Radiator BTU calculator

Fit a radiator that's too small and the room never gets warm on a cold day; go too big and you waste money and space. This calculator gives you a quick estimate of the heat output a room needs, in BTU and watts, so you can pick the right radiator. Here's how it works and where its limits are.

How to work out the BTU for a room

The quick method is the room's volume times a factor for what the room is used for. Volume is length times width times height, and the factor reflects how warm that type of room needs to be.

BTU = length × width × height × room factor.

A 4m by 3m living room with 2.4m ceilings is 28.8 cubic metres. At about 153 BTU per cubic metre for a living room, that's roughly 4,400 BTU per hour, or around 1,290 watts. The calculator does that and lets you switch the room type.

BTU or watts?

Radiators are rated in both, and they convert directly: 1 watt is about 3.412 BTU per hour. So a 1,290 watt radiator is roughly a 4,400 BTU one. Size in whichever unit your radiators are listed in, and use the other to double-check.

Room type factors

Different rooms want different warmth, so the factor changes with the room. These are the rough figures the calculator uses.

Adjust for glazing, orientation and insulation

The base figure assumes an average room. Nudge it up or down for the things that change how much heat a room loses:

• Large windows or patio doors: add roughly 10 to 20%, as glass loses heat fast.
• A north-facing room: add a little, since it gets no solar gain.
• Poor insulation or an older property: add more; a well-insulated new build can take a touch less.

Size up, not down

When you're between radiator sizes, go up. A slightly oversized radiator can be run cooler on the valve or the thermostat, so it costs you nothing in practice, whereas an undersized one simply can't heat the room when it's cold outside. Match or exceed the figure the calculator gives.

Where this tool stops

This is a quick sizing guide to help you choose a radiator, not a full heating design. A proper central heating system is sized with a room-by-room heat-loss calculation to a standard like BS EN 12831, which accounts for every wall, window, floor and the air changes in the room. For a whole system, or where it really matters, have a heating engineer run that calculation.

From sizing to a priced job

Once you know the outputs, the radiators and parts go on the quote and the labour on top. Put it all into a clean, itemised quote with the quote generator, price the whole job with how to price a job so you actually make a profit, and raise the invoice when it's done.

This tool gives a quick estimate to help you choose a radiator and is not a substitute for a proper heat-loss calculation. Room factors and heat losses vary, so treat the result as a guide and get professional advice for a full heating system.