BTU Calculator
Enter your room dimensions and construction details to find out exactly how many BTUs your radiator needs to output. Written by a qualified Gas Safe engineer.
Room dimensions
Room properties
Enter your room height, width, and length above to see your BTU result here.
How it works
- ✓ Room volume is calculated from your dimensions
- ✓ Base heat loss (153 BTU/m³) is adjusted for insulation, glazing, and room type
- ✓ Window area adds additional heat loss on top
- ✓ Based on UK design conditions (−1°C outside, 21°C inside)
Common questions
What does BTU stand for?
BTU stands for British Thermal Unit — it's a measure of heat energy. In the context of radiators, it refers to the amount of heat output per hour (BTU/h). The higher the BTU, the more heat the radiator produces.
What is a K1 vs K2 radiator?
K1 (or Type 11) is a single-panel radiator. K2 (Type 22) is a double-panel radiator with convector fins — it produces roughly double the heat output for the same physical size. For most UK living rooms, a K2 is the right choice.
Should I add a buffer to the result?
Many plumbers add a 10–20% buffer on top of the calculated BTU to account for unusually cold days, high ceilings, or future changes. This is especially worth doing for bathrooms and hallways.
Does this work for underfloor heating?
The BTU requirement is the same regardless of heat source, but underfloor heating distributes heat differently. The watt output per m² is what you'd use to size an underfloor system — divide the watts result by your floor area.
Why do heat pumps need bigger radiators?
A gas boiler typically heats water to 70–80°C. A heat pump runs at 35–55°C. Lower flow temperature means each radiator emits less heat, so you need a larger surface area to achieve the same BTU output.