Estimate the heat loss and radiator or boiler size needed for a room from its dimensions, insulation level and target temperature.
This is a quick volume-based estimate of how much heat a room loses, and therefore the radiator or heater output you need. It gives a sensible starting figure; for a full design a room-by-room heat loss survey (SAP / Manual J) is recommended.
It gives a good ballpark figure for sizing a radiator or heater. A detailed survey accounts for each wall, window, floor and air change, but the volume method is fine for quick planning.
Living rooms are typically designed for 21°C (70°F), bedrooms around 18°C (64°F) and bathrooms 22–23°C. Use the temperature you actually want to maintain.
Use the coldest design temperature for your area – around -1 to -3°C (28–30°F) for much of the UK, or your local winter design temperature in the US. This ensures the heating copes on the coldest days.
1 kW equals about 3,412 BTU per hour. The calculator shows both so you can size radiators (often rated in watts/BTU) or heat pumps and furnaces (often rated in kW or BTU).
Yes. A well-insulated room can lose less than half the heat of a poorly insulated one of the same size, which is why the insulation level has such a large effect on the result.
