Updated 23 October 2026. Worked examples use the same method as our BTU calculator, and the radiators shown are real Flatty and Rondy sizes.
A radiator that is too small never gets the room warm, however long the boiler runs. One that is too big costs more and takes up more wall, but on a modern condensing boiler or heat pump it is rarely a problem, because extra surface area lets the system run cooler. So the practical rule is to work out what the room loses, then pick a radiator that matches it or comes in slightly above.
This guide shows how to do that by hand, why the answer changes with your boiler's flow temperature, and what that looks like in real sizes from our horizontal and vertical designer ranges. If you would rather skip the arithmetic, the BTU calculator does all of it in about a minute.
The short version
1. Work out the room's heat requirement in BTU/h or watts (1 watt = 3.412 BTU/h) with the calculator or the method below.
2. Add roughly 10% if the ceiling is 2.7m or higher.
3. Pick a radiator whose ΔT50 output is within about 10% of that figure. Between two sizes, go up.
4. Check your boiler's flow temperature. If it is well below 75°C, the radiator gives less than its label says (table further down).
How the calculation works
Our calculator starts from floor area, then adjusts for what surrounds the room. Floor area is a reasonable starting point because most UK rooms have similar ceiling heights, but it does not see height, which is why step 2 above is a manual correction.
The starting figure is 190 BTU/h per m² of floor (about 56 W/m²) for a living room. Multiply it by each of the factors below, then add the heat lost through the glass.
| What changes | Option | Factor |
|---|---|---|
| Room type | Living room | × 1.00 |
| Bedroom | × 0.88 | |
| Hallway or kitchen | × 0.82 | |
| Bathroom | × 1.12 | |
| Floor below | Heated room | × 1.00 |
| Suspended wooden floor | × 1.10 | |
| Concrete ground floor | × 1.18 | |
| Above the room | Heated room | × 1.00 |
| Loft, no insulation | × 1.40 | |
| Loft, about 50mm insulation | × 1.22 | |
| Loft, 100mm or more | × 1.08 | |
| Flat roof, uninsulated | × 1.35 | |
| Flat roof, insulated | × 1.12 | |
| Wall construction | Cavity wall | × 1.00 |
| Insulated cavity wall | × 0.94 | |
| Solid brick, 220mm | × 1.06 | |
| Solid brick, 105mm | × 1.10 | |
| Timber frame | × 1.14 | |
| External walls | Each one | + 10% |
| Windows | Per m² of glass | + 200 BTU/h (double glazed), 340 to 380 BTU/h (single glazed) |
The wall factors are not ranked the way you might expect: 105mm solid brick comes out worse than 220mm because there is less masonry between you and the outside.
Four rooms, worked through
Double bedroom, 3.5m × 3m, one external wall
First-floor room over a heated living room, cavity wall, 1.5m² of double glazing.
10.5m² × 190 = 1,995 → × 0.88 (bedroom) = 1,756 → × 1.10 (one external wall) = 1,931 → + 300 (glass) = 2,231, which rounds to 2,200 BTU/h (about 645W).
A Flatty 600 × 770mm single panel is within 2% of that.
Flatty white horizontal radiator
600 × 770mm, single panel
Ground-floor living room, 6m × 4.5m, two external walls
Concrete floor, heated bedroom above, insulated cavity walls, 3.5m² of double glazing.
27m² × 190 = 5,130 → × 1.18 (concrete) = 6,053 → × 0.94 (insulated cavity) = 5,690 → × 1.20 (two external walls) = 6,828 → + 700 (glass) = 7,528, which rounds to 7,500 BTU/h (about 2,200W).
One Rondy 600 × 1440mm double panel is within 2% of that. If you would rather spread the heat, two Rondy 600 × 1140mm single panels give 7,654 BTU/h between them, and let you put one under each window.
Rondy white horizontal radiator
600 × 1440mm, double panel
Bathroom, 2.5m × 2m, one external wall
Heated rooms above and below, cavity wall, 0.8m² window.
