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U-value calculator

Add the layers of your wall, roof or floor — plaster, blocks, insulation, brick — and get the U-value and R-value to EN ISO 6946, the extra insulation a target needs and the heat lost through it.

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1Wall, roof or floor

Filled in from EN ISO 6946 for your choice: 0.13 m²K/W for walls, 0.10 for roofs (heat flowing up), 0.17 for floors (heat flowing down).

0.04 m²K/W to outside air. Towards an unheated cellar, crawl space or garage the inside value is used on that side too.

2Layer 1 (inside)

Order does not matter for the U-value. Leave a layer empty, or choose “No layer”, to skip it.

Use the λ (lambda) printed on your product or its data sheet when you have it — it beats the typical value.

3Layer 2
4Layer 3
5Layer 4
6Layer 5 (outside)
7Compare with a target

Used to turn the missing R-value into a thickness. 0.035 for a mineral wool or EPS board, 0.022 for PIR. US: λ = 0.144 ÷ R per inch.

8Heat loss (optional)

Optional: with a temperature difference it gives the heat flowing through.

For example 20 °C inside and 0 °C outside is 20 K (36 °F).

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Type your measurements in the empty boxes. Still needed:

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How do I calculate a U-value? Using the calculator

  1. 1Choose what you are working out: an outside wall, a roof, a floor, or a wall or floor next to an unheated space. The surface resistances fill in by themselves.
  2. 2Add the layers from inside to outside: pick each material (its typical λ fills in) and type its thickness. Change λ to the value on your product if you know it.
  3. 3Leave layers you don't need empty, or choose “No layer”.
  4. 4Pick a target — Part L, the German GModG, the Dutch Bbl or the French rules — or type your own, to see whether the construction passes and how much insulation is missing.
  5. 5Optional: enter the area and the temperature difference to see how much heat flows through it.

U-value formula (EN ISO 6946)

  1. 01R of each layer = thickness (m) ÷ λ (W/m·K)
  2. 02Rc = R1 + R2 + … (the layers only)
  3. 03R_T = Rsi + Rc + Rse
  4. 04U = 1 ÷ R_T (W/m²K)
  5. 05US R-value = R (m²K/W) × 5.678
  6. 06Heat flow = U × area × temperature difference (W)

Each layer resists heat in proportion to its thickness and in inverse proportion to its thermal conductivity λ: 100 mm of mineral wool at λ 0.035 gives R = 0.1 ÷ 0.035 = 2.86 m²K/W, while 100 mm of brick at λ 0.77 gives only 0.13. Add the layers up, then add the two surface resistances — the thin films of still air on each face, set by EN ISO 6946: 0.13 inside a wall, 0.10 under a roof, 0.17 above a floor and 0.04 outside. The U-value is one over the total.

A lower U-value (or a higher R-value) means less heat lost. U is in watts per square metre for every degree of difference between inside and outside, so a wall of U 0.25 loses 0.25 × 20 = 5 W through each square metre when it is 20 °C warmer inside. In the US, R-values are given in ft²·°F·h/BTU: multiply a metric R by 5.678 to get it, so R 3.5 m²K/W is about R-20.

This is the simple method for layers that are the same all over. Where timber studs, rafters or metal ties cross the insulation, EN ISO 6946 averages the paths and adds corrections, and building control calculations do that for you — the result here is the best case for the insulated part.

U-value calculation example

A cavity wall of 15 mm plaster, 100 mm aerated concrete blocks, 100 mm mineral wool (λ 0.035) and 102 mm brick has layer R-values of 0.029 m²K/W, 0.667 m²K/W, 2.857 m²K/W and 0.132 m²K/W.

With Rsi 0.13 m²K/W and Rse 0.04 m²K/W, R_T is 3.86 m²K/W (21.9 R (US) in US units) and the U-value is 0.259 W/m²K.

Part L asks for 0.18 W/m²K on a new wall in an existing home, so this wall needs 1.7 m²K/W more — about 59 mm of extra insulation at λ 0.035. Over 10 m² with 20 K between inside and outside, 52 W flows through it.

Tips before you buy

  • Take λ from the product's label or data sheet (EN 13162 to EN 13171 for insulation): two boards that look the same can differ by 20 %.
  • Insulation does most of the work. Doubling the bricks or blocks barely changes the U-value; adding 50 mm of insulation often does more than the whole masonry wall.
  • Lofts and roofs: insulation squashed by boards or stored boxes loses much of its R-value. Use loft legs if you need to store things.
  • Keep a vapour control layer on the warm side of timber walls and roofs, and ask a professional before insulating solid walls on the inside, where condensation can form.
  • For a quote or building control, ask for a U-value calculation from the insulation maker — most do them free, with the thermal bridges included.

Frequently asked questions

How do you calculate a U-value?

Divide each layer's thickness in metres by its λ value to get its R-value, add them up, add the surface resistances (0.13 + 0.04 for a wall), and take one over the total. A wall with layers adding up to R 3.69 has R_T = 0.13 + 3.69 + 0.04 = 3.86, so U = 1 ÷ 3.86 = 0.26 W/m²K.

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

The R-value is the resistance to heat flow (higher is better); the U-value is the heat that gets through (lower is better). They are each other's inverse: U = 1 ÷ R_T, where R_T includes the surface resistances. An R-value can be for one layer, the U-value is always for the whole element.

What U-value does a wall need in the UK?

In England, Approved Document L 2021 asks for 0.18 W/m²K for a new wall in an extension, and the limiting value for a new home is 0.26 (the whole-house check works with about 0.18). When you renovate a solid wall with internal or external insulation, the target is 0.30. Scotland, Wales and Northern Ireland have their own figures.

What is a good U-value?

For walls, 0.15–0.20 W/m²K is very good and 0.30 is a typical renovation result; an uninsulated solid brick wall is about 2. Roofs and lofts are usually 0.11–0.16 in a new home, and passive houses aim for about 0.10–0.15 everywhere.

How do I convert an R-value to a U-value?

Add the surface resistances to the R of the layers and take one over the total: R 4.0 for a wall gives 1 ÷ (0.13 + 4.0 + 0.04) = 0.24 W/m²K. A US R-value must first be divided by 5.678: R-20 is 3.52 m²K/W.

What is the Rc-value?

Rc is the Dutch name for the thermal resistance of the construction from surface to surface — the layers without the surface resistances (NTA 8800). The Bbl sets minimum Rc values: 4.7 for facades, 6.3 for roofs and 3.7 for floors of a new home. U = 1 ÷ (Rsi + Rc + Rse).

Does an air gap count in the U-value?

A still, unventilated air gap counts: EN ISO 6946 gives about 0.18 m²K/W for a gap of 25 mm or more in a wall. A well-ventilated cavity does not: leave it and every layer outside it out, and use the inside surface resistance in place of the outside one.

Why is my U-value lower than the insulation maker's figure?

Makers' U-value calculations include the thermal bridges — timber studs, rafters, wall ties and fixings — that let heat bypass the insulation. This calculator gives the U-value through the insulated layers only, which is the best case.

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Formulas last reviewed: September 30, 2026 ·How we check our formulas

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