A U-value (thermal transmittance) is the rate of heat loss through one square metre of a building element for each degree of temperature difference between inside and outside. It is measured in watts per square metre per kelvin (W/m²K).

How is a U-value calculated?

Using the method in BS EN ISO 6946: add the thermal resistance of each layer to the standard internal and external surface resistances, then take the reciprocal. This calculator does that sum for you.

What U-value do I need for Building Regulations?

It depends on the work. In England under Approved Document L (2021), a new-build wall must not exceed 0.26 W/m²K, a new thermal element in an extension should reach about 0.18, and upgrading a solid wall targets 0.30. Use the compliance dropdown to check against the relevant figure.

Is a lower or higher U-value better?

Lower is better. A lower U-value means less heat escapes, so the element performs better thermally and the building is cheaper to heat.

Does this calculator account for thermal bridging?

No. It gives the plane U-value through the layers you enter. It does not include repeating thermal bridges such as timber studs or mortar joints, or junction bridges. For a full compliant calculation with bridging corrections, use accredited software or a thermal assessor.

Can I use this for a solid ground floor?

Not accurately. Solid ground floors lose heat partly to the ground, which depends on the floor's perimeter-to-area ratio (BS EN ISO 13370), not just the layers. This tool is intended for walls, roofs, and exposed or suspended floors over outside air.

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

An R-value is thermal resistance, where higher is better, and it applies to a single layer or material. A U-value is the whole-element transmittance and is the reciprocal of the total resistance, where lower is better.

U-Value Calculator UK

A U-value tells you how much heat escapes through a wall, roof or floor: the lower the number, the better the element keeps warmth in. This UK U-value calculator uses the standard BS EN ISO 6946 method. You build up your construction one layer at a time, pick each material (or type in your own thermal conductivity), and it returns the U-value in W/m²K along with a Building Regulations compliance check. It works for walls, pitched and flat roofs, and exposed floors, and it is metric-first throughout. Unlike the free calculators run by insulation manufacturers, this one is not tied to any product range, so nothing here is steering you toward a particular brand of board.

Layers

Element type

Rsi = 0.13 m²K/W · Rse = 0.04 m²K/W (BS EN ISO 6946)

Layers (outside → inside)

Compliance check — Approved Document L target

Reference targets — Approved Document L (England, 2021 edition). Wales, Scotland (Section 6) and Northern Ireland differ. Always confirm the current edition for your project.

Your Results

Enter your construction layers to calculate

Surface resistance reference (BS EN ISO 6946)

Element	Rsi (m²K/W)	Rse (m²K/W)
Wall (horizontal)	0.13	0.04
Pitched roof (up)	0.10	0.04
Flat roof (up)	0.10	0.04
Exposed floor (down)	0.17	0.04

These are built in automatically — you only need to enter the actual construction layers.

How this calculator works

It adds up the thermal resistance (R-value) of every layer in your build-up, plus the fixed internal and external surface resistances for the element type you chose, then takes the reciprocal to get the U-value. Each solid layer's resistance is its thickness in metres divided by its thermal conductivity (λ). Air cavities use a fixed resistance from the standard rather than a calculated one. Change any material or thickness and the result updates straight away.

Takeaway: pick the element type first so the surface resistances are right, then build the layers from outside to inside.

Formulas used

Layer resistance (solid): R = (thickness in mm ÷ 1000) ÷ λ

Air cavity: fixed R (0.18 unventilated, 0.09 slightly ventilated)

Total resistance: R_total = Rsi + Rse + sum of all layer R-values

U-value: U = 1 ÷ R_total

Takeaway: insulation does the heavy lifting because a low λ over even a modest thickness produces a large resistance.

Common thermal conductivity (λ) values

Material	λ (W/m·K)
Brick, outer leaf	0.77
Aircrete block	0.15
Dense concrete block	1.13
PIR / PUR board	0.022
Phenolic board	0.020
Mineral wool	0.040
EPS	0.036
Plasterboard	0.21
Cement render	1.00
Softwood timber	0.13

Takeaway: these presets are loaded in the material list, but always use the λ on your product's data sheet where you have it, since insulation grades vary.

Worked example — modern cavity wall

A new-build cavity wall: 102.5 mm facing brick, a 50 mm unventilated cavity, 100 mm PIR partial-fill board, 100 mm aircrete inner block, finished with 12.5 mm plasterboard.

Layer	R (m²K/W)
External surface (Rse)	0.04
Brick: 0.1025 ÷ 0.77	0.133
Air cavity (unventilated)	0.18
PIR: 0.100 ÷ 0.022	4.545
Aircrete block: 0.100 ÷ 0.15	0.667
Plasterboard: 0.0125 ÷ 0.21	0.060
Internal surface (Rsi)	0.13
R_total	5.75 m²K/W → U = 0.17 W/m²K

That comfortably passes the 0.18 limiting wall value for a new dwelling.

Takeaway: the 100 mm of PIR contributes about 4.5 of the 5.75 total resistance, which is why board thickness, not blockwork, decides whether you pass.

Frequently asked questions

Related tools

Loft Insulation Calculator UK — work out the rolls, layers and cost needed to reach the recommended 270 mm depth
Loft Conversion Cost Calculator UK — budget a loft conversion including insulation upgrades
Stud Wall Calculator UK — timber and plasterboard for a partition or insulated stud wall
Plasterboard / Drywall Calculator — sheet counts for walls and ceilings
Dew Point Calculator UK — check the same build-up for condensation and mould risk
Timber Calculator UK — lengths, volume and board feet for framing and battens

Guides worth reading

How to Calculate a U-Value: UK Guide with Worked Example — the method, target values and where paper U-values differ from site performance

Disclaimer: This calculator is for planning and estimating only. It returns the plane U-value and does not include thermal bridging, air leakage, or ground heat loss. For Building Regulations submission, condensation risk analysis, or any element where compliance matters, use accredited software or a qualified thermal assessor and confirm the current Approved Document (or Section 6 / Technical Booklet) for your jurisdiction.