Answer
What size radiator for a 3m x 4m bedroom?
A 3m × 4m bedroom needs roughly 3,931 BTU (1,152 W) of heat — choose a radiator rated at that output or higher (measured at Δt50).
This is a guide based on standard heat-loss factors. Insulation, glazing, exterior walls and orientation change the figure — the calculator below adjusts for all of them.
| Room | 3m × 4m bedroom (~12 m²) |
|---|---|
| Required output | 3,931 BTU (1,152 W) |
| Choose a radiator rated | 3,931 BTU or higher |
Fine-tune with the BTU calculator → · Find a matching radiator →
2026 figures · BTU uses standard UK heat-loss methodology; running costs use the Ofgem price cap · always confirm with a professional.