Thermal Resistance Calculator — R = L/(kA)
Find the thermal resistance of a wall, slab, or insulating layer and the resulting heat flow rate. Enter thickness, thermal conductivity, area, and surface temperatures to get R in K/W, heat loss in watts, and the RSI/imperial R-value used in building standards.
m
W/(m·K)
m²
°C
°C
R = L / (k × A) — resistance to heat flow through the entire slab
- 1
Temperature difference ΔT = Th − Tc
20 − -10 = 30 - 2
Product k × A
0.04 × 1 = 0.04Denominator of the resistance formula. - 3
Thermal resistance R = L ÷ (k × A)
0.1 ÷ 0.04 = 2.5000
How does this calculator work?
Thermal resistance R = L/(k·A) (K/W) measures a slab's opposition to heat flow. The steady-state heat transfer rate is Q̇ = ΔT/R (watts). Per-unit-area R-value = L/k (m²·K/W). Higher R means better insulation. Enter thickness, conductivity, area, and temperatures to get all three quantities.
Formula
How this is calculated
Fourier's law of heat conduction states that the steady-state heat flow rate through a flat slab is Q̇ = k·A·ΔT/L, where k is thermal conductivity (W/m·K), A is the cross-sectional area (m²), L is the thickness (m), and ΔT is the temperature difference across the slab. Rearranging this as Q̇ = ΔT/R defines thermal resistance R = L/(k·A) in units of K/W — the thermal analogue of electrical resistance (V = IR). A higher R means less heat flows for a given temperature difference.
The R-value quoted on insulation products is the resistance per unit area, R_val = L/k (m²·K/W), often called the RSI value. To convert to imperial units (h·ft²·°F/BTU), multiply by 5.6783. For example, 100 mm of glass-wool insulation (k ≈ 0.04 W/m·K) has an RSI of 2.5 m²·K/W, equivalent to an imperial R-value of about R-14.
This calculator assumes one-dimensional, steady-state conduction through a homogeneous, flat slab with constant material properties. Real walls involve layered materials (add R-values in series), convection at surfaces, and temperature-dependent conductivity. For layered assemblies, sum each layer's R_val before multiplying by area.
Frequently asked questions
For layers in series (e.g. brick + insulation + plasterboard), thermal resistances add directly: R_total = R₁ + R₂ + … (just like electrical resistors in series). Calculate R_val = L/k for each layer and sum them before dividing ΔT to get total heat flow Q̇ = ΔT / R_total.
The R-value (RSI) is a material property per unit area: L/k. Thermal resistance R = L/(k·A) is the property of a specific panel of that area. Heat flow Q̇ = ΔT/R uses the full resistance, not the per-area value.
No — R = L/(kA) applies to flat slabs only. For cylindrical geometry (pipe insulation), thermal resistance is R = ln(r_outer/r_inner)/(2πkL_pipe). The overall approach is the same, but the geometry changes the formula.
Also known as
TG we-Calculate Editorial Team. (2026). Thermal Resistance Calculator — R = L/(kA) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/thermal-resistance-calculator
TG we-Calculate Editorial Team. "Thermal Resistance Calculator — R = L/(kA)." TG we-Calculate. 2026. https://we-calculate.com/calculator/thermal-resistance-calculator.
TG we-Calculate Editorial Team, "Thermal Resistance Calculator — R = L/(kA)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/thermal-resistance-calculator
@misc{wecalculate_thermal_resistance_calculator, title = {Thermal Resistance Calculator — R = L/(kA)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/thermal-resistance-calculator}}, year = {2026}, note = {TG we-Calculate} }
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