Thermal Conductivity Heat Flow Calculator
Heat flow through a slab from Fourier’s law — conductivity, area, thickness and temperature difference — with U-value, R-value and daily energy loss.
How it works
Q/t = k · A · ΔT ÷ d · U = k ÷ d · R = d ÷ k
Fourier’s law says conduction scales with conductivity, area and temperature gradient. Doubling thickness halves the loss; swapping brick for mineral wool cuts it twenty-fold. U-values (the building-industry figure) are just k/d for a single layer; for a wall of several layers add the R-values and invert. Real walls also have surface films that add about 0.17 m²·K/W.
Worked example
12 m² brick wall, 22 cm thick, 20 °C difference: 0.8 × 12 × 20 ÷ 0.22 = 873 W, or 21 kWh a day.
Frequently asked questions
How do I combine layers?
R_total = Σ(dᵢ/kᵢ); then Q/t = A·ΔT ÷ R_total. Insulation dominates because its R is so large.
Is the US R-value the same?
US R-values use ft²·°F·h/BTU; multiply SI R by 5.678.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.