How this is worked out
Heat leaves a room through its outside walls and windows, and with air that leaks out and is replaced by cold air. For each, heat loss in watts is:
- Walls and windows: U-value × area × temperature difference.
- Ventilation: 0.33 × air changes per hour × room volume × temperature difference.
The temperature difference is the room’s target minus the outside design temperature. The room targets (21°C for living rooms, 18°C for bedrooms, 22°C for bathrooms) and the air change rates follow the worked example in OpenEnergyMonitor’s guide to the MCS method, which uses the older CIBSE figures for air changes.
U-values we use
| Element | U-value (W/m²K) |
|---|---|
| Solid brick (no cavity), older house | 2.1 |
| Cavity wall, not filled | 1.6 |
| Cavity wall, filled with insulation | 0.7 |
| Built since about 2012 to modern rules | 0.28 |
| Single glazed | 4.8 |
| Double glazed, before about 2002 | 2.8 |
| Double glazed, recent A-rated | 1.4 |
The wall figures come from BRE’s paper on wall U-values for existing homes, which quotes the current RdSAP defaults for older solid and cavity walls. The window figures are typical values used in energy assessments and are less exact, because frames and glass vary a lot.
Worked example
A 4.5m × 3.6m living room with a 2.4m ceiling, one 4.5m outside wall with 2.5 m² of older double glazing, unfilled cavity walls, at −3°C outside. The difference is 24°C.
Wall: 1.6 × (10.8 − 2.5) × 24 = 319W. Windows: 2.8 × 2.5 × 24 = 168W. Ventilation: 0.33 × 1.5 × 38.9 × 24 = 462W. Total about 950W, or 3,200 BTU/h.
Choosing the radiator
- Radiator outputs are usually listed at ΔT50. If your boiler runs at a low flow temperature, or you’re planning a heat pump, look for the output at ΔT30 or ask the maker for a correction factor. It can be roughly half the ΔT50 figure.
- Two smaller radiators under two windows often heat a room more evenly than one large one.
- This is a single-room estimate. For a new boiler or heat pump, an installer should do a full heat loss calculation for the whole house.
Questions people ask
How many BTUs do I need for a room?
It depends on the room’s size, how much outside wall and window it has, how well insulated it is and how warm you want it. In our worked example a 4.5m × 3.6m living room with one outside cavity wall comes to about 950W, or 3,200 BTU/h. A corner room with solid walls and single glazing can need more than twice that, so work it out for your own room.
How do I convert watts to BTU?
Multiply watts by 3.412 to get BTU per hour. A 1,500W radiator is about 5,100 BTU/h.
What does ΔT50 mean on a radiator?
It’s the difference between the average water temperature in the radiator and the room, which manufacturers use to rate output. ΔT50 assumes water at about 70°C. At the lower flow temperatures used by heat pumps and condensing boilers turned down for efficiency, the same radiator gives out much less heat.
Why is my answer higher than other calculators?
We use the older CIBSE air change rates, which allow for a fairly draughty room. That leans towards a radiator that can cope on a cold day. A full room-by-room heat loss survey to BS EN 12831, as heat pump installers do, will often give a lower figure.
Sources
- BRE: CONSP-16 Wall U-values for existing dwellings (PDF)
- OpenEnergyMonitor: Heat loss calculation (MCS method, CIBSE air change rates)
- MCS heat load calculator: Heated rooms page
Trade review pending. A named tradesperson will check this calculator, and their name and qualifications will appear here.
Figures last checked 25 October 2026. Read how we calculate and our disclaimer. Found a mistake? Tell us.