What Size Radiator Do I Need? (BTU Guide)
Updated June 2026 · Estimates only — verify before quoting
A radiator that is too small never warms the room; one that is too big wastes money. "BTU" (British Thermal Units per hour) is just the heat output a radiator gives — and the right figure depends mostly on the size and type of room.
// Do the maths instantly
Radiator BTU Calculator
Work out the heat output (W and BTU) any room needs.
Open the Radiator BTU CalculatorThe quick method
Work out the room volume (length × width × height in metres), then multiply by a heat factor for the room type. Living rooms and bathrooms need more heat per cubic metre than bedrooms. Convert watts to BTU by multiplying by 3.412.
- Lounge / dining: ~70 W per m³
- Bedroom / kitchen: ~50 W per m³
- Bathroom: ~80 W per m³
- Add for north-facing rooms, large windows or poor insulation
What pushes the BTU up
North-facing rooms, French doors and patio windows, rooms above unheated spaces, and older solid-wall properties all lose more heat — so add a margin. The calculator below lets you pick the room type and gives both watts and BTU.
Frequently asked questions
How many BTUs do I need to heat a room?
As a rough guide, multiply the room volume (m³) by 50–80 W depending on room type, then ×3.412 for BTU. A typical 4×4×2.4m lounge needs roughly 9,000–11,000 BTU.
Is it better to oversize a radiator?
A small margin is sensible, but a much-oversized radiator wastes energy and space. Size to the room and let the thermostatic valve control the output.
From estimate to winning quote
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