Radiator BTU Calculator

Radiators are sold by output, but the output a room needs is set by the room - how big it is, how well it is wrapped, how much glass it carries and how cold it gets outside. Get that wrong and the room is either baking or the install is short.

Radiator BTU Calculator builds a quick heat-loss estimate for a single room so you can pick a radiator that actually covers it.

• Room heat loss: Walls, windows and infiltration.
• Insulation choices: Good, average or poor fabric.
• Climate presets: Outdoor design temps built in.
• Dual units: BTU/h, kW and watts shown.
• Live update: Results while you type.
• Export: Text, CSV or JSON report.

1. Enter the room footprint (length and width) and ceiling height.
2. Choose insulation, glazing and climate.
3. Set desired indoor temperature.
4. Read the required output in BTU/h and kW.
5. Export the result if needed.

Example 1. A 12x10 ft room with an 8 ft ceiling, average insulation, double glazing, and a 60 F delta needs roughly 3,400 BTU/h - about a 1.0 kW radiator.

Example 2. The same room poorly insulated with single glass and 40 sq ft of windows jumps to over 6,000 BTU/h (1.8 kW).

• Quick room check: Right-sized radiators fast.
• Honest estimate: Standard heat-loss method.
• Dual units: BTU/h, kW and watts together.
• Field ready: No chart hunting.
• Free and private: No uploads, no logging.