U-Value Calculator

This u-value calculator works out the U-value of a wall, roof or floor from its layers, then shows the heat lost through it in watts and how it compares with Building Regulations in England. It is made for UK and European homeowners, self builders and students who want to see how brick, block, insulation and plaster add up, using the layer method set out in BS EN ISO 6946.

Quick answer: Divide each layer’s thickness in metres by its thermal conductivity (lambda) to get its resistance, add the surface resistances (0.13 inside and 0.04 outside for a wall), then take 1 divided by the total. A brick, 100 mm mineral wool and dense block wall with plaster comes out at about 0.343 W/m²K in this u-value calculator.

What Is a U-Value Calculator?

A U-value calculator is a tool that adds up the thermal resistance of each layer in a building element and converts the total into a U-value, the heat in watts that passes through one square metre for each degree of temperature difference. In plain words: resistance of a layer (R) = thickness in metres / lambda. Total R = inside surface resistance + every layer’s R + outside surface resistance. U-value = 1 / total R.

The surface resistances come from the BRE conventions for U-value calculations (BR 443), which follow BS EN ISO 6946: 0.13 m²K/W inside a wall, 0.10 under a roof and 0.17 above a floor, with 0.04 outside. An unventilated cavity counts as a fixed resistance of 0.18 m²K/W. Heat loss through the element is then U-value x area x temperature difference.

How Do You Use the U-Value Calculator?

1. Choose the Element: a wall, a roof or ceiling, or a floor over outside air.
2. Working from outside to inside, pick Layer 1 material and enter Layer 1 thickness in millimetres, then do the same for layers 2 to 5. Set unused layers to “None (layer not used)”.
3. If your material is not listed, choose “Other: enter my own lambda” and type the value from the product data sheet into Layer 1 own lambda (or the matching layer).
4. For heat loss, enter the Area of the element, the Inside temperature and the Outside temperature.
5. Read the U-value, the heat loss in watts and the Building Regulations check, then open “Show the formula and working” to see each layer’s resistance.

How Do You Calculate a U-Value?

Add the thermal resistance of every layer, including the inside and outside surfaces, and divide 1 by the total: for a wall, U = 1 / (0.13 + each thickness / lambda + 0.04). For example, 100 mm of mineral wool with a lambda of 0.040 has a resistance of 0.1 / 0.040 = 2.5 m²K/W, far more than the 0.133 of a 102.5 mm brick leaf.

This is why insulation does almost all the work in a modern wall. The Wikipedia article on thermal transmittance shows the same method for a cavity wall. A full calculation for Building Regulations also adds corrections for wall ties, mortar joints in lightweight blockwork, timber studs and air gaps in the insulation. Those corrections usually make the real U-value a little worse, so treat this tool’s figure as a good first estimate rather than a compliance certificate.

What U-Value Do Building Regulations Require?

In England, a new wall or floor added to an existing home needs a U-value of 0.18 W/m²K or better, and a new roof 0.15 or better. These limits are in Table 4.2 of Approved Document L, Volume 1, which also sets 1.4 for new windows. For a whole new dwelling, the backstop limits in Table 4.1 are 0.26 for walls, 0.16 for roofs and 0.18 for floors, although new homes must also pass an overall energy calculation.

When you renovate an existing wall worse than 0.70, the target is 0.55 with cavity insulation or 0.30 with internal or external insulation, where that is practical. Scotland, Wales and Northern Ireland publish their own figures, and outside the UK national energy codes apply, so check locally. The calculator compares your result with the England figures for a new element in an existing home.

What Is a Good U-Value for a Wall?

A good wall U-value today is 0.18 W/m²K or lower, the figure for a new wall in an existing home in England; an uninsulated solid brick wall is about 2.1 and an unfilled cavity wall about 1.6. You can check this with the calculator: 215 mm of brick plus plaster gives about 2.12, and brick, a 50 mm air cavity, dense block and plaster gives about 1.68.

Lower U-values mean smaller heat losses, so you can fit smaller radiators or a smaller heat pump. Our radiator size calculator uses U-values to work out the watts each room needs, and the heat pump size calculator shows how insulation changes the size of unit you need.

Thermal Conductivity (Lambda) of Common Building Materials

These are the lambda values built into the calculator. Lower lambda means a better insulator. For insulation products, always use the declared lambda from the manufacturer’s data sheet, because it varies between products.

Worked Example: An Extension Wall

Example: a builder suggests a cavity wall of 102.5 mm brick, 100 mm of mineral wool, 100 mm dense block and 13 mm of plaster for a 10 m² extension wall. The target is 21°C inside on a minus 3°C day, a 24°C difference.

• Layer resistances: brick 0.1025 / 0.77 = 0.133, mineral wool 0.1 / 0.040 = 2.5, block 0.1 / 1.13 = 0.088, plaster 0.013 / 0.57 = 0.023.
• Total R: 0.13 + 2.744 + 0.04 = 2.914 m²K/W.
• U-value: 1 / 2.914 = 0.343 W/m²K, above the 0.18 limit for a new wall.
• Heat loss: 0.343 x 10 m² x 24 = 82 W.

The defaults reproduce this. Switching Layer 2 to PIR board brings the U-value to about 0.202, and also switching Layer 3 to aerated concrete block brings it to about 0.182, still just above 0.18, so the next step is to ask the builder for a thicker board or a manufacturer’s full calculation. For the rest of the room’s heating, see our heating calculators.

U-Value Calculator FAQs

Checked October 2026. Sources: GOV.UK: Approved Document L, conservation of fuel and power, BRE BR 443 Conventions for U-value calculations (CIBSE), Wikipedia: Thermal transmittance, Wikipedia: List of thermal conductivities, Celotex GA4000 product data.