What size radiator do I need? BTU and sizing guide

Updated 6 November 2026 · By the Radiator Installation team

Short answer: The right radiator size depends on the heat output a room needs, measured in watts or BTU per hour, which comes from the room's size, insulation, windows, outside walls and target temperature. As a rough guide, small bedrooms often need 2,000 to 3,500 BTU and living rooms 5,000 to 8,000 BTU. Radiators for heat pumps need to be larger because they run at lower water temperatures.

Watts and BTU

Radiator output is quoted in watts or BTU per hour. One watt is about 3.41 BTU per hour, so a 1,500 W radiator gives roughly 5,100 BTU per hour. Outputs are normally quoted at a standard test temperature known as Delta T50.

Rough guide by room

Room	Typical heat output needed
Small bedroom	About 2,000 to 3,500 BTU
Double bedroom	About 3,500 to 5,500 BTU
Living room	About 5,000 to 8,000 BTU
Kitchen	About 3,000 to 6,000 BTU
Bathroom	About 1,500 to 3,500 BTU

These are rough figures only. A proper heat loss calculation is always more accurate, especially for older, draughty or extended homes.

What increases the size needed

Solid walls or little insulation
Large or single-glazed windows
More than one outside wall
North-facing rooms
High ceilings
Lower flow temperatures, such as with a heat pump

Sizing for heat pumps

A heat pump usually heats the water to around 45 to 50°C, compared with 70°C or more for many older boiler systems. At those lower temperatures a radiator gives out roughly half its quoted Delta T50 output, so many rooms need bigger or double-panel radiators to stay warm.