How this calculator works
Fourier law relates heat flux to a temperature gradient. For a flat homogeneous layer with constant conductivity and no internal heat generation, the steady gradient is linear, so its magnitude is the face temperature difference divided by thickness. Heat flux is power per area, while thermal conductivity is a material property. Surface convection and contact resistance need additional resistance terms.
Formula
q = k(Th - Tc)/L; k = qL/deltaT; L = k deltaT/q.
Worked example
A layer with k = 0.5 W/(m K), L = 0.1 m and a 20 K face difference carries 100 W/m². A measured 60 W/m² across 0.2 m and 30 K gives k = 0.4 W/(m K).
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Frequently asked questions
Does this include wall area?
Heat flux is already per square metre. Multiply by area to obtain heat rate.
Why can flux be negative?
A negative signed temperature difference reverses the specified direction of heat flow.
Does conductivity vary with temperature?
Often. Use an appropriate mean value or integrate the temperature-dependent conductivity.