How the room load is worked out
All six surfaces of the room are treated as one envelope, with an average U-value for the insulation level you choose. Fabric loss is that U-value multiplied by the total surface area and by the design temperature difference.
Poor uses 2.0 W/m²K and 1.5 air changes, average 1.0 and 1.0, and good 0.5 and 0.5.
Ventilation is added on top, using the standard air factor and the air change rate for the same preset. The two together are the load the heater has to meet.
Worked example
A room 4 metres by 3 metres with a 2.4 metre ceiling has 57.6 square metres of surface. At the average insulation preset, 1.0 W/m²K across that surface at a 21 K difference gives 1,210 W, and one air change an hour adds 200 W, so the room needs about 1,409 W, which is 4,808 BTU per hour.
A radiator is rated at a standard water and room temperature, so a radiator quoted at 1,400 W delivers less when the flow temperature is lower. Check the rating at your system temperature.
A room figure, not a house figure
This page sizes one room. A whole house needs every room, the internal walls between heated and unheated spaces, and the hot water load, which is what the heat loss and boiler size pages do.
An open plan space behaves as one room, but a room above a garage or with two external walls loses more than the same floor area in the middle of the house. The presets cannot see that.
For a room with a lot of glazing, the average U-value will understate the loss. Treat the answer as a starting point and add a margin.
Limits of this calculator
The presets are averages for a whole envelope. They cannot represent a room with one insulated wall and three single-glazed ones.
It does not model thermal bridging, solar gain, or heat from occupants and appliances, all of which shift the real load.
It does not choose a product. Radiator output depends on the flow temperature and the emitter type, and a heater's rated output is not the same as its delivered output.
For a whole-home figure with real U-values, use the UK heat loss calculator.
Inputs for the BTU calculator
Room length
Length of the room in metres.
Room width
Width of the same room in metres.
Ceiling height
Floor to ceiling height in metres.
Insulation level
Sets the average U-value and the air change rate used in the calculation.
Design temperature difference
Indoor temperature minus the outdoor design temperature, 21 K for a UK winter.
CALCULATOR QUESTIONS
UK btu calculator questions
How many BTU do I need per square foot?
There is no fixed figure. It depends on insulation, glazing and the design temperature, which is why this page uses a stated U-value instead.
Does a radiator rated at this output deliver it?
Only at its rated flow temperature. At a lower flow temperature the output falls, often by a third or more.
Why is the ventilation loss so large?
Every air change carries heat out, and older homes leak more than the presets assume at the poor setting.
Can I use this for a whole house?
Not reliably. Use the heat loss calculator, which works element by element.