Room Heat Loss Calculator
Estimate the kW heating needed for any room based on its size, insulation level, windows, exterior walls and local climate. Use the result to correctly size radiators, heat pumps or boilers.
Room Dimensions
Insulation
Openings & Exposure
Temperatures
Heating Result
How it works
The calculator multiplies room volume by an insulation factor (W/m³), adds fixed losses for each window and exterior door, applies an exposure multiplier for exterior walls and unheated spaces, then scales for climate by the ratio of your temperature difference to a reference ΔT of 20 °C. A 15% safety margin gives the recommended installed kW.
Frequently Asked Questions
How do I calculate the kW needed to heat a room?+
Multiply the room volume (m³) by an insulation factor (typically 40–60 W/m³), then add extra output for each exterior wall, window and external door. Finally adjust for your climate by scaling the result according to the difference between your desired indoor temperature and the coldest outdoor temperature.
How many kW do I need per square metre?+
As a rule of thumb, a well insulated modern room needs around 50–70 W per m², an average home needs 80–100 W/m², and a poorly insulated older room can need 120–150 W/m² or more. This calculator refines that estimate using your exact dimensions and insulation.
Should I oversize my radiator or boiler for heat loss?+
Add about 10–20% safety margin to the calculated kW so the system can recover quickly on the coldest days. Avoid oversizing by more than that, as it wastes energy, increases running costs and can cause short cycling on modern boilers and heat pumps.