Work out the BTU/hr needed to heat a room from its size, type, insulation and external walls, with an optional radiator cost estimate.
Optional: estimate radiator cost
Related tool: This calculator gives the BTU/hr a room needs and an optional radiator cost. If you just want a quick radiator size with glazing and orientation options, try our Radiator BTU Calculator.
BTU (British Thermal Unit) is a measure of heat output. The required BTU for a room depends on its volume, insulation, external walls and intended use. This calculator provides a tailored estimate for your specific space, giving a figure you can shop with — radiators and towel rails are usually rated in BTU/hr.
Different rooms and insulation levels influence heat loss. Bedrooms typically need less heat than bathrooms or living rooms. Good insulation reduces the BTU requirement, while poor insulation (older single glazing, little wall insulation) increases it. Each external wall adds around 10% because it loses heat to the outside.
Once you know your room's BTU requirement, select a radiator with a similar or slightly higher output. Oversizing by a small margin is wise — a bigger radiator run cooler is more efficient, especially with a heat pump — but avoid excessive oversizing. If one radiator can't meet the figure, use two: their outputs simply add up.
A metric bedroom 4.5 m × 3.5 m × 2.4 m with good insulation and 2 external walls:
Switch between metric (metres) and imperial (feet) at the top. The calculator converts imperial measurements automatically and gives the result in BTU/hr and kW, which are used worldwide for heating output.
Measure the room length, width and height, choose the room type and insulation level, then this calculator estimates the required heat output. It multiplies the room volume by a heat factor and adjusts for room type, insulation and external walls.
The BTU needed depends on the room volume, insulation, number of external walls and intended use. Enter your details above to get a figure tailored to your room, in both BTU/hr and kW.
A typical bedroom needs fewer BTUs than living spaces. For an average insulated bedroom the requirement is usually around 3,000 to 6,000 BTU/hr depending on size and insulation. Enter your measurements for an exact figure.
A small, well-insulated bedroom may need as little as 2,000 to 4,000 BTU/hr. Use the calculator for a personalised estimate based on your exact measurements and insulation level.
Cooling (air conditioning) uses a similar method but adds factors such as sun exposure and occupancy. This tool is for heating, but the room volume it works out is a useful starting point for estimating cooling BTU needs too.