Heat Transfer Calculator

R-value and U-value are the same physics, inverted.

R-value and U-value are the same physics, inverted. Steady-state conduction through a slab, with the R-value and U-value that the building trade uses.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Material

Thickness

Area

m²

Warm side

°C

Cold side

°C

Hours per year at this difference

Heat flow

88 W

U = 0.4 W/m²K · R = 2.5 m²K/W

Heat flow88 W

Heat flux8.8 W/m²

U-value0.4 W/m²K

R-value2.5 m²K/W

Temperature gradient220 °C per metre

MaterialMineral wool

Energy over a year440 kWh

Fourier's law: heat flow is conductivity times area times temperature difference, divided by thickness. Everything follows from the conductivity, which varies by a factor of ten thousand between PIR foam and copper. R-value and U-value are the same physics repackaged for the building trade. R is thickness over conductivity and U is its reciprocal, so insulation is sold on R — bigger is better — while heat loss is calculated on U, where smaller is better. Confusing them inverts the answer. This is steady state, which means it describes the eventual equilibrium rather than what happens on a cold morning. Real walls have thermal mass and take hours to respond, which is why a heavy masonry building feels different from a lightweight one with the same U-value.

How the Heat Transfer Calculator works

Steady-state conduction through a slab, with the R-value and U-value that the building trade uses. Insulation is sold on R and heat loss is calculated on U, and confusing them inverts the answer.

Also known as: heat through a wall calculator · r value to u value · how much heat does insulation save · conduction through a material

Where to go next

The Heat Transfer question rarely arrives on its own. These are the ones that usually come with it:

Wall U-Value Calculator — R-values add. U-values never do.
Latent Heat Calculator — Boiling costs five times more than heating to boiling.
Heat Loss Calculator — Fabric plus ventilation, with the assumption stated.
Speed Distance Time Calculator — Any one of the three from the other two, in four units.

Sources

Frequently asked questions

What is Fourier's law?

Heat flow equals conductivity times area times temperature difference, divided by thickness. Everything about conduction follows from it, and the conductivity varies by a factor of ten thousand between PIR foam and copper.

What is the difference between R-value and U-value?

R is thickness divided by conductivity — resistance, so bigger is better. U is its reciprocal — how much heat gets through, so smaller is better. They are the same physics packaged for different audiences.

Does doubling insulation halve the heat loss?

Through that layer, yes. Across a whole wall, no — the other layers and the surface resistances do not change, so the improvement is always less than proportional. Diminishing returns set in quickly past about 200 mm.

Why does my wall not behave like this calculation?

Because this is steady state. Real walls have thermal mass and take hours to respond, so a heavyweight masonry building feels quite different from a lightweight one with the same U-value.

What is a good U-value?

Current new-build standards typically require walls below 0.18 W/m²K and roofs below 0.13. A solid uninsulated brick wall is around 2.0, which is more than ten times worse.

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