HEATLOSSPROHeat loss per m² by house age
The fastest estimate there is: floor area × a typical heat loss per m². Good for a first idea of the heat pump size, not for buying one.
In short: at about 21 °C inside and −3 °C outside, a house loses roughly 80–100 W/m² if built before 1945 and not improved, 60–80 W/m² from 1945–1990, 40–50 W/m² from 1990–2015, and 30 W/m² or less if newer. A 100 m² 1960s semi-detached house needs about 8 kW.
Quick estimate
Typical heat loss per m²
For a semi-detached house at 21 °C inside and −3 °C outside. Detached houses and bungalows lose about 15–20% more; mid-terraced houses and flats less.
| Built | W/m² | 100 m² house |
|---|---|---|
| Before 1919, solid walls | 100 | 10.0 kW |
| 1919–1944 | 90 | 9.0 kW |
| 1945–1975 | 80 | 8.0 kW |
| 1976–1990 | 65 | 6.5 kW |
| 1991–2005 | 50 | 5.0 kW |
| 2006–2015 | 40 | 4.0 kW |
| 2016 or newer | 30 | 3.0 kW |
| Passive house | 15 | 1.5 kW |
Why it is only a first guess
Heat loss depends on what the house is made of, not when it was built. Two 1930s houses can differ by a factor of two: one with filled cavities, a well-insulated loft and new windows, the other untouched. Shape matters too: a long, narrow house or one with a big extension has more outside wall per m² of floor.
For a heat pump, the whole-house figure is not enough either. Each room needs its own figure, to check its radiator or underfloor heating at the heat pump's low flow temperature. That is what a room-by-room calculation gives. In the HeatLoss Pro designer you trace the floor plan and set the constructions, and every room's heat loss appears as you draw, in about 5 minutes for a typical house. Drawing is free; a full calculation with a PDF report costs €1.
Frequently asked questions
What is a typical heat loss per m²?
At design conditions (about 21 °C inside and −3 °C outside), roughly 80–100 W/m² for a house built before 1945 and not improved, 60–80 W/m² for one from 1945–1990, 40–50 W/m² for 1990–2015, and 30 W/m² or less for a new house. A detached house loses more than a terraced one of the same size, because more of its walls face outside.
How do I estimate the heat loss of my house?
Multiply the heated floor area by a W/m² figure for its age and type. A 100 m² semi-detached house from the 1960s at about 80 W/m² needs about 8 kW on the coldest day. Check it against your gas use, and calculate it room by room before buying a heat pump.
How accurate is a W/m² estimate?
Not very: two houses of the same age can differ by a factor of two, depending on insulation, glazing, draughts and shape. Use it to get a feel for the size of heat pump, then confirm it with your gas bill and a room-by-room heat loss calculation.
How many kW heat pump do I need for a 100 m² house?
Usually 3–5 kW for a modern or well-insulated house, 6–9 kW for a house from 1945–1990, and 9–12 kW for an older, uninsulated one, at UK design temperatures. Colder climates need more. The heat pump must give that output at the design outdoor temperature, according to its datasheet.
Do the whole house in 5 minutes
Trace your floor plan and see every room's heat loss, the heat pump size and the radiator check as you draw. Free to try, nothing to install, no sign-up.
Open the designer free