How the BTU calculator works
The calculator starts from the room volume and a heat requirement per m³ that depends on how warm the room is kept (about 21°C for living rooms, 18°C for bedrooms). It then adjusts for insulation and exposure:
The radiator sizes assume 600mm-high steel panel radiators. Type 11 is single panel, single convector; Type 21 is double panel, single convector; Type 22 is double panel, double convector. Outputs are approximate at ΔT50, and the calculator scales them for lower-temperature systems.
Typical heat needed by room (average insulation)
| Room | Size | Watts | BTU/hr |
|---|---|---|---|
| Small bedroom | 3 × 2.5m | ~720W | ~2,460 |
| Double bedroom | 4 × 3.5m | ~1,340W | ~4,590 |
| Living room | 5 × 4m | ~2,400W | ~8,190 |
| Bathroom | 2.5 × 2m | ~670W | ~2,290 |
Single external wall, 2.4m ceilings. Bathrooms are often fitted with a heated towel rail, which may give less heat than its size suggests. Check its output.
Worked example: a 4m × 4m corner lounge
The room is 4m × 4m × 2.4m = 38.4m³. A living room kept at 21°C needs about 50W per m³, so 1,920W, and two external walls add 10%: 2,112W, or about 7,200 BTU/hr. Single-panel radiators would need to be over 2m long. A 600 × 1,200mm Type 22 covers it, or a 600 × 1,600mm Type 21. Many people split a load like this between two radiators, which heats the room more evenly.
Switch the system to heat pump (ΔT30) and the same room needs almost twice the radiator size, beyond any single 600mm-high panel listed. That's why heat pump installs often include larger or extra radiators.
Why flow temperature matters
Catalogue outputs are measured at ΔT50: the radiator's average temperature is 50°C above the room's. A modern condensing boiler works most efficiently with cooler water, and a heat pump runs cooler still. At lower temperatures the same radiator gives out much less heat, so a room that was warm on an old boiler can feel cool after an efficient new system is set up properly. Sizing radiators for a lower flow temperature now keeps your options open for a heat pump later.
Getting the most from radiators
- Position: under a window, or on the coldest wall, counters cold draughts. Avoid putting a sofa or long curtains in front of a radiator.
- TRVs: thermostatic radiator valves let each room settle at its own temperature, so a slightly oversized radiator isn't a problem.
- Balancing and bleeding: if radiators near the boiler are hot and distant ones are lukewarm, the system may need balancing. If a radiator is cold at the top, bleed it.
- Insulate first: topping up the loft or fitting better glazing lowers the heat each room needs. The loft insulation calculator and loft insulation guide are a good place to start.
Common sizing mistakes
- Using floor area only. High ceilings add a lot of volume. A 3m ceiling needs 25% more heat than a 2.4m one.
- Swapping like for like on a new system. If the flow temperature is going down, the old radiator sizes may no longer be enough.
- Relying on a towel rail. Chrome towel rails often give out far less heat than a panel radiator of the same size. Check the watts on the spec sheet.
Once you've sized each room, add them up and check the boiler with the boiler size calculator. These are estimates for choosing radiators, not a substitute for a heating engineer's room-by-room heat loss calculation. More home calculators are in heating, insulation and measuring.