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Basics
Radiator sizing: how much heat does a room need?
A radiator that is too small leaves a room cold; one that is too large wastes money and space. This guide explains how to estimate heat demand, read radiator output ratings and choose a size.
1. Estimate the heat demand
The simplest estimate multiplies the room volume by a factor for how well the building is insulated:
| Building | Watts per m³ (estimate) |
|---|---|
| Well insulated | about 30 |
| Average | about 45 |
| Poorly insulated / old | about 60 |
Add about 15% for rooms with two or more exterior walls. A 20 m² room with a 2.7 m ceiling in an average building needs about 20 × 2.7 × 45 ≈ 2,430 W. The Radiator size calculator does this calculation.
2. What changes the result
Large windows, high ceilings, north-facing rooms, cold floors and draughts increase heat loss. Better insulation reduces it; see R-value and U-value explained: insulation basics. For a new system or a heat pump, a room-by-room heat loss calculation by a heating engineer is the reliable method.
3. Output ratings and ΔT
Radiator output is stated at a test temperature difference between the average water temperature and the room, usually ΔT50 (for example 75 °C water in a 20 °C room) under EN 442. If your system runs cooler, output drops sharply: at ΔT30 a radiator gives only about half its ΔT50 rating. Heat pumps and condensing boilers work best at low water temperatures, so they often need larger radiators.
4. Radiator types and length
Panel radiators are labelled by panels and fins: type 11 (one panel, one fin set), 21 and 22 (two panels). A 60 cm high type 22 radiator gives very roughly 1,700 W per meter of length at ΔT50; check the manufacturer’s table for exact values. You can split the demand between two radiators in large rooms.
5. Where to place it
Under a window is traditional, as the radiator warms the cold air falling from the glass. Keep furniture and long curtains from covering it, and leave space below for air to flow. Fit thermostatic radiator valves so each room can be controlled.
Common mistakes
- Choosing by room area alone, ignoring ceiling height and insulation.
- Reading the ΔT50 output for a low-temperature system.
- Oversizing all radiators far beyond the need.
- Hiding radiators behind furniture or curtains.
- Not fitting thermostatic valves.
Frequently asked questions
How many watts do I need per square meter?
Often 70–150 W/m² depending on insulation and ceiling height; using volume gives a better estimate.
What does ΔT50 mean?
The radiator’s rated output when the average water temperature is 50 °C above room temperature.
Do heat pumps need bigger radiators?
Usually yes, because they run at lower water temperatures, which lowers radiator output.
What is a type 22 radiator?
A panel radiator with two panels and two sets of convector fins, a common choice for higher output.
How do I convert watts to BTU/h?
Multiply by about 3.412. 2,000 W ≈ 6,800 BTU/h.