U-value calculator
Thermal transmittance of a wall or roof from its layers.
How it is calculated
R of each layer = thickness (m) ÷ conductivity λ (W/m·K). Total R = Rsi + ΣR + Rse. U = 1 ÷ total R (W/m²·K). Surface resistances follow ISO 6946 for the heat-flow direction.
Check locally: A simplified layer calculation: it does not include thermal bridges, fixings or air gaps beyond the layers you add.
Questions
What is the difference between U and R?
R is resistance to heat flow (higher is better); U is 1 ÷ total R, the heat flow per square metre per degree (lower is better).
Is anything I enter saved or sent?
No. The calculation runs in your browser. Nothing you type is sent anywhere. You can copy the result, print it or download it as a CSV.