The short answer
Work out the heat the room needs, then choose radiators whose rated output adds up to it. As a quick guide, multiply the room's volume in cubic metres by about 30 W if it's well insulated, 45 W if average or 60 W for solid walls and single glazing, then adjust for room temperature, outside walls and big windows. A 4 × 3.5m living room with a 2.4m ceiling needs about 1,500 W, or 5,200 BTU.
Radiator size calculator
Enter the room's measurements and what it's like. The result is the heat the room needs on a cold day, and the radiator output to look for.
How to work out radiator size by hand
- Multiply length × width × ceiling height in metres to get the room's volume in m³.
- Multiply by about 30 W for a well-insulated room (newer build, insulated walls, double glazing), 45 W for an average one, or 60 W for solid walls and single glazing.
- Adjust for the room's temperature. The figures above are for a living room at 21°C; bedrooms, kitchens and halls at 18°C need about an eighth less, and a bathroom at 22°C about 4% more.
- Add about 10% for a corner room with two outside walls, 20% for three, and take off about 15% for a room with none. Add 15% for large windows or French doors and 10% if it faces north.
- Multiply the watts by 3.412 if you want BTU per hour.
| Room | Size | Insulation | Heat needed |
|---|---|---|---|
| Living room, one outside wall | 4 × 3.5m (33.6 m³) | Average | about 1,500 W (5,200 BTU) |
| Double bedroom | 3.5 × 3m (25.2 m³) | Average | about 1,000 W (3,400 BTU) |
| Bathroom | 2.5 × 2m (12 m³) | Average | about 560 W (1,900 BTU) |
| Victorian front room, corner, 2.8m ceiling | 4.5 × 4m (50.4 m³) | Solid walls | about 3,300 W (11,350 BTU) — use two radiators |
It's an estimate. A room-by-room heat loss calculation to BS EN 12831, which a heating engineer can do, is the accurate way — and worth having if you're fitting a heat pump.
What does ΔT50 mean on a radiator?
Radiator outputs are quoted at a temperature difference, or Delta T (ΔT), between the water in the radiator and the room. ΔT50 — water averaging 70°C in a 20°C room — is the usual rating. Run the water cooler, as condensing boilers and heat pumps do best, and the same radiator gives out less:
| ΔT | Typical water temperature | Share of the ΔT50 output |
|---|---|---|
| ΔT60 | 80°C | about 127% |
| ΔT50 | 70°C | 100% — the usual rating |
| ΔT40 | 60°C | about 75% |
| ΔT30 | 50°C (typical heat pump) | about 51% |
So for a heat pump, choose radiators rated at roughly twice the heat the room needs. Always check which ΔT a maker quotes before you compare two radiators — some towel rails are rated at ΔT60, which makes them look a quarter more powerful.
Type 11, 21 or 22?
The type is how many panels and sets of fins a radiator has. A Type 22 gives nearly twice the heat of a Type 11 the same size, so if the wall space is short, go up a type rather than wider. Our Type 11 vs 21 vs 22 guide explains the difference.
| Radiator | Sizes in stock | From (inc VAT) |
|---|---|---|
| Type 11 — single panel, single convector | 60 | £19.40 |
| Type 21 — double panel, single convector | 53 | £27.99 |
| Type 22 — double panel, double convector | 57 | £28.94 |
| Heated towel rails | 31 | £68.86 |
Where should the radiator go?
- Under the window is still the best place in most rooms: it warms the cold air falling off the glass before it crosses the floor.
- Split big rooms between two radiators on different walls for even heat, and so neither has to be enormous.
- Keep it clear. A sofa in front or long curtains over the top can cut the heat getting into the room.
- Fit thermostatic radiator valves (TRVs) so each room holds its own temperature, leaving the room with the wall thermostat on a lockshield or manual valve.
Bathrooms need their heat too: a towel rail covered in towels gives out far less, so check the heated towel rail guide before relying on one alone.
Questions
- How many BTU do I need per square metre?
- With a 2.4m ceiling, roughly 250 BTU per m² (about 70 W) in a well-insulated room, 370 BTU (about 110 W) in an average one, and 490 BTU (about 145 W) for solid walls and single glazing. Higher ceilings need proportionally more.
- Can a radiator be too big?
- Not in a harmful way. With a TRV it simply reaches temperature faster and then throttles back. It costs more to buy, but a bigger radiator lets the boiler run cooler and more efficiently.
- Do heat pumps need bigger radiators?
- Usually, yes. A heat pump runs the radiators at around ΔT30, where they give about half their ΔT50 output, so you need radiators rated at roughly twice the room's heat loss — often by going from Type 11 to Type 22.
- What size radiator do I need for a bathroom?
- A typical 2.5 × 2m bathroom needs about 560 W (1,900 BTU) at 22°C. Bathrooms are kept warmer than other rooms and have a lot of tiled surface, so don't undersize — and remember towels on a rail cut its output.
Related guides
- Heating & boilersType 11 vs Type 21 vs Type 22 Radiators: What's the Difference?
- Heating & boilersHeated Towel Rail Guide: Sizes, BTU Output and Chrome vs White
- Heating & boilersHow to Bleed and Balance Radiators
- Heating & boilersWhat Size Combi Boiler Do I Need? kW Explained