Rule-of-thumb sizing. For accurate results run a full room-by-room heat loss calculation.
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This free radiator BTU calculator estimates the heat requirement of a room in both watts and BTU/h, so you can pick a radiator size on the spot. It is built for UK heating engineers and plumbers quoting radiator swaps, sizing replacements or sanity-checking an existing installation.
Enter the room's length, width and height in metres, pick the room type and note whether it has large windows. The calculator works out the room volume and multiplies it by a watts-per-cubic-metre factor for the room's use — 50 W/m³ for a lounge or dining room, 45 for a bathroom, 40 for a bedroom and 30 for a kitchen or hall — with a 20% uplift for rooms with large glazed areas.
The result is a rule-of-thumb figure suitable for like-for-like replacements and quick quoting. For new systems, extensions or anything with unusual construction, run a full room-by-room heat loss calculation instead.
Heat requirement (W) = length × width × height × room factor × glazing factor; BTU/h = W × 3.412
Length × width × height gives the room volume in cubic metres. The room factor is the watts of heat needed per m³ for that type of room: 50 for lounge/dining, 45 for bathroom, 40 for bedroom, 30 for kitchen/hall. The glazing factor is 1 for average windows or 1.2 for large glazed areas, adding 20% to cover the extra heat loss. Multiplying watts by 3.412 converts to BTU/h, the unit most radiator catalogues still quote.
Enter the length, width and height in metres. Height defaults to 2.4 m, the typical UK ceiling height — adjust it for older properties or rooms with high ceilings, as volume drives the whole calculation.
Choose lounge/dining (50 W/m³), bathroom (45 W/m³), bedroom (40 W/m³) or kitchen/hall (30 W/m³). The factor reflects the comfort temperature normally expected in each type of room.
Select average glazing, or large windows to apply a 20% uplift. Big glazed areas lose noticeably more heat, so patio doors, bay windows or floor-to-ceiling glass warrant the higher factor.
The calculator shows the room volume in m³ and the heat requirement in both watts and BTU/h. Choose a radiator with an output at or slightly above the requirement at your system's operating temperatures.
This is a rule-of-thumb sizing method based on room volume and use. It gives sensible results for typical UK rooms with average insulation and is well suited to radiator replacements and quick quotes, but it does not model fabric U-values, air changes, party walls or exposed floors.
For accurate results — particularly for full system design, new builds, extensions, poorly insulated properties or low-temperature systems such as heat pumps — run a full room-by-room heat loss calculation instead. Undersized emitters on a low-flow-temperature system will never reach design output.
When comparing against radiator catalogues, check the ΔT the outputs are quoted at. Manufacturer outputs are commonly rated at ΔT50; a system running cooler flow temperatures will deliver less than the catalogue figure, so a correction factor or larger radiator may be needed.
Rule-of-thumb sizing. For accurate results run a full room-by-room heat loss calculation.
Multiply the room volume (length × width × height in metres) by a factor for the room's use — 50 W/m³ for a lounge, 45 for a bathroom, 40 for a bedroom, 30 for a kitchen or hall — and add 20% for large windows. That gives watts; multiply by 3.412 for BTU/h. A typical 4.5 × 3.6 × 2.4 m lounge works out around 1,944 W, or roughly 6,600 BTU/h.
They are two units for the same thing: heat output. One watt equals 3.412 BTU/h. UK heat loss calculations are normally done in watts, while many radiator catalogues still list outputs in BTU/h, so this calculator shows both to save you converting.
It is fine for like-for-like replacements and quick quotes in typically insulated rooms. It is not a substitute for a full room-by-room heat loss calculation, which accounts for wall construction, insulation, air changes and orientation — use one for new systems, extensions, or anything unusual, and always for heat pump design.
Pick a radiator at or slightly above the calculated requirement rather than dramatically oversizing. A margin helps rooms heat up faster and suits lower flow temperatures, which improves condensing boiler efficiency. Remember catalogue outputs are usually rated at ΔT50 — if your system runs cooler, the real output will be lower than the listed figure.
Yes — this method is volume-based, so height matters as much as floor area. The calculator defaults to 2.4 m, but a 3 m Victorian ceiling adds 25% to the room volume and therefore 25% to the heat requirement compared with the default, so always measure the actual height.
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