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.
£293.00
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
£332.00
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
£104.99
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.
How this calculator works
The calculator multiplies your room length by width by height to get the volume in cubic metres, then applies a watts-per-cubic-metre figure based on the room type you choose. Room types range from 40 W/m³ for a well-insulated bedroom to 100 W/m³ for a single-glazed conservatory. The result in watts is then converted to BTU per hour using the standard conversion factor of 1 W = 3.412 BTU/hr.
The room-type figures are averages for a typical UK property with double glazing and moderate insulation. If your room has single glazing, high ceilings (over 2.7m), external walls on multiple sides, or poor loft insulation, add 20-30% to the calculated BTU. Conversely, a room with triple glazing, external wall insulation and a modern boiler can often get away with 10-15% less.
Typical radiator sizes and outputs
| Radiator size | BTU output (approx) | Suitable for room volume | Typical room |
|---|---|---|---|
| 600 x 400mm (compact) | 1,500-2,000 BTU | 10-15m³ | Small ensuite, box room |
| 600 x 600mm (small) | 2,500-3,000 BTU | 15-25m³ | Small bedroom, study |
| 600 x 1000mm (medium) | 4,000-5,000 BTU | 25-40m³ | Standard bedroom, dining room |
| 600 x 1200mm (large) | 5,500-7,000 BTU | 40-60m³ | Large bedroom, living room |
| 600 x 1400mm (extra large) | 7,500-9,000 BTU | 60-80m³ | Large living room, kitchen-diner |
| Double panel (600 x 1200) | 8,000-10,000 BTU | 60-80m³ | Large room, high heat loss |
Worked example: A 4m x 3m x 2.4m living room
A living room 4m long, 3m wide and 2.4m high has a volume of 4 × 3 × 2.4 = 28.8m³. Using the living room rate of 60 W/m³, the required heat output is 28.8 × 60 = 1,728 W. Converting to BTU: 1,728 × 3.412 = 5,896 BTU/hr, rounded to the nearest 100 = 5,900 BTU/hr.
A single 600 x 1200mm radiator rated at 5,500-6,500 BTU would be the minimum for this room. If the room has large single-glazed windows or is on an external wall with no insulation, step up to a 600 x 1400mm radiator (7,500+ BTU) or consider two smaller radiators on opposite walls to distribute heat more evenly.
Common mistakes when choosing radiators
- Buying a radiator based on the number of columns or the height alone. A single-panel 600 x 1200mm radiator might output 3,500 BTU, while a double-panel version of the same size outputs 6,000+ BTU. Always check the actual BTU or wattage rating, not just the physical size.
- Forgetting that modern condensing boilers run at lower flow temperatures (50-55°C) than old boilers (70-80°C). A radiator sized for an old boiler will struggle to heat the room with a modern system. If you're upgrading your boiler, size radiators for the lower flow temperature or add thermostatic radiator valves (TRVs) to balance the system.
- Blocking the radiator with furniture or long curtains. A radiator needs free airflow around it to convect heat into the room. A sofa pushed up against it can reduce its effective output by 30-50%. Keep at least 10-15cm clearance in front, and use short curtains that stop above the radiator, not long ones that drape in front of it.
- Ignoring the room's heat loss profile. A room with a large bay window, external wall on two sides, or a tiled floor needs more heat than a similar-sized room on an internal wall with carpet. The calculator's room-type average is a starting point, not a precise heat loss survey.
What to look for when buying radiators
Radiator material affects how quickly it heats up and how long it stays warm. Aluminium radiators heat up fast and respond quickly to thermostat changes, but they also cool down fast — good for rooms you only heat occasionally. Steel radiators heat more slowly but retain heat longer, giving a more even, steady warmth. Cast iron radiators take hours to heat up but stay warm for a long time after the boiler switches off — they're a traditional choice for period properties but heavy and expensive.
Double-panel radiators have roughly twice the surface area of a single-panel radiator of the same size, so they output significantly more heat in the same wall space. If you need high BTU in a narrow space, a double-panel or double-panel-plus-convector (type 22) is usually the answer. A type 21 is double-panel without convector fins; a type 22 has fins between the panels for extra surface area.
Finally, think about the finish. White is the standard and reflects heat efficiently. Coloured or designer radiators can be a feature, but dark colours absorb more heat and can reduce output by 5-10% compared to white. If you want a statement radiator, size up slightly to compensate, or choose a white high-output model and paint the wall behind it instead.
More questions answered
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.
What's the difference between single panel, double panel and convector radiators?
A single-panel (type 1) radiator has one sheet of steel. A double-panel (type 2) has two sheets with water flowing through both, roughly doubling the output for the same width. A double-panel-plus-convector (type 22) adds fins between the panels for even more surface area and heat output. For the same wall space, a type 22 outputs roughly 2.5-3 times more heat than a type 1.
Should I replace all my radiators when I upgrade my boiler?
Not necessarily. Modern condensing boilers run at lower flow temperatures (50-55°C) than old boilers (70-80°C), so existing radiators may be undersized for the new system. However, you don't always need to replace every radiator — start with the rooms that struggle to heat up, and add thermostatic radiator valves (TRVs) to control the temperature room by room.
What is a thermostatic radiator valve (TRV)?
A TRV is a self-regulating valve that screws onto the side of a radiator and automatically adjusts the hot water flow to maintain the room at your chosen temperature. They're cheap (from £10-£20 each), easy to fit, and can save 10-20% on heating bills by preventing overheated rooms. Fit them on every radiator except the one in the room with the main thermostat.
Why is my radiator hot at the top but cold at the bottom?
This is usually caused by sludge or rust building up at the bottom of the radiator, blocking hot water from reaching the lower sections. It's common in older systems with steel radiators and untreated water. The fix is to power-flush the heating system to remove the debris, or fit a magnetic filter on the boiler return to catch future sludge. In severe cases, the radiator may need replacing.