Copper Pipe Sizes Explained — OD, Nominal Bore & Insulation Sizing Guide

Copper Pipe Sizes in the UK — Complete Guide

Understanding copper pipe sizes is essential for selecting the correct pipe insulation. In the UK, copper pipe is measured by its outside diameter (OD) — and the bore of the pipe insulation must match the OD of the pipe. This guide explains UK copper pipe sizes, the difference between OD and nominal bore, and how to select the right insulation for each size.

UK Copper Pipe Sizes at a Glance

UK copper tube to EN 1057 is specified by outside diameter. For standard metric copper tube, the stated size is the actual OD — so 15mm copper tube has a 15mm OD and requires 15mm bore insulation. The table below covers the standard EN 1057 metric copper tube sizes stocked in the UK.

Note: 76.1mm is the declared OD for this large-format copper tube size. Insulation bore dimensions and tolerances vary by product, so check the manufacturer's stated pipe-size compatibility before ordering insulation for 76.1mm tube.

Pipe Size, Bore and Wall Thickness — What the Numbers Mean

Pipe insulation is described by two measurements: the bore (internal diameter, which fits over the pipe OD) and the wall thickness (the depth of insulation material). These are different measurements and should not be confused.

For example, a 22mm × 19mm insulation tube has a 22mm internal bore to fit a 22mm OD pipe and a 19mm insulation wall. The overall outside diameter of the insulated pipe will be considerably larger than 22mm — 22mm + (2 × 19mm) = 60mm overall. This matters when insulation needs to fit within a duct, chase or ceiling void, or when calculating clearances in plant rooms.

Wall thickness selection depends on the application, pipe size, service temperature and the applicable specification. See our BS 5422 Standards Guide and Wall Thickness Guide for guidance.

How is Copper Pipe Measured in the UK?

UK copper pipe is measured by its outside diameter (OD) in millimetres — a key factor in BS 5422 and Part L compliance. This means:

• 15mm copper pipe has an OD of exactly 15mm
• 22mm copper pipe has an OD of exactly 22mm
• 28mm copper pipe has an OD of exactly 28mm
• And so on for all standard sizes

This is straightforward for copper pipe — the size stated is the actual OD. When ordering pipe insulation, you need insulation with a bore (internal diameter) that matches the pipe OD.

Imperial Copper Pipe — Legacy Sizes and Terminology

Older UK installations may use imperial copper tube, which was specified to different dimensional standards from modern metric EN 1057 tube. Imperial and metric copper tube are not the same dimensions — do not assume that a pipe described as "½ inch" has the same OD as modern 15mm metric copper tube.

The table below shows commonly associated legacy descriptions, but these are approximate associations only. Always measure the actual OD of existing pipework before ordering insulation, particularly on older installations where imperial tube may be present.

If the measured OD does not match a standard insulation bore, contact our technical team before ordering. We can help identify a compatible insulation product for the measured pipe OD and application.

OD vs Nominal Bore — What's the Difference?

Outside diameter (OD) is the actual measured external diameter of the pipe. For UK copper pipe, the stated size is the OD.

Nominal bore (NB) is a historical designation used for steel and iron pipe that does not correspond to the actual pipe OD. It originated from the internal bore of early cast iron pipes and has been retained as a naming convention even though modern pipe dimensions differ from the nominal value.

This causes confusion when insulating steel pipework:

Insulation for steel pipe should be selected using the actual pipe OD and the insulation manufacturer's stated pipe-size compatibility. Do not assume that the nearest nominal insulation bore is suitable solely from the rounded size description.

Always measure the actual OD of steel pipework before ordering insulation.

Standard UK Copper Pipe Sizes

15mm Copper Pipe

The most common domestic pipe size. Used for central heating flow and return pipework, hot and cold water supplies, and radiator connections.

• OD: 15mm
• Insulation bore required: 15mm
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

Shop 15mm pipe insulation · 15mm guide

22mm Copper Pipe

Standard domestic heating main size. Used for central heating flow and return mains, larger hot water circuits and connections to cylinders and boilers.

