In short: the calculation takes three steps. 1) Multiply the floor area of the room by the watts per m² for your type of building (from 100-120 W/m² for an uninsulated property down to 30-50 W/m² for a high-performance new build). 2) Apply the corrections: climate, bathroom, corner room, top floor, glazing. 3) Divide the result by the output of a single section, declared at Δt 50 to EN 442, and round up. Example: a 20 m² living room in a 1980s house in a cold region → about 2,200 W (7,500 BTU/h) → 17 aluminium sections at 600 mm pipe centres.
«How many sections do I need?» is the question every radiator replacement starts with, and the answer «the same as before» is the shortcut that leads to two mirror-image mistakes: a room that never gets to temperature, or two hundred euros spent on sections that will never be needed. The old cast iron radiator was sized for a house with no insulation and timber windows: if you have changed the windows, that number no longer applies.
Here is how to work out what size radiator you need the way it is actually done: two tables, the corrections that matter, a fully worked example, the watts-to-BTU conversion and the single most important warning of all — the one about Δt, which wrecks the sums of anyone with a heat pump. Tubular steel and cast aluminium models, built up to the number of sections you need, are in the steel and aluminium radiators collection.
Two ways to calculate, and when you need which
There is the design calculation, standardised under EN 12831: losses are assessed through every wall, floor, window and thermal bridge, plus air changes, giving the exact heat load room by room. That is what a heating engineer does, and it is what you need for a new system, a formal submission or a grant application.
And then there is the practical calculation in watts per square metre, which is the one in this guide: it approximates, but within an acceptable margin when you are replacing emitters in an existing system that already works. The rule: if you are redoing the system from scratch or moving to low temperature, get the real calculation done. If you are swapping radiators while keeping the boiler and pipework, the method below is more than enough.
Step 1 — The room's heat requirement in watts
Measure the floor area of the room in square metres and multiply it by the coefficient on the row that describes your house. The values assume a standard ceiling height of 2.5 m.
| Type of building | Watts per m² | BTU/h per m² | How to recognise it |
|---|---|---|---|
| Uninsulated period property | 100-120 W/m² | 340-410 | Solid walls with no cavity, timber or aluminium single-glazed windows, no external insulation |
| Partially insulated (roughly late 1970s to mid 1990s) | 80-100 W/m² | 270-340 | Cavity wall, original double glazing, nothing done since |
| Renovated | 55-75 W/m² | 190-260 | External or cavity insulation, new thermally broken frames, low-E double glazing |
| New build / high performance | 30-50 W/m² | 100-170 | High-performance envelope, triple glazing, mechanical ventilation |
Within each band, take the high value if the room has more than one external wall or large windows, the low one if it is an internal, sheltered room.
WATTS OR BTUs? They measure the same thing. 1 W = 3.412 BTU/h, so to convert watts into BTU/h multiply by 3.412, and to go the other way divide by the same figure. European data sheets quote watts, British ones usually quote BTU/h: 2,000 W is 6,824 BTU/h, and a 5,000 BTU/h radiator is a 1,465 W one. Whichever unit you use, keep the whole calculation in that unit.
IF THE CEILING IS NOT 2.5 m: work by volume instead of floor area. Divide the table value by 2.5 to get watts per cubic metre (example: 90 W/m² ÷ 2.5 = 36 W/m³) and multiply by the real volume of the room. In period properties with 3.5-4 m ceilings the difference is enormous, and ignoring it is the mistake that sends the whole calculation off the rails.
Step 2 — The corrections that change the result
Apply the percentages that concern you to the figure you have, one after the other: each one is worked out on the result of the previous step, not on the starting number. That is how the worked example below does it.
- Climate. In a mild, coastal or southern location you can take 10-20% off; in a cold inland or northern region add 10-15%; in mountain locations add 20-30%. If your country publishes design outside temperatures, use those as your reference.
- Bathroom: +10-15%. Not a whim: bathrooms are heated to a comfort temperature of 24 °C against 20 °C for other rooms.
- Corner room (two external walls): +10-15%.
- Top floor under an uninsulated roof: +10-20%. Ground floor over an unheated cellar or void: +5-10%.
- Large glazed areas (patio doors, full-height glazing): +10%.
- Wall adjoining unheated spaces (garage, stairwell, empty flat): +10%.
- Bedroom: you can take 5-10% off, people sleep better at 18-19 °C.
Step 3 — From watts to number of sections
Now you need the watts produced by a single section of the radiator you have chosen. The figure is on the data sheet and is declared to EN 442 at Δt 50 K: that means the difference between the mean water temperature in the radiator and the room temperature is 50 degrees (in practice: 75 °C flow, 65 °C return, room at 20 °C).
These are the orders of magnitude. They are indicative values: output varies between manufacturers even at the same size, so the number to use for the final sum is always the one on the data sheet of the model you are buying.
| Type of radiator | Size | Watts per section at Δt 50 | BTU/h per section |
|---|---|---|---|
| Cast aluminium | 500 mm pipe centres (h ~580 mm) | 105-125 W | 360-425 |
| 600 mm pipe centres (h ~680 mm) | 125-145 W | 425-495 | |
| 700 mm pipe centres (h ~780 mm) | 145-165 W | 495-565 | |
| 800 mm pipe centres (h ~880 mm) | 165-180 W (real example: 172 W) | 565-615 | |
| Tubular steel | 2 columns, h 670 mm | 45-55 W | 155-190 |
| 3 columns, h 670 mm | 65-70 W (real example: 67.2 W) | 220-240 | |
| 4 columns, h 670 mm | 80-90 W | 275-305 | |
| Cast iron columns | h ~680 mm, 2-3 columns | 90-140 W | 305-480 |
The final formula is trivial: number of sections = watts required ÷ watts per section, rounding up. If the result is an extremely long radiator (more than 18-20 sections), it is better to split it into two emitters on different walls: it heats more evenly and it fits under the windows.
