Radiator & BTU Calculator: What Size Radiator Do I Need?
Find out how much heat a room loses on a cold day, in watts and BTU, and what size radiator to buy for a gas or oil boiler or a heat pump. Works for homes in the UK, Ireland and eight other European countries.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Size your radiator
Radiator & BTU Calculator
Work out how much heat a room loses on a cold day, in watts and BTU, and what size radiator to buy for a gas or oil boiler or a heat pump.
1 The room
Heating design temperatures for older homes (CIBSE). Newer homes use 21 °C in every room except bathrooms.
SizeLength
Width
Ceiling height
Walls to the outside only. Walls to other heated rooms don't count.
Windows and glass doorsHow many
Width
Height
2 How it's built
Outside walls Windows3 Your area and heating
The outdoor temperature heating is designed for: colder only about 35 hours a year (CIBSE Guide A).
Heating systemThe rated output (at ΔT50) from the label or the maker's website. Leave empty to skip.
Radiator output to buy
0 W
0 BTU/h
rated output at ΔT50, for a boiler at 70 °C
Heat the room loses
0 W
Radiator gives out
0%
Inside vs outside
0 °C
Where the heat goes
Radiators that would do
| Type (600 mm high) | Length | Rated output |
|---|
Typical outputs at ΔT50 from a maker's datasheet (Stelrad Compact). Taller radiators give more from the same length. Check the exact model before you buy.
With a heat pump instead
How it's worked out: heat lost = area × U-value × temperature difference for each outside wall, window, loft ceiling and ground floor, plus air leaks (0,33 × air changes an hour × room volume × temperature difference). Ground floors use the yearly average outdoor temperature instead of the coldest one. Radiator output to buy = heat lost ÷ (temperature difference between the radiator water and the room ÷ 50)^1,3, because radiators are rated at ΔT50 (EN 442). U-values from RdSAP 10; room temperatures and air changes from the CIBSE Domestic Heating Design Guide; design outdoor temperatures from CIBSE Guide A (UK) and the ASHRAE Handbook 2021 (elsewhere). 1 W = 3,412 BTU/h.
Estimates only. Real heat loss depends on how your home is built, how airtight it is and the weather. Before fitting a heat pump, get a room-by-room heat loss survey from a certified installer.
How it works
From heat loss to radiator size
A radiator has to replace the heat a room loses on a cold day. The calculator works out that heat loss, then the radiator output that delivers it at your heating system's water temperature. The sources and assumptions behind it are on our How we calculate page.
How much heat the room loses
Heat escapes through outside walls, windows, the ceiling to a cold loft and the floor, and with air leaking in and out. Each is worked out from its area, how well it’s insulated and the difference between inside and a cold winter day, using the same method heating engineers use.
What ΔT50 means
Radiators are sold with their output at ΔT50: water about 50 °C warmer than the room, as with a boiler running hot. Run the water cooler and the radiator gives out less, so the calculator scales the output to your system.
Why heat pumps need bigger radiators
A heat pump is most efficient with water at about 35–50 °C. At 45 °C, a radiator gives out only about a third of its rated output, so the same room needs a radiator roughly two and a half times bigger than with a boiler.
Examples
What size radiator does a room need?
The same room can need a radiator two to three times bigger in an older home than in a newer one, and bigger again with a heat pump. These examples use the calculator’s own formula.
Illustrative only: London (design temperature −3 °C), one outside wall with one window, 2.4 m ceilings. Older home: unfilled cavity walls, double glazing from before 2002, an uninsulated ground floor, 100 mm of loft insulation and average draughts. Newer home: well insulated walls, newer double glazing, an insulated floor, 270 mm of loft insulation and few draughts. Radiator sizes are rated outputs at ΔT50. Your rooms will differ, so use the calculator above.
Choosing a radiator
From a number to a radiator
Once you know the output you need, pick a radiator that gives at least that much at ΔT50, and think about the heat loss itself.
Pick a type and size
Single panel (type 11), double panel single fin (type 21) and double panel double fin (type 22) radiators of the same size give very different outputs. Taller and longer radiators give more. Check the rated output in watts or BTU on the maker’s data, and round up.
Getting ready for a heat pump
Radiators sized for a boiler are often too small for a heat pump. Swapping a few for double-panel or longer models is common. Before you buy a heat pump, get a room-by-room heat loss survey from a certified installer.
Cut the heat loss first
Draught-proofing, loft insulation and better glazing shrink the radiator you need and your bills. The calculator shows where each room loses most heat. Our Heating Cost Compare shows what that heat costs with each kind of heating.
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Common questions
Frequently asked questions about radiator sizes
Common questions about BTU, watts, ΔT50 and radiators for heat pumps.
How many BTU do I need for my room?
It depends on the room’s size, how well it’s insulated, its windows and how cold your area gets. A typical 4 × 4 m living room in an older home loses about 820 W (2,800 BTU/h) on a cold day in London, and needs a radiator rated about 970 W (3,300 BTU/h) for a gas boiler. Use the calculator for your own room.
What does ΔT50 mean?
It’s the temperature difference between the radiator water and the room that radiator outputs are tested at (EN 442): water about 50 °C warmer than the room, as with a boiler running at around 70 °C. If the water is cooler, the radiator gives out less than its rated output.
Will my radiators work with a heat pump?
Often some of them need changing. A heat pump works best with water at about 35–50 °C. At 45–50 °C a radiator gives out roughly a third to just under a half of its rated output, and less still at 35 °C. Rooms that were comfortable with a boiler may need a bigger, double-panel or extra radiator. A heat loss survey by a certified installer will tell you which.
Is a bigger radiator more expensive to run?
No. Your running cost depends on how much heat the room loses, not on the size of the radiator. A bigger radiator can heat the room with cooler water, which lets a condensing boiler or a heat pump run more efficiently.
Watts or BTU: which should I use?
Either: they measure the same thing. 1 watt is about 3.41 BTU per hour. UK radiator shops often quote BTU, while most of Europe uses watts. The calculator shows both.
Why do bathrooms need bigger radiators?
Bathrooms are designed to be warmer, about 22 °C, and towels hung on a towel rail block some of its heat. Extractor fans also pull warm air out. So a bathroom often needs more output than its small size suggests.
Are the results exact?
No. They are estimates based on typical U-values, the coldest temperature your area is designed for, and average air leakage. Real homes vary, so round up when choosing a radiator, and get a professional heat loss survey before fitting a heat pump.