Short answer: a room needs enough heat to replace what it loses, and what it loses depends on the walls, windows, floor and ceiling, how much air the room changes, and how warm you want it. There is no honest figure per square metre. A 20 m² lounge comes out at roughly 3,600 BTU/hr in a modern insulated house and over 6,000 BTU/hr in a solid-walled, single-glazed 1930s semi. Same floor area, nearly double the radiator.
This guide walks through the method the BTU Calculator uses, with three rooms worked in full, so you can see where the number comes from.
BTU, watts and kW
A BTU, a British thermal unit, is the old imperial unit of heat, and the UK heating trade still quotes radiator outputs in BTU per hour. The conversion is fixed:
| 1 W | 3.412 BTU/hr |
| 1 kW | 3,412 BTU/hr |
| 1,000 BTU/hr | 293 W |
The calculation itself works in watts, because the building physics is metric; the BTU figure is a conversion at the end.
The method
The approach is the standard simplified one used across the trade. Each surface loses heat in proportion to its area and its U-value, the rate at which heat passes through a square metre of it for each degree of temperature difference. Air changes lose heat in proportion to the room’s volume. Add those up, scale by the room’s target temperature, apply the factors, add a margin.
1. Windows. Area × U-value of the glazing. A wood or plastic double-glazed unit is taken as 2.9; single glazing in a wood frame as 6.4, more than double.
2. Walls. (Wall area − window area) × U-value of the wall, for each external wall. Insulated cavity 0.92; uninsulated brick cavity 1.5; 220 mm solid brick 2.1; 105 mm solid brick 3.0; timber frame 0.4.
3. Ceiling. Floor area × the U-value of what is above: nothing for a heated room, 0.35 for a roof space with 100 mm or more of insulation, 2.7 for an uninsulated roof space.
4. Floor. Floor area × a coefficient for the floor type, which is lower for larger rooms and for rooms with one external wall: nothing for a heated room below, about 0.45 to 1.22 for concrete, 0.96 or 1.27 for a suspended timber floor.
5. Air changes. Floor area × height × 0.33 × a multiplier for how much the room is ventilated: 1.0 for a living room, 0.5 for a bedroom, 1.5 for a kitchen or hallway, 2.0 for a bathroom.
6. Temperature. The sum of the five losses is per degree. Multiply by the target temperature for the room type: 22 °C living room, 19 °C bedroom, 17 °C kitchen or hallway, 23 °C bathroom.
7. Factors. ×1.15 for a north-facing room; ×1.20 for French doors; then ×1.15 as a safety margin on everything.
That gives watts. Multiply by 3.412 for BTU/hr.
Three rooms, worked
A: modern lounge
4 m × 5 m × 2.4 m high. One external wall on the 4 m side, uninsulated brick cavity (U 1.5). One 2 m² double-glazed window (U 2.9). Heated room above; concrete floor below.
| Loss | Working | W per °C |
|---|---|---|
| Window | 2 × 2.9 | 5.8 |
| Wall | (2.4 × 4 − 2) × 1.5 | 11.4 |
| Ceiling | heated above | 0 |
| Floor | 0.45 × 20 (concrete, > 16 m², one wall) | 9.0 |
| Air | 20 × 2.4 × 0.33 × 1.0 | 15.8 |
| Total | 42.0 |
× 22 °C = 925 W; × 1.15 margin = 1,063 W, which is 3,627 BTU/hr or 1.06 kW.
B: the same room in a 1930s semi
Same dimensions. 220 mm solid brick (U 2.1), the same 2 m² window but single glazed in a wood frame (U 6.4), suspended timber floor, and it faces north.
| Loss | Working | W per °C |
|---|---|---|
| Window | 2 × 6.4 | 12.8 |
| Wall | 7.6 × 2.1 | 16.0 |
| Ceiling | heated above | 0 |
| Floor | 0.96 × 20 (timber, > 16 m²) | 19.2 |
| Air | as before | 15.8 |
| Total | 63.8 |
× 22 °C = 1,404 W; × 1.15 north-facing = 1,614 W; × 1.15 margin = 1,856 W, which is 6,332 BTU/hr or 1.86 kW.
Same floor area. The single glazing alone more than doubles the window loss, and the timber floor doubles the floor loss. This is why a per-square-metre rule of thumb is not worth having.
C: a small bedroom under the roof
3 m × 3.5 m × 2.4 m. Two external walls, insulated cavity (U 0.92). One 1.5 m² double-glazed window. Roof space above with 100 mm or more of insulation (U 0.35); heated room below.
| Loss | Working | W per °C |
|---|---|---|
| Window | 1.5 × 2.9 | 4.4 |
| Walls | (2.4 × 3 − 1.5) × 0.92 + 2.4 × 3.5 × 0.92 | 13.0 |
| Ceiling | 10.5 × 0.35 | 3.7 |
| Floor | heated below | 0 |
| Air | 10.5 × 2.4 × 0.33 × 0.5 | 4.2 |
| Total | 25.2 |
× 19 °C = 478 W; less a 50 W allowance for the heated room below; × 1.15 margin = 491 W, which is 1,675 BTU/hr or 0.49 kW. A single small panel.
The BTU Calculator runs exactly this arithmetic from the inputs you give it and shows the breakdown, so you can see whether it is the glazing or the walls driving the number, which is also the answer to “what would insulating it save?”
Choosing the radiator
Radiator outputs in catalogues are quoted at a standard temperature difference between the water and the room, written ΔT50. On a conventional boiler system running around 75 °C flow, that is about right and you pick a radiator whose quoted output meets or slightly exceeds the room figure.
On a system running cooler, which is every heat pump and an increasing number of condensing boilers, the same radiator gives much less than its catalogue figure, and the shortfall is not linear. That is a separate subject with its own arithmetic, and a guide to sizing radiators for a heat pump is coming; the BTU Calculator’s heating system setting applies the correction for you.
Where this method stops
This is the trade’s simplified method, and it is deliberately generous, which is what you want when the cost of undersizing is a cold room and the cost of oversizing is a slightly larger panel. It is the right tool for replacing or adding a radiator in an existing house.
It is not a full heat loss calculation. For a new heating system, a heat pump installation, or anything where the boiler or pump itself is being sized, a heating engineer’s room-by-room calculation to BS EN 12831, with the actual U-values of the building and the local design temperature, supersedes it. Use this to sanity-check that calculation, not to replace it.
Method, U-values and factors are as implemented in BTU Calculator v1.1 and the worked examples were computed from that code on 11 September 2026. U-values are typical figures for the construction types named, not measured values for any particular building. Plan CD is not affiliated with any heating manufacturer or certification body.