• OD: 22mm
• Insulation bore required: 22mm
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

Shop 22mm pipe insulation · 22mm guide

28mm Copper Pipe

Used in larger domestic heating systems and light commercial heating mains. Common for primary circuits in properties with high heat demand.

• OD: 28mm
• Insulation bore required: 28mm
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

Shop 28mm pipe insulation · 28mm guide

35mm Copper Pipe

Large domestic and light commercial heating pipe. Used in primary circuits for larger properties and light commercial heating mains.

• OD: 35mm
• Insulation bore required: 35mm
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

Shop 35mm pipe insulation · 35mm guide

42mm Copper Pipe

Standard commercial heating pipe. Used in commercial heating mains, plant room pipework and hot water primary circuits in commercial buildings.

• OD: 42mm
• Insulation bore required: 42mm
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

Shop 42mm pipe insulation · 42mm guide

54mm Copper Pipe

Large bore commercial pipe. Used in commercial heating mains, large hot water distribution systems and plant room headers in medium to large commercial buildings.

• OD: 54mm
• Insulation bore required: 54mm
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

Shop 54mm pipe insulation · 54mm guide

76.1mm Copper Pipe

Very large bore commercial pipe. Used in district heating mains, large commercial heating systems and industrial applications. Note: the EN 1057 declared OD for this size is 76.1mm. Insulation bore dimensions and tolerances vary by product, so check manufacturer pipe-size compatibility before ordering.

• OD: 76.1mm
• Insulation bore required: check product-specific fit
• Wall thickness: select according to the application, insulation product and applicable specification — see our BS 5422 Standards Guide

View 76mm pipe insulation products · 76mm guide

How to Measure Your Pipe for Insulation

1. Measure the outside diameter (OD) — use a tape measure or vernier caliper around the outside of the pipe. For copper pipe, the OD will match the standard size (15mm, 22mm, 28mm etc.)
2. For steel pipe — measure the actual OD, not the nominal bore. Use the table above and the insulation manufacturer's stated pipe-size compatibility
3. Select the correct bore insulation — for standard metric copper sizes, match the insulation bore to the pipe OD; for steel and larger/non-standard sizes, verify product-specific compatibility
4. Select the correct wall thickness — based on application, operating temperature and applicable specification

Read our full guide: How to Measure Pipe Insulation Size

Shop Pipe Insulation by Size

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Frequently Asked Questions

Is copper pipe measured by OD or bore?

UK copper pipe is measured by outside diameter (OD). 15mm copper pipe has an OD of 15mm, 22mm copper has an OD of 22mm, and so on. When ordering pipe insulation, the bore of the insulation must match the OD of the pipe for standard metric copper sizes.

What is the difference between nominal bore and outside diameter?

Nominal bore (NB) is a historical designation used for steel and iron pipe that does not correspond to the actual pipe OD. For example, 25mm NB steel pipe has an actual OD of approximately 33.7mm. Select insulation using the actual OD and the insulation manufacturer's stated pipe-size compatibility rather than assuming a rounded bore size.

What size pipe insulation do I need for 22mm copper pipe?

22mm copper pipe has an OD of 22mm, so you need 22mm bore pipe insulation. Wall thickness depends on the application and applicable specification — see our BS 5422 Standards Guide and Wall Thickness Guide for guidance.

What is the most common copper pipe size in UK homes?

15mm and 22mm are the most common domestic copper pipe sizes. 15mm is used for radiator connections and branch pipework; 22mm is used for heating mains and larger hot water circuits.

Can I use the same insulation on copper and steel pipe?

The same insulation material families may be available for both copper and steel pipework, but sizing must be based on the actual pipe OD and the product manufacturer's stated pipe-size compatibility. Steel nominal bore sizes do not directly correspond to actual OD.

For specification guidance, thickness selection and compliance context, visit the BS 5422 Resource Centre.

Related Guides

• What Thickness Pipe Insulation Do I Need?
• BS 5422 Pipe Insulation Standards Guide
• How to Measure Pipe Insulation Size
• Part L Compliance for Pipe Insulation
• Armaflex Self-Seal vs Standard Pipe Insulation