estimate output (BTU/h + kW) - including low-temperature performance.
Instant estimate
Your radiator
Outputs are guidance estimates. Manufacturer ratings vary by model and ΔT.
mm
mm
Lower flow temperatures reduce radiator output - often significantly.
BTU
If you know the room BTU, we’ll check fit.
Used for “suitable room size” guidance.
Guidance only. For final selection, confirm manufacturer output tables at your intended ΔT.
Your output
Estimated output
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Suitable room (guide)
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Low-temp suitability
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Fit check
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Enter room BTU to compare
Tip: If you’re aiming for 55°C flow temperature or below, radiators usually need to be larger than “old boiler” sizing.
Want us to sanity-check your sizing?
We’ll confirm the right radiator output for the room and the system temperature you’re aiming for.
- Comfort: faster warm-up, fewer cold spots.
- Efficiency: right output at the right flow temperature.
- Future-proofing: allowance for low-temp or heat pump readiness.
- No guesswork: we’ll confirm against your room and layout.
Good to know: we’ll include your calculator summary so our engineer already has the basics.