This central heating boiler BTU calculator will give you an estimated guide to the heat output in BTU and Watts needed for one room only. To calculate for an entire property, repeat the calculation for each room and note down the results, total them up and you will have the required BTU (or kW) for the whole house.
Outdoor temperatures, external walls, insulation, heated adjacent rooms (i.e. above, below and to the side) all play a part in determining boiler BTU output. The boiler should be sufficient to supply all connected radiators to the sum of their maximum demand.
It’s worth noting that you should aim to have a boiler that provides more output than you require, this is simply because its better for the boiler to be “ticking over” to supply your heat than to be “flogging itself to death”.
As a rough guide, measure the Height, Length and Width of the room and input them into the calculator below. Then click on whether you measured in feet or metres to get your BTU output.
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As a rough guide, measure the Height, Length, and Width of the room and input them into the calculator below. Then click on whether you measured in feet or meters to get the BTU required.