What a BTU figure actually tells you
A BTU - British Thermal Unit - is a measure of heat. One BTU is roughly the energy needed to raise a pound of water by one degree Fahrenheit, which is a strange, imperial, faintly archaic thing for a modern heating industry to still be using. But it is what radiator manufacturers print on the box, so it is what you have to work with.
What the number on this page tells you is the heat a room loses on a cold day. That is the same as the heat you must put back in to hold it at a comfortable temperature. Size the radiator to match that figure and the room settles. Come in under it and the room never quite gets there, no matter how long the heating runs.
Why volume, not floor area
The most common sizing mistake is working from floor area. Two rooms with the same footprint but ceilings of 2.4m and 3.2m are not the same job - the taller one has a third more air to heat and a third more wall to lose it through.
Heat fills a volume. That is why this calculator asks for a height, and why the rule of thumb you may have heard - so many watts per square metre - will let you down in a Victorian front room with high ceilings, or in a loft conversion with a sloped ceiling and a cold roof above.
Getting each input right
Measuring the room
Measure to the inside face of the walls, in metres. For an L-shaped room, split it into two rectangles, calculate each, and add them. Bay windows are worth including - they are usually the coldest part of the room.
Judging your insulation honestly
This is where people flatter themselves. A rough guide for UK housing stock:
- Poor - solid brick, typically pre-1930, with no cavity to fill. Walls under about 26cm thick are usually solid.
- Average - cavity walls, built from roughly the 1930s onwards, possibly filled at some point. Most UK homes land here.
- Good - a genuinely modern build, or a house that has been properly retrofitted.
If you are not sure, choose Average. Undersizing a radiator is a much more expensive mistake to fix than oversizing one.
External walls, orientation, and what’s above
Every external wall is a surface losing heat to the outside. A corner room with two of them is a harder job than a room in the middle of the house with one.
North-facing rooms get no meaningful solar gain, all winter, all day. They run colder than a south-facing room of identical construction, and they are the ones people complain about.
What sits above the room matters more than most people expect. A bedroom under an unheated loft loses heat upwards through the ceiling. A room under a flat roof loses more still.
The ΔT trap - the bit nobody tells you
This is the most important section on this page, and it is the one that most radiator calculators skip entirely.
Radiator manufacturers quote heat output at a stated ΔT - the difference between the average water temperature in the radiator and the air temperature in the room. The UK standard, under BS EN 442, is ΔT50: flow at 75°C, return at 65°C, room at 20°C. Every reputable retailer quotes at ΔT50, and this calculator returns a ΔT50 figure so you can compare like for like.
Two things follow, and they both cost people money.
First: watch for inflated figures. A radiator advertised at ΔT60 or ΔT70 will show a bigger BTU number for exactly the same piece of metal. The conversion factor from ΔT50 to ΔT60 is about 1.26 - so a 1,000 BTU radiator becomes a “1,264 BTU” radiator without anything physical changing. If a listing does not state its ΔT, be suspicious.
Second, and far more importantly: heat pumps do not run at ΔT50.
A heat pump is most efficient at low flow temperatures - typically 35°C to 45°C, against a 20°C room. That is a ΔT of around 30, sometimes lower. And radiator output falls away sharply as ΔT drops. The relationship is not linear; it follows an exponent of about 1.3 under EN 442.
In practice, a radiator rated at 5,000 BTU at ΔT50 will deliver roughly half that at ΔT30.
Read that again, because it is the reason so many heat pump retrofits disappoint. The heat pump gets blamed. The heat pump is usually not the problem. The radiators - sized years ago for a boiler running at 75°C - are simply too small to deliver the required output at a heat pump’s flow temperature.
Common mistakes
- Sizing to floor area instead of volume
- Measuring the outside of the building rather than the inside of the room
- Forgetting what is above the room
- Trusting a headline BTU figure without checking the ΔT it was quoted at
- Sizing every radiator perfectly and never checking the boiler can carry the total
When to stop and get a proper survey
This calculator is an estimate. It is a good, careful one, and for swapping a single radiator on an existing gas system it is enough.
It is not a room-by-room fabric heat loss calculation. It does not model U-values, air changes, or thermal bridging.