Radiator Size Calculator
Enter the room's size, its use, how many outside walls it has and the type of glazing. The calculator works out the heat output you need and how long each type of panel radiator must be to give it.
- Watts and BTU
- Panel type and length
- Boiler or heat pump
Heat pumps run cooler, so radiators need to be about twice as big for the same room.
A radiator has to replace the heat a room loses. Rooms with more outside walls, single glazing or tall ceilings lose more. A double convector radiator gives nearly twice the heat of a single panel the same size.
Radiator size chart
Living room, one outside wall, 2.4 m ceiling. Search for a room size.
| Room (m) | Heat (W) | BTU | Type 11 (mm) | Type 22 (mm) |
|---|---|---|---|---|
| 3 × 3 | 1188 | 4054 | 1131 | 660 |
| 4 × 3 | 1584 | 5405 | 1509 | 880 |
| 4 × 4 | 2112 | 7206 | 2011 | 1173 |
| 5 × 4 | 2640 | 9008 | 2514 | 1467 |
| 6 × 4 | 3168 | 10809 | 3017 | 1760 |
| 6 × 5 | 3960 | 13511 | 3771 | 2200 |
Lengths are for 600 mm high radiators with a boiler at ΔT50.
Room size and radiator
Living room, 2.4 m ceiling. Slide the room length.
The radiator is drawn against the wall at the length a double convector would need.
How to size a radiator
Room volume first. Step through it.
Room volume
L × W × H.
ΔT50 means the radiator is 50 °C warmer than the room. Most catalogues quote output at ΔT50.
Single, double or double convector
Panel types. Compare them.
One panel, one fin layer.
Two panels, one fin layer.
Two panels, two fin layers.
Type 22 radiators stick out further from the wall, about 100 mm instead of about 60 mm.
Before buying
Radiator checks. Tick each item.
Tick each item as you go.
Put radiators under windows where you can. They warm the cold air falling off the glass.
What you get
Radiator sizing. Each tile explains one result.
In watts.
For US catalogues.
For each type.
Your choice.
Boiler or heat pump.
Two smaller radiators heat a long room more evenly than one big one.
Frequently Asked Questions
No question matches that search.
How do I work out what size radiator I need?
Multiply the room volume by a factor for its use, adjust for outside walls and glazing, then choose a radiator with that output.
How many watts does a room need?
A living room needs about 50 watts for every cubic metre, a bedroom about 40.
What is the difference between a single and a double radiator?
Type 11 has one panel and one fin layer, type 22 has two of each and gives nearly twice the heat.
Do heat pumps need bigger radiators?
Yes. They run cooler, so radiators need to be roughly twice as large for the same heat.
How do I convert watts to BTU?
Multiply watts by 3.412.
Where should a radiator go in a room?
Under a window or on an outside wall, where the room loses the most heat.
What does delta T mean on a radiator?
The difference between the water temperature and the room. Outputs are usually quoted at ΔT50.
A warm room, every room
Work out the heat the room needs, then choose the panel type and length to give it. With a heat pump, size up so the room stays warm at lower water temperatures.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.