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Radiator BTU Calculator

Work out radiator btu instantly with clear inputs, formula shown and shareable results.

About Radiator BTU Calculator

Radiator sizing uses watts per cubic metre by room type: about 45 for a living room, 40 for a bedroom, 35 for a kitchen where appliances add heat, and 55 for a bathroom where higher temperatures are wanted. Each large window adds roughly 300 W of loss.

Formula

Radiator output

watts = volume x room type factor + 300 W per large window; BTU/h = watts x 3.412

Worked example

With the default values below, Radiator BTU Calculator returns Radiator output required of 7,943. Change any input above and every figure here is recalculated in your browser.

Inputs used

Room area
16 m²
Ceiling height
2.4 m
Room typeone of Living room, Bedroom, Kitchen, Bathroom
Living room
Large windows
2 windows

Results

Radiator output required
7,943 BTU/h
Radiator output
2,328 W
Approximate radiator size
3.33 m²
Room type factor applied
45.000 x

Step by step

  1. 1Values used
  2. 2Radiator output
  3. 3Radiator output required= 7,943 BTU/h
  4. 4Radiator output= 2,328 W
  5. 5Approximate radiator size= 3.33 m²
  6. 6Room type factor applied= 45.000 x

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does a kitchen need less?

Cooking, the fridge and other appliances contribute significant heat, so the radiator only has to make up the difference.

Are quoted radiator outputs comparable?

Only at the same delta T. Outputs are usually quoted at delta T 50, and a modern condensing boiler running at lower flow temperatures delivers considerably less.

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