Radiator BTU Calculator
Choosing an undersized radiator means a room that never quite warms up, while an oversized one wastes energy and money. BTU output should be matched to the room's volume and how well it holds heat — a draughty conservatory needs far more output per square metre than a small, well-insulated bedroom. Enter your room dimensions below to get a BTU target for shopping.
Work out the radiator BTU you need
Enter your room dimensions and pick the room type that best matches it.
Well insulated, moderate heat needs
For a 28.8m³ bedroom / study, you need a radiator (or combination of radiators) rated at approximately:
5,000 BTU/hr
That's around 1440W of heat output. This is a guide figure — older properties with poor insulation or single glazing may need more.
Recommended products
A few well-reviewed radiators to help you hit your target BTU output.
£79.99
Pros
- Pressed steel construction with a clean white finish
- Standard bracket spacing fits most UK wall systems
- Compatible with any combi or system boiler
Cons
- Valves and thermostatic head sold separately
£129.00
Pros
- Double panel with double convector fins for stronger heat output
- Designer flat-panel look fits modern UK interiors
- Compatible with combi or system boiler central heating
Cons
- Heavier than single panel radiators — needs sturdy wall fixings
£189.99
Pros
- No plumbing required — wall mounted or freestanding on a socket
- Digital thermostat with daily and weekly timer
- Ideal for extensions or rooms off the central heating loop
Cons
- Running cost higher than a gas-fed wet radiator
- Lower review count than other picks
Frequently asked questions
What does BTU mean for radiators?
BTU stands for British Thermal Unit — the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. Radiators are rated by how many BTUs per hour they output, and matching that output to your room size is what keeps a room comfortably warm without wasting energy.
Is it better to round up or down?
Always round up. An undersized radiator will run constantly at full output and still struggle to heat the room on cold days, while a slightly oversized radiator simply reaches temperature faster and can be turned down with a thermostatic valve.
Does this account for windows and insulation properly?
This calculator uses room-type averages as a starting point. Rooms with large single-glazed windows, high ceilings, or solid (uninsulated) walls will need more BTU output than the estimate suggests — in those cases, lean toward the next size up or get a room-by-room heat loss survey from an installer.
Can I use multiple smaller radiators instead of one large one?
Yes — the BTU figure is a total for the room, so you can split it across two radiators (for example, on opposite walls) as long as their combined output meets or exceeds the total.