How to use it
- Enter the floor area and ceiling height of the room.
- Choose the insulation level, or enter your own heat loss factor.
- Pick the system's supply and return temperatures.
- Read the catalog output you need at ΔT50 or ΔT60.
Two steps: heat loss, then radiator output
1. Heat loss. The room's heat demand is estimated from its volume (or floor area) and a factor for the insulation level: about 0.7 W per cubic foot for a well-insulated room, 1 W for average and 1.4 W for poor insulation. You can override the factor if you know your house better.
2. Radiator output at your temperatures. Panel radiators are rated at standard conditions (EN 442): ΔT50 means 75 °C supply, 65 °C return, 20 °C room (167/149/68 °F). At lower water temperatures, as with a condensing boiler or a heat pump, the same radiator gives off much less heat. The calculator converts between the two with the EN 442 formula:
Φ = Φn × (ΔT ÷ ΔTn)ⁿ, with n ≈ 1.3 for panel radiators,
where ΔT is the logarithmic mean temperature difference between the water and the room.
Worked example
A 170 sq ft room with 8 ft ceilings and average insulation loses about 1,350 W (4,600 BTU/h) on a design day. With a gas boiler running at 158/131 °F supply/return and the room at 68 °F (70/55/20 °C), the mean temperature difference is about 42 K, so a radiator delivers only about 80% of its ΔT50 rating. You therefore need a radiator rated at about 1,690 W at ΔT50.
At heat pump temperatures (113/95 °F) the same room would need a radiator rated more than three times higher, which is why heat pump retrofits often mean bigger radiators.
Typical system temperatures
| Supply / return | Typical system | Output vs. ΔT50 rating (room 68 °F) |
|---|---|---|
| 194/158 °F (90/70 °C) | Older boiler | about 125% |
| 167/149 °F (75/65 °C) | EN 442 rating point (ΔT50) | 100% |
| 158/131 °F (70/55 °C) | Gas boiler | about 80% |
| 131/113 °F (55/45 °C) | Condensing boiler | about 51% |
| 113/95 °F (45/35 °C) | Heat pump | about 30% |
You can also type your own supply, return and room temperatures.
Choosing a panel type
European-style steel panel radiators come in types named by panels and fin rows: type 11 (one panel, one row of fins), type 21, type 22 (two panels, two rows of fins, the most common) and type 33 (three panels). For a given wall length, more panels mean more output. The calculator suggests a type from the required rating; look up the exact height and length in the manufacturer's output table, in the ΔT50 (or ΔT60) column.
Already have a radiator? Enter its catalog rating at the bottom to see how much it actually gives off at your temperatures and what percentage of the room's heat loss that covers.
Accuracy
The heat loss factor is a rule of thumb for the insulation level, not a room-by-room calculation such as ACCA Manual J in the US or EN 12831 in Europe, which account for each wall, window and air change. Use this tool to compare options, check whether an existing radiator is big enough, or see the effect of lowering the water temperature. The exponent n varies between about 1.25 and 1.35 by make and model; the default of 1.3 is typical for panels. 1 W = 3.412 BTU/h.
Frequently asked questions
What does ΔT50 mean on a radiator?
It's the rating condition: average water temperature 50 K above the room (75 °C supply, 65 °C return, 20 °C room). Many US-sold panel radiators list output at this point too.
Will my radiators work with a heat pump?
At heat pump water temperatures (about 113 °F) a radiator gives only about 30% of its ΔT50 output, so you may need larger radiators or better insulation. The calculator shows the numbers for your room.
How many BTU do I need per square foot?
It depends on insulation and climate; roughly 20–40 BTU/h per square foot is a common range for heating. The calculator uses volume and insulation for a better estimate.
What is the radiator exponent n?
It describes how output falls as water temperature drops. Panel radiators are about 1.3; it's in the manufacturer's data.
Can I use it for baseboard heaters?
The heat loss part applies to any heater. The output conversion is for panel radiators rated to EN 442; US finned-tube baseboard is rated differently, per foot at a given water temperature.
Last updated: November 12, 2026