A simplified BS EN 12831 design heat loss calculator. Tells you the kW figure your heat pump needs to cover, the annual heating demand, and which fabric upgrades will move the number most. Free, no sign-up.
Property
Fabric
Location
Result
Tell us about your property
Approximate values are fine — the calculator gives a design heat loss number per BS EN 12831 methodology.
All habitable floor area combined. UK 3-bed semi typical 90m². 4-bed detached 130m². Bungalow 80m².
Default 2.4m for most modern UK homes. Victorian terraces 2.7m+.
UK average 20%. Modern open-plan 30%. Older terraces 12-15%.
Pre-1919 (solid wall)
1919-1944
1945-1980 (cavity wall)
1981-2002
2003+ (modern)
Current fabric state
None
Some (≤200mm)
Full (≥270mm)
None
Cavity wall filled
Internal wall (IWI)
External wall (EWI)
Single glazing
Double glazing
Triple glazing
Uninsulated
Insulated (retrofit)
Modern (built post-2003)
Your location
Determines design ΔT (BS EN 12831) and annual heating degree days. Internal temperature 21°C assumed.
South (London, SW)
Midlands (E/W Mid)
North England
Scotland
How we calculate this. U-values from BS 5250 / Part L 2010 reference table, varied by construction era and insulation status. Heat loss formula Q = U × A × ΔT (steady-state, BS EN 12831). Ventilation losses 0.33 × volume × ACH × ΔT, with ACH varying by era. Annual demand uses degree-day method (HDD per region: 1,850-2,500 kKh). Heat pump sizing applies +15% oversize factor for design margin. This is a screening calculator, not an MCS-compliant heat loss calculation. An MCS installer will do a room-by-room calculation with manufacturer-specific heat emitter outputs.