Often modest radiator output is enough, especially in newer or better insulated homes.
Sizing guide
Radiator size by room
Use room size, room type and heat-loss clues as the starting point. Copying the radiator in the next room often leads to underheating or a room that only feels comfortable after running the system for too long.
Last reviewed: 16 April 2026 - Practical sizing guide, not an installer heat-loss design
Needs climb faster with larger windows, exposed walls and higher ceilings.
This is where rough rules are most likely to undershoot. Treat quick estimates with care.
Heater room size calculator
Useful for a quick room-heating baseline when you are comparing with plug-in electric heat.
Gas vs electric heating
Read this if the real decision is about the heating approach, not just the emitter size.