Radiator BTU Calculator: What Size Radiator Do I Need?

A radiator is sized by the heat the room loses, not by the wall it has to fill. This calculator takes the room's volume, how it is used, and how leaky it is, and returns the output you need in watts and BTU per hour, then converts that into a radiator length you can actually shop for. The step most people skip changes the answer more than anything else: radiators are advertised at a flow temperature of ΔT50, and a system running cooler than that delivers only a fraction of the printed figure.

The result is a room-by-room rule of thumb built on volume and exposure, which is how merchants, plumbers and radiator suppliers size a replacement in an existing house. It is not a full room-by-room heat loss calculation to BS EN 12831, and it should not be used to size a heat pump or a whole system: that needs measured wall build-ups, air change rates and a proper survey.

Watts, BTU and why both are quoted

Watts and BTU per hour measure the same thing in different units, and one watt is 3.412 BTU per hour. British radiator listings still lead on BTU out of habit, so a 1,500 W radiator is sold as roughly 5,100 BTU. Nothing changes between the two numbers, so pick whichever your supplier prints and ignore the other. What does change the answer is the temperature the radiator is tested at.

The ΔT trap that oversizes half the country's radiators

Panel radiators are rated to EN 442 at ΔT50, meaning the average water temperature in the radiator sits 50 degrees above the room. That suited an old boiler firing at 80 degrees flow and 60 return into a 20 degree room. Turn the flow down to save gas, or fit a heat pump, and the same radiator emits far less. The correction factors published against the ΔT50 rating are 0.75 at ΔT40, 0.51 at ΔT30 and 0.30 at ΔT20, so a radiator on a heat pump gives about half its catalogue output, and a room needing 1,500 W wants a radiator rated near 3,000 W on the box.

That is the single most useful thing to know before you order. If there is any chance of a heat pump later, size now at the lower flow temperature and you will not be lifting floorboards twice. The calculator does this conversion for you: the first figure is the heat the room needs, the second is the rated output to shop for.

Choosing the panel type

Type 11 is a single panel with one convector fin set, slim and low output, useful in a hallway or where a door swing is tight. Type 21 adds a second panel, Type 22 is a double panel with double convectors and is the standard fit in most rooms, and Type 33 is a triple that packs a large output into a short wall. The length estimate above uses typical outputs for each type at the height you selected, so treat it as a shortlist rather than a specification, then check the manufacturer's figure for the exact model before ordering.

If the suggested length runs past about 2.4 metres, split the load across two radiators on different walls, which is what the calculator suggests when it happens. A very long single panel is awkward to fit, looks heavy in a small room, and heats one end of the space while the other stays cold. If even a Type 33 comes back longer than the wall you have, the room is asking for more heat than panel radiators can deliver at that flow temperature: insulate first, raise the flow temperature, or look at underfloor heating.

Position, and the myth about the window

Radiators went under windows because single glazing dropped a cold curtain of air down the wall and the radiator cancelled it out. With modern glazing that cold draught is much weaker, so the position is now a design decision. Under the window keeps the useful wall free for furniture. On an internal wall works fine in an insulated room and can be tidier. What matters is that nothing traps the output: a sofa pushed against a radiator, a full-length curtain hanging over it, or a shelf sitting tight on top will all send heat into the fabric instead of the room.

Bathrooms and towel rails

Bathrooms carry a higher allowance because the target temperature is higher and everything in the room is wet. Towel rails are the usual fit, and they are weak heaters for their size: a ladder rail that fills a wall may only deliver a few hundred watts. Check the rated output rather than the dimensions, and be careful with finishes, because chrome and coloured rails are rated lower than the same rail in white and manufacturers publish the figures separately. In a small bathroom that gap decides whether the room is warm in February.

Once the heating is sized, the rest of the room specification follows: our lighting calculator works out the lumens and downlights for the same space, the tile calculator covers floors and walls, and the London renovation cost calculator prices the whole job. For rooms where the radiator competes for wall space, see our small bathroom design ideas and hallway design ideas.