5m² × 190 = 950 → × 1.12 (bathroom) = 1,064 → × 1.10 = 1,170 → + 160 (glass) = 1,330, which rounds to 1,300 BTU/h (about 380W).
This is a job for a towel rail rather than a panel radiator. Towels hung over the rails block some of the heat, so treat the rail's rated output as the best case and choose one with a margin. Our towel rail BTU guide covers the sizing, and the heated towel rails and electric towel rails collections list outputs on every product page.
Open-plan kitchen-diner, 7m × 4m, three external walls
For an open-plan space with a dining area, use the living room setting, because the cooking only warms one end. Concrete floor, heated room above, insulated cavity, 5m² of double glazing.
28m² × 190 = 5,320 → × 1.18 = 6,278 → × 0.94 = 5,901 → × 1.30 (three external walls) = 7,671 → + 1,000 (glass) = 8,671, which rounds to 8,700 BTU/h (about 2,550W).
Two Flatty 600 × 1470mm single panels give 8,542 BTU/h in total, 2% under. One on each end wall heats a long room more evenly than a single large radiator would.
Flatty white horizontal radiator
600 × 1470mm, single panel (x2)
Choosing the radiator: Flatty and Rondy sizes
Both ranges are tested to BS EN 442, and outputs are given at ΔT50. The figures below are the same in white and anthracite.
Horizontal, 600mm high
| Model | Size | Panels | Output (ΔT50) |
|---|---|---|---|
| Rondy | 600 × 360mm | Single | 1,210 BTU/h (355W) |
| Flatty | 600 × 770mm | Single | 2,236 BTU/h (656W) |
| Rondy | 600 × 1140mm | Single | 3,827 BTU/h (1,122W) |
| Flatty | 600 × 1470mm | Single | 4,271 BTU/h (1,252W) |
| Flatty | 600 × 980mm | Double | 4,606 BTU/h (1,351W) |
| Flatty | 600 × 1470mm | Double | 6,909 BTU/h (2,026W) |
| Rondy | 600 × 1440mm | Double | 7,368 BTU/h (2,161W) |
Vertical, 1800mm high
| Model | Size | Panels | Output (ΔT50) |
|---|---|---|---|
| Rondy | 1800 × 300mm | Single | 2,697 BTU/h (791W) |
| Flatty | 1800 × 350mm | Single | 2,885 BTU/h (846W) |
| Flatty | 1800 × 420mm | Single | 3,462 BTU/h (1,015W) |
| Rondy | 1800 × 540mm | Single | 4,856 BTU/h (1,424W) |
| Flatty | 1800 × 420mm | Double | 5,335 BTU/h (1,565W) |
| Rondy | 1800 × 600mm | Double | 7,973 BTU/h (2,338W) |
| Flatty | 1800 × 630mm | Double | 8,001 BTU/h (2,346W) |
The reason to go vertical is wall width. A Flatty 1800 × 420mm single gives about 8.2 BTU/h for every millimetre of width, where a 600 × 1470mm horizontal single gives about 2.9. If you have a tall, narrow gap and a big requirement, vertical wins. If you have a long, low wall under a window, horizontal does. More on that in our vertical radiators guide.
Single or double panel? A double panel gives roughly 60% more output in the same width and height, at the cost of 13 to 15mm more depth. Our single vs double panel guide goes through when it matters.
ΔT50 is a label, not what your radiator delivers
Every output in the tables above is measured at ΔT50. That means water at 75°C flow and 65°C return in a room at 20°C, so the average water temperature is 50°C above the room. Older catalogues sometimes quote ΔT60 (90°C flow, 70°C return). To compare, multiply a ΔT60 figure by 0.79 to get ΔT50.
