A radiator needs to put out enough heat to replace what the room loses through its walls, windows, floor and ceiling, plus air leakage.
Quick estimate
A practical starting point is room volume (m³) × a factor: about 45 W/m³ for living rooms at 21°C and 35 W/m³ for bedrooms at 18°C. Increase it for extra external walls, single glazing, poor insulation or a north-facing aspect. BTU = watts × 3.412.
ΔT50 vs real flow temperatures
Radiator outputs are usually quoted at ΔT50 (water about 50°C hotter than the room). Modern condensing boilers run cooler for efficiency, and heat pumps much cooler, often ΔT 25–30. At those temperatures a radiator gives perhaps a third to half its ΔT50 rating, so radiators often need to be larger for heat pumps.
Do a proper heat loss
For heat pumps and new systems, a room-by-room heat loss calculation (for example to BS EN 12831, as MCS installers use) is the professional standard.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.