How many watts of heating your room needs, and the right heater size for winter.
How this works
Heat loss per m³ of room at a 20°C indoor-outdoor gap: well insulated 25 W, average 40 W, poorly insulated 60 W. Scale by your real temperature gap and add 15% for large single-glazed windows.
Questions people ask
How do I use the Room Heat Loss and Heater Size tool?
Fill in the fields, starting with “Room length”, “Room width” and “Ceiling height”, and change any other options that match your home. The answer, a drawing and practical tips update as you type, and the Copy link button saves your numbers in a link you can share.
Is the Room Heat Loss and Heater Size tool free?
Yes. The Room Heat Loss and Heater Size tool is free, with no sign-up, and works on phones, tablets and computers. The results are estimates to help you plan, so check with a qualified professional before structural, electrical, gas or plumbing work.