What size radiator do I need?
Enter your room's measurements and we'll work out the heat output it needs, in BTU and watts, using manufacturers' published figures.
Enter your room's measurements.
Quick guide: typical rooms
BTU per hour each room needs, using Viessmann's figures. The last column is the catalogue (ΔT50) size to buy if your heating runs at heat pump temperatures (ΔT30), using Stelrad's 0.513 factor.
| Room | BTU/h needed | Watts | ΔT50 size for ΔT30 |
|---|---|---|---|
| Small bedroom, double glazed 3 × 3 m, 2.4 m ceiling | 2,333 | 684 | 4,547 |
| Double bedroom, double glazed 4 × 3.5 m, 2.4 m ceiling | 3,629 | 1,063 | 7,074 |
| Double bedroom, single glazed 4 × 3.5 m, 2.4 m ceiling | 4,435 | 1,300 | 8,646 |
| Living room, double glazed 5 × 4 m, 2.4 m ceiling | 6,480 | 1,899 | 12,632 |
| Large living room, double glazed 6 × 4.5 m, 2.4 m ceiling | 8,748 | 2,564 | 17,053 |
Round up to the next radiator size. Use the calculator above for your own measurements.
BTU or watts?
They measure the same thing: heat output. UK radiator catalogues usually give both. One BTU per hour is 0.2931 watts, so multiply BTU by 0.2931 to get watts, or watts by 3.412 to get BTU. A 1,000 W radiator gives about 3,412 BTU per hour.
How it works
- Work out the room's volume: length × width × height, in metres. Viessmann
- Multiply by Viessmann's figure for the room: 108 for a double-glazed bedroom, 132 for a single-glazed bedroom, 135 for a double-glazed living room. That gives the heat the room needs, in BTU per hour. Viessmann
- Radiator catalogues quote output at ΔT50. If your heating runs cooler, a radiator gives out less heat, so you need a bigger one. We divide by Stelrad's correction factor: 0.747 at ΔT40 and 0.513 at ΔT30. Stelrad, Worcester Bosch
- Watts = BTU per hour × 0.2931. NIST, Stelrad
An estimate, not a design
Manufacturers are clear that a rule of thumb like this is a starting point. Stelrad says to round up to the next available radiator size and to consult a heating engineer if you're unsure. A proper heat loss calculation takes into account walls, windows, insulation and how exposed the room is.
Viessmann doesn't publish a figure for kitchens, bathrooms or single-glazed living rooms. For a bathroom, which is designed to be warmer, we use the living room figure as the closest published guide.
Common questions
How many BTU do I need for a room?
Viessmann's rule of thumb is the room's volume in cubic metres multiplied by 108 for a double-glazed bedroom, 132 for a single-glazed bedroom or 135 for a double-glazed living room. A 4.8 × 4 × 2.4 m double-glazed bedroom needs about 4,977 BTU per hour.
What does ΔT50 mean on a radiator?
It's the standard condition radiator outputs are quoted at. If your heating runs cooler, for example with a heat pump or a lowered flow temperature, the radiator gives out less. Stelrad's factor at ΔT30 is 0.513, so a radiator gives about half its ΔT50 output.
How do I convert BTU to watts?
Multiply BTU per hour by 0.2931. To go the other way, multiply watts by 3.412.
Sources
- Viessmann: How many BTU to heat a room?
- Stelrad: Radiator output correction factors
- Worcester Bosch technical bulletin TB0029 (radiator correction factors)
- Stelrad: Why Delta T matters for radiator output
- NIST: Conversion factors (Btu per hour to watts)
- Stelrad: How to convert BTU to kW
- Stelrad: BTU calculator and heat loss
Last reviewed October 2026. Spotted something that doesn't match your manual? Email hello@boilercodes.co.uk and we'll check it. How we check