Radiator BTU Requirement Calculator

About this calculator

Radiator sizing starts with a room's heat loss, which depends on its volume and how much heat it loses relative to that volume — a factor that varies by room type because of window area, exposure and typical temperature demands. The estimate is required BTU = room volume × room-type heating factor, with living rooms and bedrooms needing a lower factor than kitchens, bathrooms or conservatories, which lose heat faster through glazing, extraction and exterior walls.

Homeowners and heating engineers size radiators this way before buying because an undersized radiator never brings the room to the target temperature no matter how long it runs, while an oversized one wastes energy overheating the space and cycles the boiler inefficiently — getting the BTU output within the right range matters more than most people assume when shopping by radiator length alone.

This calculator takes your room's volume and its room type and returns the BTU and watt output your radiator needs, so you can pick a radiator sized to actually heat the space rather than guessing from its physical dimensions.