Radiator size calculator with ΔT correction
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Rooms | Living room: room heat loss 1,000 W, room temperature 21 °C |
| Flow temperature | 45 °C |
| Return temperature | 40 °C |
| Radiator exponent n | 1.3 |
| Radiators you're considering | 600 × 1200 double: output at δt50 2,400 W; 600 × 1400 double: output at δt50 3,100 W |
Buy at least 2,996 W at ΔT50 (10,222 BTU/h)
| Result | Value |
|---|---|
| Output to buy at ΔT50 (all rooms) | 2,996 W (10,222 BTU/h) |
| Mean water temperature | 42.5 °C |
| ΔT (mean water − room) | 21.5 K |
| Correction factor (ΔT ÷ 50)^n | 0.334 |
| Total room heat loss | 1,000 W (3,412 BTU/h) |
| Room | Heat loss | ΔT | Factor | Buy at ΔT50 | Smallest of your sizes that's enough |
|---|---|---|---|---|---|
| Living room | 1,000 W | 21.5 K | 0.334 | 2,996 W | 600 × 1400 double (gives 1,035 W) |
Show the working
- Mean water temperature: (45 + 40) ÷ 2 = 42.5 °C
- Living room: ΔT = 42.5 − 21 = 21.5 K; factor = (21.5 ÷ 50)^1.3 = 0.334; 1,000 W ÷ 0.334 = 2,996 W at ΔT50
How it works
- Mean water temperature = (flow + return) ÷ 2
- ΔT = mean water temperature − room temperature
- Correction factor = (ΔT ÷ 50)^n, where n is the radiator exponent from the maker (often about 1.3)
- Output to buy at ΔT50 = room heat loss ÷ correction factor
Radiator catalogues quote output at ΔT50 (BS EN 442). At the lower flow temperatures of a condensing boiler or heat pump, a radiator gives much less, so you need a bigger one.
Our exponent n is a typical figure; use the maker's own n or their correction table where they give one.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Radiator exponent n | 1.3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Radiator exponent n, default 1.3 as stated in brief v2 H2 ("typical from makers"). Stored as an assumption until the Researcher cites a maker's data sheet (e.g. Stelrad, Zehnder) and switches it to typical. |
Regulations and standards
- BS EN 442 (radiator outputs rated at ΔT50)
Explained: BS EN 442 (radiators).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.