We work out how much heat your room loses on a cold winter day, the same way a heating engineer does. We add up the heat lost through each outside wall, window, door, floor and ceiling (using typical U-values for each type of construction), plus the heat lost through draughts and fresh air. We then add a 20% margin so the room warms up quickly from cold. Your result is never lower than the standard size-based guide most UK radiator retailers use (room volume × 4–5 BTU per cubic foot), so you won't end up with a radiator that's too small.

Each room is heated to the recommended temperature for that room (21°C for living rooms, 22°C for bathrooms, 18°C for bedrooms, kitchens and hallways), on a typical cold winter day for your part of the UK.

All results are given at ΔT50, the UK standard for radiator outputs, so you can compare them directly with the BTU figures on our product pages. If you have a heat pump or a low-temperature boiler, your radiators run cooler, so we increase the ΔT50 output you need to make up for it.

This calculator is a guide. For a new build, an extension or a full heating system design, we'd always recommend a room-by-room heat-loss survey by a qualified heating engineer.