Calculate Your Room’s Radiator Size
Use our free BTU calculator to work out the exact radiator size your room needs. Enter your room dimensions and details such as windows, walls and insulation, then click the button below. We’ll instantly show you the heat output required in both BTUs and watts.
Calculate Your Room’s Radiator Size
Use our free BTU calculator to work out the exact radiator size your room needs. Enter your room dimensions and details such as windows, walls and insulation, then click the button below. We’ll instantly show you the heat output required in both BTUs and watts.
Calculate BTU
BTU Calculator
Determine the ideal radiator size for your room. Adjust the sliders and select the options below to calculate the correct BTU output and wattage required. You can add multiple rooms and save or share your results.
* 2025 heating regulations compliant
Expert Opinions
“Fitting programmable thermostats, room thermostats and TRVs can cut wasted energy. A simple controls upgrade can save a typical UK household a noticeable amount each year.”
“Turning your thermostat down by just one degree can shave real money off your annual heating costs. Good glazing and insulation also make radiators work far more efficiently.”
Key Features
- Room-by-room BTU result in BTU/hr and Watts
- Accounts for windows, glazing, external walls, insulation
- Quick sliders for length, width, height
- Share or save your results (print, email, PDF, WhatsApp)
How It Works
- Click Start Calculating or scroll to the calculator.
- Add a room and set room type / insulation.
- Use the sliders for length, width and height (m).
- Add windows (height & width) and pick glazing if present.
- Click Calculate to see the BTU and Watts required.