A note on tubular steel: at the same height, more columns means more output but also more depth. That is why tall, narrow tubular models (h 1,800-2,000 mm) solve rooms where there is no space under the window: few sections, a lot of surface.
The warning that upends every calculation: Δt
The declared wattage applies at Δt 50, that is, with a traditional boiler system sending water out at 75 °C. If your system runs at a lower temperature — a well-set condensing boiler and, above all, a heat pump — real output collapses, and not by a little. The formula is W(Δt) = W₅₀ × (Δt÷50)n, with an exponent n around 1.3 for aluminium and steel radiators. In practice:
| Flow / return | Δt | Real output against the plate figure |
|---|---|---|
| 75 / 65 °C | 50 K | 100% |
| 65 / 55 °C | 40 K | about 75% |
| 55 / 45 °C | 30 K | about 51% |
| 50 / 40 °C | 25 K | about 41% |
| 45 / 35 °C | 20 K | about 30% |
Put bluntly: with a heat pump running at 50 °C flow, a radiator delivers less than half of what the data sheet says. If you are switching to a heat pump and keeping the radiators, this is the table to do your sums with — and in that case the EN 12831 design calculation is no longer optional.
A complete worked example
The room: a 20 m² living room, 2.5 m ceiling, a flat built in 1985 and never insulated, in a cold inland region, one external wall with a large patio door.
- Base: partially insulated building → 90 W/m² × 20 m² = 1,800 W
- Cold region: +10% → 1,980 W
- Large glazed area: +10% → 2,178 W, rounded to 2,200 W (about 7,500 BTU/h)
- Radiator chosen: cast aluminium, 600 mm pipe centres, 135 W per section at Δt 50
- Sections: 2,200 ÷ 135 = 16.3 → 17 sections
In practice: a 17-section radiator is about 1.4 metres long, so either you have the right wall or it is better to split it into two emitters of 8 and 9 sections. And if the same flat moved tomorrow to a heat pump with 50 °C flow, those 17 sections would become around 40: which is exactly why low-temperature systems are designed, not improvised.
Better to oversize or undersize?
If you have to get it wrong, get it slightly wrong on the high side: a 10-15% margin is handled perfectly well by thermostatic valves, which simply close earlier, and it covers you on the coldest days of the year. An undersized radiator, on the other hand, works flat out and never reaches temperature: it uses more, it does not warm you, and no amount of control can fix it.
The point at which excess becomes waste is around 25-30%: beyond that you are paying for sections that will never come into play and, on a condensing boiler, you risk cycling that is too short.
If the radiator is cold at the top, it is not an output problem
Before concluding that a radiator is undersized, rule out the most obvious and most frequent cause: air in the circuit. A radiator that is warm at the bottom and cold in its upper half has half its surface out of use, so it delivers half of what it should — and no calculation in the world compensates for that. Five minutes of maintenance fixes it: see how to bleed radiators.
In the same way, if the radiator is cold at the bottom the problem is sludge, not watts: the system needs flushing. Do your calculation on a healthy system, otherwise you will be sizing emitters against faults instead of square metres.
FAQ: common questions about sizing
How many radiator sections do I need per square metre?
There is no single number, because it depends on the output of the section. As an order of magnitude, in a partially insulated house with cast aluminium radiators at 600 mm pipe centres you need about 0.7 sections per m² (90 W/m² ÷ 135 W per section). In the same house, with 3-column tubular steel at 670 mm, you need roughly twice as many: about 1.3 sections per m².
What is the output of a cast iron radiator?
A cast iron column section around 680 mm tall sits roughly between 90 and 140 W at Δt 50, depending on the number of columns. Bear in mind, though, that cast iron radiators have very high thermal inertia and, after decades of internal deposits, often deliver less than their original rating.
How do I convert watts to BTUs for radiators?
Multiply the watts by 3.412 to get BTU/h, divide by 3.412 to go the other way. A 1,500 W radiator is a 5,118 BTU/h radiator. Only mind the units: BTU/h is a power, like the watt, not an energy.
Does the calculation apply to a bathroom towel rail too?
The method is the same, with two provisos: add 10-15% because bathrooms aim for 24 °C, and remember that a towel rail, for the same space taken up, delivers less than a conventional radiator because it has less exchange surface. If you go for the electric version the reasoning changes again: see heated towel rail, electric or central heating.
Can I fit a new radiator on the old connections?
In most cases yes, if the pipe centres match (500, 600, 700 and 800 mm are the standard sizes). Measure the distance from centre to centre of the connections before ordering: that figure decides whether fitting takes half an hour or half a day.
Why does my new radiator heat less than the old cast iron one?
Almost always because it was chosen by eye, with the same number of sections as the previous one. Cast iron and aluminium have very different outputs per section: the comparison has to be made on total watts, never on the number of sections.
In conclusion
Radiator sizing fits in one line: square metres × watts per m² for your type of house, plus the corrections, divided by the output of one section at Δt 50. The two things that separate a right answer from a wrong one are honesty in classifying your own house (new insulation changes everything) and checking the Δt of your own system.
Once you have the number, you will find the modular models — tubular steel in 2, 3 and 4 columns, cast aluminium in standard pipe centres, vertical designs for narrow walls — in the central heating radiators collection.
Related guides: how to bleed radiators · heated towel rail: electric or central heating · how to bleed a heated towel rail.
