What Size Radiator Do I Need? Watts and BTU for Each Room (UK)
A radiator needs to give out as much heat as the room loses. A 4 × 3.5 m living room with a 2.4 m ceiling, double glazing and one external wall needs about 1,848 watts (6,305 BTU per hour). Radiators are labelled with their output at ΔT50, so look for that figure, or the next size up. A low-temperature heat pump delivers only about half the labelled output, so the same room would need a radiator labelled about 3,590 W. The radiator size calculator works out your own room.
Heat needed for common rooms
Double glazing, cavity insulation, one external wall, 2.4 m ceiling, at the usual room temperatures:
| Room | Heat needed | BTU per hour | Radiator for a heat pump (ΔT50 label) |
|---|---|---|---|
| Small bedroom (3 × 2.5 m) | 866 W | 2,956 | 1,683 W |
| Double bedroom (4 × 3.5 m) | 1,617 W | 5,517 | 3,141 W |
| Box room or study (2.5 × 2.5 m) | 722 W | 2,463 | 1,402 W |
| Living room (5 × 4 m) | 2,640 W | 9,008 | 5,129 W |
| Large lounge (6 × 4.5 m) | 3,564 W | 12,160 | 6,924 W |
| Kitchen (4 × 3 m) | 1,386 W | 4,729 | 2,693 W |
| Bathroom (2.5 × 2 m) | 688 W | 2,346 | 1,336 W |
| Hall (4 × 1.5 m) | 627 W | 2,139 | 1,218 W |
What changes the answer
For the 4 × 3.5 m living room:
| Condition | Watts | BTU/h |
|---|---|---|
| Double glazing, cavity insulation, one external wall (the standard) | 1,848 W | 6,305 BTU/h |
| Single glazing | 2,218 W | 7,566 BTU/h |
| Triple glazing | 1,663 W | 5,675 BTU/h |
| Solid walls, not insulated | 2,218 W | 7,566 BTU/h |
| Well insulated (new build) | 1,571 W | 5,360 BTU/h |
| Corner room (two external walls) | 2,125 W | 7,251 BTU/h |
| Inside room (no external walls) | 1,571 W | 5,360 BTU/h |
As a bedroom at 18°C the same room needs 1,617 W. Single glazing adds about a fifth (2,218 W), and solid uninsulated walls add the same.
Watts to BTU
| Watts | BTU per hour |
|---|---|
| 1,000 W | 3,412 |
| 1,500 W | 5,118 |
| 2,000 W | 6,824 |
| 2,500 W | 8,530 |
| 3,000 W | 10,236 |
Multiply watts by 3.412 to get BTU per hour, or divide BTU by 3.412 to get watts.
Choosing the radiator
- Look at the ΔT50 output on the label or the table. Some sellers show only a higher figure, such as the ΔT60 or ΔT75 output, which is misleading.
- Panel type: a double panel with fins (type 22) gives out more heat from a given width than a single panel, and a tall radiator more than a low one.
- Position: under a window is best. The radiator offsets the cold air coming off the glass.
- Thermostatic valves let you set the temperature of each room, and avoid overheating a room that is slightly oversized.
- Heat pump? Aim for the larger output, or add underfloor heating, because the system runs at a lower flow temperature.
Before you decide
- Insulate first: better loft insulation and draught-proofing cut the heat the room needs. See the loft insulation calculator.
- Balance the system and bleed the radiators so they heat evenly.
- For a whole-house system, ask an installer for a room-by-room heat loss calculation. This calculator is an estimate for one room.
- Boiler and gas work must be done by a Gas Safe registered engineer.
Estimates only. The factors are rules of thumb: a heat loss calculation by an installer is more accurate.
Frequently asked questions
What size radiator do I need for a living room?
A 4 × 3.5 m living room with double glazing and one external wall needs about 1,848 watts (6,305 BTU/h). Choose a radiator with that output at ΔT50, or a little more.
How many watts per square metre?
There is no fixed figure, because the height, glazing and insulation change it. This room, with a 2.4 m ceiling, needs about 132 W per square metre of floor, more for single glazing or solid walls and less for a well-insulated room.
How do I convert BTU to watts?
Divide the BTU per hour by 3.412. A radiator of 6,000 BTU gives out about 1,760 watts.
What is ΔT50?
The standard rating for radiators: the average water temperature in the radiator is 50°C above the room temperature, which is roughly a flow of 75°C and a return of 65°C in a 20°C room.
Why do heat pumps need bigger radiators?
A heat pump runs at a lower flow temperature, so a radiator gives only about 51% of its ΔT50 output at ΔT30. The living room would need a radiator labelled about 3,590 W instead of 1,848 W.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.