Radiator BTU and Watts: A Practical Sizing Guide

Radiator output is normally stated in watts or British thermal units per hour (BTU/h). Both describe the rate of heat transfer: one watt equals approximately 3.412 BTU/h. The conversion is simple, but reliable sizing requires more than changing units.

Start with room heat loss

A room loses heat through walls, windows, roofs, floors, ventilation and air leakage. A quick calculator can provide an early estimate, while a formal project should use a heat-loss method appropriate to local regulations and the building design. Insulation level, glazing area, exposure and desired room temperature can materially change the result.

Check the stated test condition

Published radiator outputs are tied to a temperature difference between the radiator water and the room. In European data this is often shown as Delta T50, but low-temperature heat-pump systems may operate at a much smaller temperature difference. Output falls as water temperature falls, so a radiator selected from a high-temperature table may need to be larger at low flow temperatures.

Allow for real installation conditions

Deep covers, restricted airflow, long curtains and furniture placed directly in front of a radiator can reduce useful heat delivery. Position, valve arrangement and system balancing also matter. Where design margins are required, apply them consistently rather than selecting an arbitrary oversized model.

Create a clear schedule

For each room, record calculated demand, selected model, size, connection, colour, stated output condition and valve requirement. This gives suppliers, installers and project managers one shared reference and reduces avoidable changes later.