♨️ Radiator Size Calculator
Enter a room's air volume and a factor for its insulation to estimate the radiator output it needs in BTU — so the radiator you choose is neither undersized and cold nor oversized and wasteful.
🧮 Size Your Radiator
What is a Radiator Size Calculator?
It works out how much heat a radiator has to put into a room. Because a radiator warms the air, sizing is based on volume: enter the room's cubic feet and a factor for how easily it loses heat, and the tool returns the BTU output to look for on a radiator's spec sheet.
Use it to replace a radiator that never quite warmed the room, plan a new heating layout, or check a plumber's recommendation. Pair it with the room volume calculator to get the cubic-foot figure, then round the output up when you choose a model.
❓ Frequently Asked Questions
How does the radiator size calculator work?
Enter the room's air volume in cubic feet and a BTU-per-cubic-foot factor that reflects the room type and how well it's insulated. The tool multiplies the two to estimate the radiator output the room needs in BTU, which you then match against a radiator's rated output.
What factor should I use?
Use a lower factor for well-insulated interior rooms and a higher one for rooms with poor insulation, large or many windows, or lots of exterior wall. Bedrooms and living rooms, north-facing rooms, and rooms above unheated spaces all tend to need a higher factor. When in doubt, size up slightly.
How do I find my room's volume?
Multiply length by width by ceiling height, all in feet. Our room volume calculator does this for you and hands you a cubic-foot figure you can drop straight into this tool.
Should I round radiator output up or down?
Round up. Pick a radiator rated at or just above the calculated output so it can hold temperature on the coldest days; a modern thermostatic valve will throttle it back the rest of the time. An undersized radiator simply can't reach the target temperature when it's freezing outside.