Answer
What size radiator for a 5m x 4m bathroom?
A 5m × 4m bathroom needs roughly 9,008 BTU (2,640 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 | 5m × 4m bathroom (~20 m²) |
|---|---|
| Required output | 9,008 BTU (2,640 W) |
| Choose a radiator rated | 9,008 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.