Radiator size calculator
Work out the radiator output a room needs to stay warm on a cold day, from its size, how it is built and where you live.
Method and tables from the MCS Heat Pump Calculator.
Result
This room needs a radiator rated at about 622 watts at ΔT50 to stay at 21°C on a cold day in LS postcodes.
watts at ΔT50
| Heat loss | 622 W |
|---|---|
| In BTU an hour | 2,124 BTU/h |
| Radiator rating at ΔT50 | 622 W, 2,124 BTU/h |
- Air leaking in and out 391 W (63%)
- Outside walls 100 W (16%)
- Windows 132 W (21%)
Show the workings
| Step | Sum | Answer |
|---|---|---|
| Temperature difference to design for | 21°C room − -2.5°C outside | 23.5 °C |
| Volume of the room | 4 m × 3.5 m × 2.4 m | 33.6 m³ |
| Ventilation | 1.5 air changes an hour × 0.33 × 33.6 m³ × 23.5°C | 390.852 W |
| Outside wall, less windows and door | 4 m × 2.4 m − 2 m² | 7.6 m² |
| Outside walls | 7.6 m² × 0.56 W/m²K × 23.5°C | 100.016 W |
| Windows | 2 m² × 2.8 W/m²K × 23.5°C | 131.6 W |
| Heat the room loses on a design day | 391 + 100 + 132 | 622.468 W |
| In BTU an hour | 622 W ÷ 0.2930711 W per BTU an hour | 2,123.949 BTU/h |
| Radiator rating needed at ΔT50 | 622 W, as the water is about 50°C above the room | 622.468 W |
What this leaves out or assumes
- Walls to other heated rooms, and to neighbours, are taken to lose no heat. The MCS method allows for some loss through party walls and to unheated rooms, so a room with those needs a little more.
- No extra is added for heating that goes off and on, an exposed site or a fireplace. An installer's full heat loss survey covers these.
Where the figures come from
- Design temperature, lounge: 21 °C. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
- Outside design temperature, LS postcodes (E Pennines (Finningley)): -2.5 °C. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
- Air changes an hour, lounge, before 2000: 1.5 per hour. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
- Heat to warm the air, per cubic metre per degree: 0.33 W/m³K. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
- U-value: Cavity wall, brick both sides, 50 mm mineral wool in the cavity, plastered: 0.56 W/m²K. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
- U-value: Wood or PVC frame, double glazed: 2.8 W/m²K. Source: MCS Heat Pump Calculator, version 1.10 (MCS). Its tables come from CIBSE Guide A and the Domestic Heating Design Guide.. Checked 13 November 2026.
- One British thermal unit (IT) per hour, in watts: 0.2930711 watts. Source: NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. Checked 13 November 2026.
How to choose a radiator from the answer
Radiators are sold by their heat output with the water 50°C warmer than the room, called ΔT50. Look for that figure in the radiator's details, in watts or BTU, and choose one at least as big as the answer above. Two smaller radiators that add up to it work as well as one big one.
Why cooler water needs a bigger radiator
A radiator gives out less heat when the water in it is cooler. With the water at a mean of 50°C in a 21°C room, a radiator gives about half its ΔT50 output, so it needs to be about twice the size. That is why heat pumps, which run cooler, often need some radiators changing. Choose the water temperature above to see the effect.
How the sum works
A room loses heat through each wall, window, door, floor and ceiling that faces outside or the ground: its area, times its U-value (how easily heat passes through it), times the temperature difference. It also loses heat to air leaking in and out: the number of air changes an hour, times 0.33 watts per cubic metre per degree, times the room's volume and the temperature difference.
The temperature difference is between the room and a cold day where you live: the outside design temperature for your postcode area, lower on higher ground. These are the figures MCS uses for heat pump installations. See all the figures.
What it does not cover
It works one room at a time and assumes the rooms around it are heated. It adds nothing for heating that goes off and on, an exposed hillside or an open fire. For a new boiler or heat pump, an installer's room-by-room heat loss survey is the way to size the whole system. This gives information, not advice.