Thermal Resistance Calculator
Calculate the conductive thermal resistance of a single material layer, R = L / (k * A). Optionally supply a temperature difference and the tool also returns the heat flow through the layer.
Enter Values
How to use this calculator
- Enter the layer thickness L in metres and the thermal conductivity k in W/m.K.
- Enter the area A in square metres over which heat is conducted.
- Optionally enter a temperature difference dT in kelvin to also get the conductive heat flow Q = dT / R.
How it works
The absolute conductive resistance of a plane layer is R = L / (k * A), measured in K/W. A larger thickness raises resistance, while a higher conductivity or larger area lowers it. If a temperature difference dT is applied across the layer, the steady-state heat flow follows Ohm's-law analogy Q = dT / R. Resistances of layers in series simply add, which makes this the building block for multilayer wall calculations.
Worked example
Worked example. For an insulation layer with L = 0.1 m, k = 0.04 W/m.K and area A = 10 m2: R = 0.1 / (0.04 * 10) = 0.25 K/W. With a temperature difference dT = 20 K, the heat flow is Q = 20 / 0.25 = 80 W.
Common mistakes
- Confusing the absolute resistance (K/W) with a per-area building R-value (m2.K/W = L / k) — this tool returns the absolute resistance for the given area.
- Using thickness in millimetres rather than metres.
- Adding resistances of layers in parallel instead of in series when they actually sit one behind the other.
Frequently asked questions
What is the difference between this R and a building R-value?
This R is the absolute resistance in K/W and depends on area. A building R-value is per unit area (m2.K/W) and equals L / k, independent of area. Multiply the building R-value by area, then invert, to relate them.
Can I add several layers?
Compute R for each layer with the same area, then add them because series thermal resistances sum. The total resistance gives the combined heat flow for a given temperature difference.
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Tip: Enter any known values to calculate the remaining results.
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