BTU calculator for radiators: what size do I need?

How the BTU calculator works

You've got a room that never quite warms up, or a new one with bare plaster and no radiator yet. The question is the same: how much heat does it lose on a properly cold day, and what radiator puts that heat back?

We answer it the way a heating designer does, one surface at a time. Each wall, window, ceiling and floor loses heat in proportion to its area, its U-value and the gap between the room's temperature and the temperature on the other side. A U-value is the watts that pass through one square metre for each degree of difference, so lower is better. A filled cavity wall from the 1930s, at 0.7, lets through less than half of what an unfilled one does at 1.5.

The U-values come from RdSAP 10, the method used for EPCs on existing homes. England, Scotland, Wales and Northern Ireland each have their own tables. The cold day comes from 14 weather stations: Manchester's is -4.5 °C, Plymouth's -1.5 °C, and only about 35 hours of a typical year are colder. A solid ground floor is the exception. It sits on earth that stays at much the same temperature all year, so it uses the region's yearly average instead.

Then we add the air that leaks out, starting at 0.5 air changes an hour for a room with outside surfaces. A fully enclosed room uses zero instead. We also count the devices you select and a small allowance for the joints where walls meet floors and ceilings, which the trade calls thermal bridging.

Take the example the calculator opens with, a 1930s semi in Manchester. The living room is 4.2 by 3.6 m: one outside wall, cavity filled, a double-glazed window, a solid floor and a heated room above. It loses about 570 W on a design-cold day. The wall is a quarter of that, and so is the air. The window is a fifth of the wall's size and loses nearly four-fifths as much as the brickwork around it.

The last step is the radiator. UK catalogues rate radiators at ΔT50: water averaging 70 °C in a 20 °C room. Yours probably isn't running at exactly that, so we turn the room's loss into the rating to look for at your temperatures. Two rooms with the same loss can need different radiators. A bathroom at 22 °C gets less from the same radiator than a bedroom at 18 °C, because the gap between water and room is smaller.

Delta T 50, and why a heat pump needs a bigger radiator

Stelrad's specification book calls ΔT50 "the UK's industry standard for heating outputs". A boiler running at 75 °C out and 65 °C back gets close to the label. A heat pump doesn't. At 45 °C flow and 40 °C return the water averages 42.5 °C, which in a 21 °C living room is a difference of 21.5 degrees. A radiator's output falls faster than that difference does. It gives about a third of its rating.

So the Manchester living room, which needs a 583 W radiator on a boiler at 75/65, needs about 1,700 W of rating on a heat pump at 45/40. A 1,000 W radiator on that wall would give about 330 W there, roughly 230 W short of the 568 W the room loses. A radiator doesn't read its own label.

That's why a heat-pump quote asks about your radiators. Type the catalogue figure of a radiator you're looking at, set your installer's flow and return temperatures, and you'll see what it gives in your room and whether it covers the loss. The correction is the formula behind Stelrad's own table: output times (ΔT divided by 50) to the power 1.3.

Who this is for

Homeowners choosing a radiator for one room: a replacement for one that never kept up, a first radiator in an extension, or a check before a heat pump goes in. It also suits anyone holding a catalogue in BTU and a quote in kilowatts who wants both in the same units. It doesn't size a boiler or a heat pump, which needs the whole house and the hot water too.

What size radiator do I need?

It's the total on the wall that heats the room. If a room needs 1,700 W at ΔT50, two radiators of 850 W do the same job as one of 1,700 W, and you can type up to three and see them added. Round up to the next size in the range you like: we don't pick a model.

We don't add a safety margin either. The cold day the room is sized for is already colder than all but about 35 hours of a typical year. Your installer may add one. Ask what it's for.

Your EPC helps with the inputs: its Walls, Roof and Windows lines usually name the construction, such as "Cavity wall, filled cavity" or "Pitched, 270 mm loft insulation". Buying in BTU? The figure sits right beside the watts.