What this calculator does

A BTU estimate helps size radiators and room heating output. This calculator uses room volume and insulation to produce a practical first estimate.

Formula used

Room volume is multiplied by a watts-per-cubic-metre allowance, then converted to BTU per hour. Cold rooms add a small allowance.

How to read the result

Heating design depends on building fabric, windows, location and system temperature. Treat this as a planning estimate before checking product guidance.

Assumptions

  • Uses metric room dimensions.
  • Converts watts to BTU per hour using 1 watt equals 3.412 BTU per hour.
  • Does not replace a heat loss survey.

Sources and checks

This calculator uses a standard public formula. Where rules or thresholds can change, source links are listed on the relevant page.

Frequently asked questions

Should I oversize a radiator?

A small margin can help, but oversizing too much can reduce comfort and efficiency. Match the room and heating system where possible.