Find the U-value (thermal transmittance) of a wall, roof or floor. Enter each material layer's thickness and thermal conductivity — the calculator adds the resistances and returns the U-value in W/m²K.
Each layer: thickness in mm and conductivity λ in W/m·K. Leave a layer's thickness at 0 to skip it.
U = 1 ÷ total resistance. Lower U-values mean better insulation. Surface resistances for the chosen element are included automatically.
The U-value, or thermal transmittance, measures how quickly heat passes through a building element. A lower U-value means less heat loss and better insulation. It is the reciprocal of the total thermal resistance — the sum of every layer's resistance plus the thin films of still air on each surface.
How the U-value is calculated
R(layer) = thickness (m) ÷ λ · R(total) = Rsi + ΣR(layers) + Rso · U = 1 ÷ R(total) Rsi and Rso are the internal and external surface resistances, set by the element type.
Worked example
A wall of 100 mm brick (λ 0.77), 100 mm insulation (λ 0.035) and 13 mm plaster (λ 0.57): the layer resistances are 0.130, 2.857 and 0.023 m²K/W. Adding Rsi 0.13 and Rso 0.04 gives R = 3.180, so U = 1 ÷ 3.180 = 0.314 W/m²K.
Typical conductivity values
Common λ values (W/m·K): brickwork ~0.77, dense concrete ~1.4, aircrete block ~0.15, mineral-wool insulation ~0.035, PIR/PUR board ~0.022, timber ~0.13, plasterboard ~0.21, gypsum plaster ~0.57. Use the manufacturer's figure where you have it, as insulation products vary.
Frequently asked questions
What is a good U-value for a wall?
Modern new-build walls typically target around 0.18–0.28 W/m²K. Older solid walls can be 1.5 or higher. Lower is better.
What are surface resistances?
Thin layers of still air cling to each face and slow heat flow. Standard values are Rsi 0.13 and Rso 0.04 for walls; roofs and floors differ slightly.
Do I use thickness in mm or m?
Enter thickness in millimetres here; the calculator converts to metres (÷1000) before dividing by conductivity.
Is this a certified U-value?
No — it is a guide using the simple layered method. Bridged constructions and certified assessments use the combined method (BS EN ISO 6946) and may differ.
A guide using the simple layered method. For building-control or certification, use a full BS EN ISO 6946 assessment that accounts for thermal bridging.
Written by the CalcPine team · Reviewed for accuracy · Last updated 23 July 2026 · Method: U = 1 / (Rsi + Σ(d/λ) + Rso).
