Thermal Resistance Calculator
What the thermal resistance calculator does
The thermal resistance calculator tells you how strongly a flat layer of material resists conductive heat flow. Enter the layer thickness in metres and the material’s thermal conductivity in W/(m·K), press Calculate, and the tool returns the thermal resistance R in m²·K/W. The higher the value, the better the layer insulates.
The formula
R = d / k
- d is the thickness of the layer in metres
- k is the thermal conductivity in watts per metre-kelvin
- R is the thermal resistance per unit area, in m²·K/W
This is the R-value used in building physics. Doubling the thickness doubles the resistance; halving the conductivity does the same. Thermal resistance is the reciprocal of the U-value for a single layer, so you can move between the two with the U-value calculator.
Thermal conductivity of common materials
| Material | k (W/m·K) | R for 100 mm (m²·K/W) |
|---|---|---|
| Copper | 400 | 0.00025 |
| Steel | 45 | 0.0022 |
| Concrete | 1.7 | 0.059 |
| Brick | 0.7 | 0.14 |
| Softwood timber | 0.13 | 0.77 |
| Plasterboard | 0.25 | 0.40 |
| Mineral wool | 0.038 | 2.6 |
| Expanded polystyrene (EPS) | 0.036 | 2.8 |
| Polyurethane foam | 0.025 | 4.0 |
| Still air | 0.026 | 3.8 (in theory, before convection) |
Values are typical at room temperature. Manufacturers publish declared conductivity for specific products; use those figures for real projects.
Worked examples
Wall insulation. 150 mm of mineral wool with k = 0.038: R = 0.150 / 0.038 = 3.95 m²·K/W.
A concrete slab. 200 mm of concrete with k = 1.7: R = 0.200 / 1.7 = 0.118 m²·K/W. The same slab with 100 mm of polyurethane underneath adds 4.0, so the floor’s total resistance rises to about 4.1 m²·K/W.
A heatsink base. A 5 mm aluminium plate (k = 205): R = 0.005 / 205 = 0.0000244 m²·K/W. Metals contribute almost nothing to resistance; in electronics the interface materials and the air side dominate.
Layers in series
For a wall made of several layers, calculate each R separately and add them: Rtotal = R1 + R2 + R3 + … Surface resistances are added as well; standard values are 0.13 m²·K/W for the inside face and 0.04 m²·K/W for the outside face of a wall. A typical cavity wall might be: outer brick 0.14, cavity 0.18, mineral wool 2.6, block 0.11, plaster 0.06, plus surfaces 0.17, for a total near 3.3 m²·K/W and a U-value of about 0.30 W/m²·K.
From resistance to heat flow
Once you have R, the conductive heat flux through the layer is q = ΔT / R in W/m², where ΔT is the temperature difference across it. For the 150 mm mineral wool example with 20 °C inside and 0 °C outside, q = 20 / 3.95 = 5.1 W/m². A 40 m² wall loses about 200 W. The Fourier heat transfer calculator does this step directly.
Imperial R-values
North American product labels use ft²·°F·h/BTU. Multiply the SI value by 5.678 to convert. R = 3.95 m²·K/W is R-22 in imperial units. The insulation R-value thickness calculator works in the other direction: it tells you how thick a layer must be to reach a target R.
Common mistakes
- Entering thickness in millimetres. 100 mm is 0.1 m. The calculator expects metres.
- Using the resistance of a bare material for a product. Foil facings, air gaps and compression change the effective value.
- Ignoring thermal bridges. Studs, fixings and mortar joints conduct heat around the insulation and reduce the real average resistance.
- Applying the formula to convection or radiation. R = d/k is for conduction through a solid only. Use the Nusselt number calculator for convective coefficients.
Related tools
The thermal conductivity converter switches between W/m·K and BTU·in/(h·ft²·°F). The heat flux converter handles W/m² and BTU/h·ft². For sizing heating equipment against the losses you calculate, see the boiler size calculator.
External references
- Engineering ToolBox: thermal conductivity of common materials
- U.S. Department of Energy: insulation and R-values
Frequently asked questions
What is a good thermal resistance for a wall?
Modern codes in cold climates call for total wall resistances of roughly 3 to 5 m²·K/W (U-values of 0.2 to 0.3 W/m²·K). Roofs are usually required to reach 5 to 7 m²·K/W.
Is thermal resistance the same as R-value?
Yes. R-value is the everyday name for thermal resistance per unit area. In the SI system the unit is m²·K/W; in the imperial system it is ft²·°F·h/BTU.
How is thermal resistance related to U-value?
U = 1 / Rtotal. A total resistance of 4 m²·K/W is a U-value of 0.25 W/m²·K.
Does temperature affect the result?
Thermal conductivity rises slightly with temperature for most insulation, by a few percent between 10 °C and 40 °C. Use the conductivity declared at the mean temperature of your application when precision matters.
Can I use this for a pipe?
Not directly. Cylindrical layers need the logarithmic form R = ln(r₂/r₁) / (2πkL). The flat-layer formula is a reasonable approximation only when the insulation is thin compared with the pipe radius.