Heat Transfer Calculator

Calculate heat transfer rates as well as rates for both conduction and convection with this easy-to-use heat transfer calculator.

Heat flow rate6.65 kW19 K across 0.002857 K/W of resistance

Conductive resistance0.002857 K/Wthickness ÷ (conductivity × area)

Thermal conductivity1.05 W/(m·K)Glass

Temperature difference19 K

Heat flux per square metre3325 W/m²

U-value175 W/(m²·K)the building-industry figure — lower is better insulated

R-value (SI)0.00571 m²·K/W0.0324 in US units (ft²·°F·h/BTU), which are about 5.68× the SI figure — the two are constantly confused

Thermal diffusivity5.0e-7 m²/sk ÷ (ρc) — how fast a temperature change propagates, as opposed to how much heat flows

Time for heat to penetrate the thickness72 sroughly L² ÷ α — 0.02 hours, and note it goes as the SQUARE of thickness

Volumetric heat capacity2.1e+6 J/(m³·K)the thermal mass per unit volume

Energy over 24 hours159.6 kWh

At twice the thickness3.325 kWconduction halves, but any surface films do not — so the total rarely halves

The same thickness of polystyrene209 W31.8× less conductive than glass

The formula

Q̇ = kA ΔT ÷ L; R = L ÷ kA; Q̇ = ΔT ÷ R

Resistances in series, like a circuit

Conduction is an exact analogue of Ohm's law: temperature difference plays the part of voltage, heat flow of current, and thermal resistance of electrical resistance. Layers in a wall add their resistances in series exactly as resistors do, which is what makes multi-layer construction calculable.

Surface films matter more than people expect. Still air clinging to each face of a pane adds resistance of its own, and on a thin conductive material those films can dominate the total. That is why a 6 mm sheet of glass — whose glass alone offers almost no resistance — still slows heat noticeably, and why double glazing works chiefly by trapping a further air layer rather than by adding glass.

Two figures describe the same thing from opposite ends. U-value is conductance per unit area, so lower is better insulated; R-value is its reciprocal, so higher is better. Be careful which R: the US figure in ft²·°F·h/BTU is about 5.68 times the SI figure in m²·K/W, and quoting one as the other is a common and large error.