Work out heater size for room instantly with clear inputs, formula shown and shareable results.
Heating requirement is volume times a heat loss factor times the temperature rise needed. The factor ranges from about 1.2 W per cubic metre per degree in a well-insulated modern room to 2.4 in a solid-wall Victorian room, and extra external walls add roughly 10% each.
Heating load
watts = volume x heat loss factor x temperature rise
It halves the heat loss factor between poor and good. Insulating first is always cheaper than sizing a bigger heater and running it for years.
Yes, using the local design external temperature. Undersizing means the room never reaches temperature on the coldest days.