Pipe Insulation Calculator — How Much Pipe Lagging Do I Need?

This calculator works out pipe insulation quantities based on your total pipe run length, pipe diameter, and chosen insulation material, and shows the typical wall thickness advised by BS 5422:2023 (the British Standard for pipe insulation) for that material.

The formula

Pieces = (Total pipe length × Wastage factor ÷ Piece length) + Elbow extras

How thickness is decided

BS 5422:2023 and Part L (the Domestic Building Services Compliance Guide) do not fix a thickness in millimetres. They set a maximum heat loss in watts per metre of pipe (at 60°C in still air at 15°C: 15mm = 7.89 W/m, 22mm = 9.12 W/m, 28mm = 10.07 W/m, 35mm = 11.08 W/m, 42mm = 12.19 W/m, 54mm = 14.12 W/m). The required wall thickness is whatever meets that limit for the product’s declared thermal conductivity (λ), so a lower-λ material can be thinner. The figures below are the BS 5422:2023 advised walls for typical domestic products — always confirm against the datasheet λ.

Typical advised thicknesses (BS 5422:2023)

• 15-35mm pipe (foam / rubber): ~19-20mm wall
• 15-35mm pipe (mineral wool / phenolic): ~20mm wall
• 42-54mm pipe (foam / rubber / mineral wool): ~25mm wall
• 42-54mm pipe (phenolic, low λ): ~20mm wall (enhanced level 25-30mm)

Frost protection is different

These walls limit heat loss. To stop a pipe freezing, BS 5422:2023 Table 30 (12-hour) needs far more, and the requirement climbs steeply at the smallest bores: a 15mm copper cold pipe needs about 50mm of mineral wool inside the building (60mm inside an unheated thermal envelope), built up in layers — whereas a 22mm pipe needs only about 20mm. The smallest pipes often cannot be frost-protected by lagging alone (trace heating or a trickle of flow may be needed).

Standard piece lengths

• Foam tubes: 1m lengths
• Mineral wool sections: 1m lengths
• Rubber (Armaflex): 2m lengths
• Phenolic foam: 1m lengths