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.
How is Copper Pipe Measured in the UK?
UK copper pipe is measured by its outside diameter (OD) in millimetres — a key factor in BS5422 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.
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:
| Steel Pipe Nominal Bore | Actual OD (approx) | Insulation Bore Required |
|---|---|---|
| 15mm NB (½ inch) | 21.3mm OD | 22mm bore insulation |
| 20mm NB (¾ inch) | 26.9mm OD | 28mm bore insulation |
| 25mm NB (1 inch) | 33.7mm OD | 35mm bore insulation |
| 32mm NB (1¼ inch) | 42.4mm OD | 42mm bore insulation |
| 40mm NB (1½ inch) | 48.3mm OD | 48mm bore insulation |
| 50mm NB (2 inch) | 60.3mm OD | 60mm bore insulation |
| 65mm NB (2½ inch) | 76.1mm OD | 76mm bore insulation |
| 80mm NB (3 inch) | 88.9mm OD | 89mm bore insulation |
| 100mm NB (4 inch) | 114.3mm OD | 114mm bore insulation |
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
- Common wall thicknesses: 9mm, 13mm (domestic); 19mm (commercial)
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
- Common wall thicknesses: 9mm, 13mm (domestic); 19mm, 25mm (commercial)
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
- Common wall thicknesses: 13mm (domestic); 19mm, 25mm (commercial)
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
- Common wall thicknesses: 13mm (domestic); 25mm, 32mm (commercial BS 5422)
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
- Common wall thicknesses: 25mm (Kooltherm BS 5422); 32mm (Armaflex BS 5422)
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
- Common wall thicknesses: 30mm (Kooltherm BS 5422); 32mm (Armaflex BS 5422)
Shop 54mm pipe insulation · 54mm guide
76mm Copper Pipe
Very large bore commercial pipe. Used in district heating mains, large commercial heating systems and industrial applications.
- OD: 76.1mm
- Insulation bore required: 76mm
- Common wall thicknesses: 30mm (Kooltherm BS 5422); 40–50mm (district heating)
Shop 76mm pipe insulation · 76mm guide
108mm Copper Pipe
Maximum standard copper pipe size. Used in large commercial hot water primary circuits, district heating and large plant room headers.
- OD: 108mm
- Insulation bore required: 108mm
- Common wall thicknesses: 40–50mm (commercial and district heating)
Shop all pipe insulation by size
How to Measure Your Pipe for Insulation
- 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.)
- For steel pipe — measure the actual OD, not the nominal bore. Use the table above to find the correct insulation bore
- Select the correct bore insulation — the bore of the insulation must match the OD of the pipe for a snug fit
- Select the correct wall thickness — based on application, operating temperature and BS 5422 requirements
Read our full guide: How to Measure Pipe Insulation Size
Shop Pipe Insulation by Size
15mm | 22mm | 28mm | 35mm | 42mm | 54mm | 76mm | 89mm | 114mm
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.
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 33.7mm and requires 35mm bore insulation. Always measure the actual OD of steel pipework before ordering insulation.
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. The wall thickness depends on the application — 9mm or 13mm for domestic heating, 19mm or 25mm for commercial HVAC.
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?
Yes, the same insulation products work on both copper and steel pipe. The key is to match the insulation bore to the actual OD of the pipe — remembering that steel pipe nominal bore sizes do not correspond to the actual OD.
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