Radiator Size Calculator
Enter the room area, ceiling height, insulation level and whether a big window or external wall leaks on it. The calculator returns the output to look for in watts and BTU/h, plus the physical radiator length at typical double-panel output.
Plumbing merchants size by wall length because that is what fits. Rooms are heated by volume through their envelope - this calculator does the volume physics and tells you why oversizing is safe and undersizing never is.
How to use
- Measure floor area and ceiling height honestly - alcoves and bays count.
- Flag the large window or external wall; 10-20% more output goes there.
- Match the watt figure on the radiator's output chart at your system temperature, not the brochure headline.
Frequently asked questions
How many watts per square metre of heating?
A room with average insulation needs roughly 100 W per m² at 2.5 m ceilings (40 W per cubic metre). Poor insulation pushes 125+, modern builds manage 75. The ceiling-height multiplier is where the 'per square metre' shortcut breaks.
Is it bad to oversize a radiator?
No - with a thermostatic valve an oversized radiator reaches the setpoint faster then modulates down, quietly and evenly. An undersized one runs flat out on the coldest day and still loses the race: the one mistake behaviour cannot fix.
What does BTU mean for radiators?
BTU/h is the same number in imperial clothes: watts times 3.412. UK spec sheets quote BTU, European ones quote watts - this calculator shows both so you can shop either catalogue.
Why is my room cold even with the radiator on?
Nine times out of ten the radiator is undersized for the room's volume or the valve is set low; the tenth time it is air-locked - bleed it. If it heats fine but the room still feels cold, check draughts and insulation before buying more iron.