Calculate thermal resistance and U-value for a building element using material thickness and thermal conductivity.
This simplified plane-layer calculation does not account for thermal bridges, repeating studs, fixings, air gaps or moisture effects. Professional compliance calculations may require a more detailed method.
How to use the U-Value Calculator
Enter the internal and external surface resistances, followed by the thickness and thermal conductivity of each solid layer. Layer 1 is required. Layers 2 and 3 are optional, but each optional layer needs both values. Use product-specific conductivity data where available.
U-value formula
| Calculation | Formula |
|---|---|
| Layer resistance | R = thickness in metres ÷ conductivity |
| Total resistance | R-total = Rsi + all layer resistances + Rse |
| U-value | U = 1 ÷ R-total |
The U-value is expressed in watts per square metre kelvin (W/m²K). A lower U-value means less heat passes through the element for the same area and temperature difference.
Choosing thermal conductivity values
Thermal conductivity varies by material, density, moisture condition and product specification. Use declared or design values from reliable product documentation. Example placeholders demonstrate the input format only and are not recommended design values.
U-values and UK building work
Energy-efficiency requirements depend on the building type, location and work being carried out. Consult the relevant national rules and the UK government’s Approved Document L guidance for England, and check the applicable requirements elsewhere in the UK.
Limits of a simple layer calculation
A wall, roof or floor may contain repeating timber or metal members, mortar joints, cavities, fixings and junctions. These can create parallel heat-flow paths or thermal bridges. A single series calculation can therefore differ from a formal assessment.
Related calculators
Estimate room or building heat demand with the Heat Loss Calculator. Estimate solar generation with the Solar Panel Calculator UK. Check cooling capacity with the Air Conditioning Size Calculator. Browse all tools on the United Kingdom Calculator homepage.
Frequently asked questions
What is a good U-value?
There is no single value for every project. The required or suitable value depends on the building element, project type, location and applicable standards.
Is a lower U-value better?
For heat retention, a lower U-value indicates lower heat transfer through the element.
Why is thickness divided by conductivity?
For a uniform layer, thermal resistance equals thickness in metres divided by thermal conductivity in W/mK.
Can I leave optional layers blank?
Yes. Leave both fields of an optional layer blank. If one value is entered, the calculator asks for its matching value.
Does this calculator include thermal bridging?
No. It calculates solid layers in series and does not model studs, fixings, junctions or other repeating heat-flow paths.