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U Value Calculator

Work out u value instantly with clear inputs, formula shown and shareable results.

About U Value Calculator

U-value is the reciprocal of the total thermal resistance, counting every layer plus the internal and external surface films. Adding 2.6, 0.35 and 0.17 gives 3.12 m2 K/W and a U-value of 0.32 W/(m2 K), so each square metre loses 6.4 W under a 20 K difference.

Formulas

U-value

U = 1 / (R1 + R2 + R(surfaces))

Heat loss

Q = U x A x dT

Worked example

With the default values below, U Value Calculator returns U-Value of 0.3205. Change any input above and every figure here is recalculated in your browser.

Inputs used

R-value of the first layerminimum 0 m2 K/W
2.6 m2 K/W
R-value of the second layerminimum 0 m2 K/W
0.35 m2 K/W
Combined internal and external surface resistancesminimum 0 m2 K/W
0.17 m2 K/W

Results

U-Value
0.3205 W/(m2 K)
Total Thermal Resistance
3.1200 m2 K/W
Standard Achieved
Older construction - upgrade worthwhile
Heat Loss At A 20 K Difference
6.410 W/m2

Step by step

  1. 1Values used
  2. 2U-value
  3. 3Heat loss
  4. 4U-Value= 0.3205 W/(m2 K)
  5. 5Total Thermal Resistance= 3.1200 m2 K/W
  6. 6Standard Achieved= Older construction - upgrade worthwhile
  7. 7Heat Loss At A 20 K Difference= 6.410 W/m2

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why include surface resistances?

Because air films on both faces genuinely resist heat flow - typically 0.13 inside and 0.04 outside, which matters for thin constructions.

What U-value do new walls need?

Around 0.18 W/(m2 K) for new-build walls under current UK standards, and 0.11-0.13 for roofs.

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