Heat Conduction Calculator (Fourier's Law)
By Santaji GadepatilinUpdated October 27, 2026
Heat flows through solids from hot to cold at a rate set by the material's thermal conductivity, thickness and area. Fourier's law is the basis of insulation design and heat exchanger calculations — see what thermal conductivity is and the main types of heat exchangers. This free heat conduction calculator solves for whichever value you leave blank and shows the formula and steps.
How to use the heat conduction calculator
Enter the material’s thermal conductivity, the area, the temperature difference across it and its thickness. You get the heat flow, U-value and R-value of that layer.
Fourier’s law
Q = k × A × ΔT ÷ d
Thermal conductivity of materials
| Material | k (W/m·K) |
|---|---|
| Copper | 385–400 |
| Aluminium | 205–237 |
| Steel | 45–50 |
| Concrete | 1.0–1.8 |
| Brick | 0.6–1.0 |
| Glass | about 1.0 |
| Water | 0.6 |
| Wood (softwood) | 0.12–0.15 |
| Mineral wool | 0.032–0.040 |
| Expanded polystyrene (EPS) | 0.033–0.038 |
| Polyurethane (PIR) foam | 0.022–0.028 |
| Still air | 0.026 |
Example
10 m² of 100 mm mineral wool (k = 0.04) with a 20 °C difference:
- Q = 0.04 × 10 × 20 ÷ 0.1 = 80 W
- U = 0.4 W/m²·K, R = 2.5 m²·K/W
The same area of 100 mm concrete would lose 3,600 W — 45 times more.
Windows
| Glazing | Typical U-value (W/m²·K) |
|---|---|
| Single glazing | 5.0–5.8 |
| Double glazing | 2.6–2.8 |
| Double, low-e, argon | 1.2–1.6 |
| Triple glazing | 0.6–0.8 |
Imperial R-values
US R-values use ft²·°F·h/BTU. Multiply SI R-values by 5.678 to convert: R 2.5 SI ≈ R-14 US.
Frequently asked questions
What is Fourier's law?
Heat flow through a material is proportional to its conductivity, area and temperature difference, and inversely proportional to its thickness — Q = kAΔT ÷ d.
What is the difference between U-value and R-value?
R-value measures resistance to heat flow (higher is better insulation). U-value is its inverse, heat flow per m² per degree (lower is better).
How do I combine layers?
Add the R-values of each layer, then U = 1 ÷ total R. Include surface resistances for walls and windows.
How much heat does a wall lose in a day?
Multiply heat flow in watts by 24 and divide by 1,000 for kWh per day.