Most homes do not run at 75/65. A condensing boiler is often set lower so the return water is cool enough to condense, and a heat pump runs cooler still. The radiator then delivers a fraction of its label:
| System | Flow / return | ΔT | Share of ΔT50 output |
|---|---|---|---|
| Rating standard | 75°C / 65°C | 50 | 100% |
| Condensing boiler | 70°C / 50°C | 40 | about 75% |
| Heat pump | 55°C / 45°C | 30 | about 52% |
If you run at ΔT40, a radiator needs to be about a third bigger than the calculation says. On a heat pump it needs to be nearly double. That is the one adjustment people miss most often. Our heat pump radiator sizing guide works through it, and the heat pump radiators collection lists suitable models.
Mistakes that cost money
- Ignoring ceiling height. A room with 3m ceilings holds a quarter more air than the same room at 2.4m, so scale the result by height ÷ 2.4.
- Taking the label at face value. A radiator that is right at ΔT50 can be a third short at ΔT40, so check the flow temperature first.
- Sizing to the gap on the wall. Work out the BTU first, then find a shape that fits. If the wall is too short, go vertical or split the output across two radiators.
- Fitting like for like. The old radiator may have been undersized from the start, or the room may have changed since (new windows, an insulated loft, an extension). Run the numbers again.
- Using a single figure per room type. Anything that ignores walls, glazing and what sits above the room will be wrong in both directions.
Measuring the room
Measure length, width and height in metres. A laser measure is quicker and more accurate than a tape, but a tape is fine if you check it twice. For an L-shaped room, split it into rectangles and add the results.
Count an external wall as any wall with outside air, an unheated garage, loft space or utility room behind it. A party wall to a neighbour who heats their home does not count.
For windows, add up the glass area of the whole room rather than counting windows. A typical window is around 1.8m² (1.2m × 1.5m). A set of double patio doors is around 3.8m² (1.8m × 2.1m) in total. For a bay, measure every pane.
Conservatories, extensions and bigger jobs
A conservatory loses far more heat than a normal room, mostly through the glass, so expect the calculator's answer to understate it. Building Regulations also expect a conservatory to be thermally separated from the house with its own heating controls, so extending the central heating into it is usually not the right route. Electric radiators are the common solution. Our WiFi electric radiators can be controlled room by room, and the conservatory heating guide covers the options. Check the rules with your local building control before starting work.
A room-by-room calculator is the right tool for replacing one or two radiators. If you are replacing a whole system, moving to a heat pump, or heating a solid-wall property where the requirement could be well above these examples, a full heat loss survey by a heating engineer (BS EN 12831) gives numbers you can rely on. Fabric improvements such as insulation and glazing change the total more than any radiator choice, and our calculator shows how much: the top-floor bedroom above, with an uninsulated loft, needs 3,000 BTU/h; with 100mm of loft insulation it needs 2,400.
Frequently asked questions
What happens if a radiator is too small?
It runs flat out and the room still does not reach temperature on cold days. Thermostatic valves stay open, the boiler works harder, and the only fix is a bigger radiator or a second one.
Can I use two smaller radiators instead of one big one?
Yes, and it often heats a room more evenly, especially a long one. The combined output needs to meet the requirement. You will need an extra pair of valves and a little more pipework. The living room example above shows both approaches.
Do I need to recalculate if I add insulation?
Yes, because it changes the factors. Insulating the cavity moves the wall factor from 1.00 to 0.94, and insulating a loft changes the top-floor factor from 1.40 to 1.08. You do not have to replace radiators straight away, but it is worth knowing if you are already renovating.
Should I use BTU or watts?
Either, as long as you compare like with like. Radiator makers in the UK publish both, and boilers are rated in kW. To convert, divide BTU/h by 3.412 to get watts. A requirement of 10,000 BTU/h is about 2,930W, or 2.93kW.
Can a towel rail count as the radiator in a bathroom?
Often, yes, if its output covers the room's requirement with some margin for towels. Check the rated output on the product page against the figure you calculated. For electric rails, the running costs guide shows what each wattage costs to run.
Got your number? Free delivery on orders over £200 (£9.99 below). Sizes and outputs are on every product page.
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