Room heat loss and radiator size (simplified, not EN 12831)
Φ = Σ(A · U · ΔT) + 0.33 · n · V · ΔT. Type your U-values (free) or pick a construction preset (Pro); Pro also applies the 1.1 factor and picks the nearest radiator from a typical output table.
Limits and citations come from the app's pack files. The calculator below is checked against the app's engine on every build · engine 1.
The same job in the app
5 × 4 × 2.5 m room, 20 °C inside and -10 °C outside: 1192 W, 60 W/m², and a type 22 radiator 600 × 900 mm.
How it's calculated
The envelope term adds up area × U-value × temperature difference for each surface that faces outside. The ventilation term is 0.33 · n · V · ΔT, with n the air changes per hour and V the room volume.
The radiator pick multiplies the room’s loss by 1.1 and takes the nearest panel from a typical output table at ΔT 50 K. It is a starting point for a quote, not a design to a standard — and the screen says so above the result.
Worked example
5 × 4 × 2.5 m, 20 °C inside, -10 °C outside, 0.5 air changes an hour: 945 W through the envelope plus 247.5 W of ventilation is 1192.5 W, or 59.6 W/m²; the app shows 1192 W. With the 1.1 factor that asks for 1312 W; the pick is a type 22, 600 × 900 mm, 1323 W at ΔT 50 K.