UK and European copper is named by its outside diameter, so 15mm copper is 15 mm (0.591") across and 22mm is 22 mm (0.866"). DN sizes name steel pipe, and DN15 is 1/2" pipe, 21.3 mm (0.840") across.
A metric size (15mm, DN20), an inch size (1/2, 3/4") or a width you measured (21.3 mm, 0.625").
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Metric copper and US copper
15mm and 22mm copper don’t match US copper. US 1/2" copper is 0.625" (15.9 mm) across and 3/4" is 0.875" (22.2 mm), so 15mm and 22mm pipe are a little smaller. Use a fitting made for the pipe you have. For US copper by type, see the copper pipe size chart.
The chart shows Table X tube, which has a 0.7 mm wall at 15mm and 0.9 mm at 22mm and 28mm. Thicker Table Y tube has a 1.0 mm wall at 15mm and 1.2 mm at 22mm and 28mm, so the outside stays the same and the inside is smaller.
Metric pipe size chart
Metric pipe size chart
| Size | Outside (OD) | OD in inches | Wall | Inside (ID) | ID in inches |
|---|---|---|---|---|---|
| 10mm | 10 mm | 0.394" | 0.6 mm | 8.8 mm | 0.346" |
| 12mm | 12 mm | 0.472" | 0.6 mm | 10.8 mm | 0.425" |
| 15mm | 15 mm | 0.591" | 0.7 mm | 13.6 mm | 0.535" |
| 22mm | 22 mm | 0.866" | 0.9 mm | 20.2 mm | 0.795" |
| 28mm | 28 mm | 1.102" | 0.9 mm | 26.2 mm | 1.031" |
| 35mm | 35 mm | 1.378" | 1.2 mm | 32.6 mm | 1.283" |
| 42mm | 42 mm | 1.654" | 1.2 mm | 39.6 mm | 1.559" |
| 54mm | 54 mm | 2.126" | 1.2 mm | 51.6 mm | 2.031" |
Lawton Tubes, Table X straight lengths. Inches are millimetres divided by 25.4; the inside is the outside less two walls.
| DN | Inch size (NPS) | Outside (OD) | OD in inches |
|---|---|---|---|
| DN15 | 1/2" | 21.3 mm | 0.840" |
| DN20 | 3/4" | 26.7 mm | 1.050" |
| DN25 | 1" | 33.4 mm | 1.315" |
| DN32 | 1-1/4" | 42.2 mm | 1.660" |
| DN40 | 1-1/2" | 48.3 mm | 1.900" |
| DN50 | 2" | 60.3 mm | 2.375" |
| DN65 | 2-1/2" | 73.0 mm | 2.875" |
| DN80 | 3" | 88.9 mm | 3.500" |
| DN90 | 3-1/2" | 101.6 mm | 4.000" |
| DN100 | 4" | 114.3 mm | 4.500" |
Steel pipe to ASME B36.10, from InfraBuild and Wheatland Tube data charts.
| US copper | Outside (OD) | OD in mm | Nearest metric copper |
|---|---|---|---|
| 3/8" | 0.500" | 12.7 mm | 12mm |
| 1/2" | 0.625" | 15.9 mm | 15mm |
| 3/4" | 0.875" | 22.2 mm | 22mm |
| 1" | 1.125" | 28.6 mm | 28mm |
| 1-1/4" | 1.375" | 34.9 mm | 35mm |
| 1-1/2" | 1.625" | 41.3 mm | 42mm |
| 2" | 2.125" | 54.0 mm | 54mm |
Copper Development Association tube tables. The nearest metric size is shown to compare.
whatadapterfor.com
DN sizes are names
DN (nominal diameter) names a pipe size in metric, the way 1/2" and 3/4" do in inches. Neither number is the width. DN15 steel pipe is 21.3 mm across and DN20 is 26.7 mm, so DN15 pipe and 15mm copper are different pipes.
The same DN can be a slightly different width in another standard. Australian medium pipe to AS 1074 is 26.9 mm across at DN20 and 76.1 mm at DN65, against 26.7 mm and 73.0 mm for ASME B36.10 steel pipe. Measure the outside when a fitting has to grip it.
Joining metric copper to a BSP thread
Metric compression fittings are sold as pipe size by thread. A 15mm x 1/2" fitting takes 15mm pipe at one end and has a 1/2" BSP thread at the other. Conex Bänninger makes its compression couplers in 15mm x 1/2", 22mm x 3/4" and 28mm x 1", among other sizes. Its female threads are ISO 228 (parallel BSP), and its male threads ISO 228 or ISO 7 (taper).
Kuterlite’s 12mm and 15mm compression fittings have 3/8" and 1/2" BSP coupling threads, so the body screws straight into a female BSP fitting or valve of that size. For a tap or an appliance, 15mm x 1/2" and 22mm x 3/4" also come as tap connectors. Tap connector sizes covers their washers and how far to turn the nut.
Across the threads, 1/2" BSP is about 20.96 mm and 3/4" BSP about 26.44 mm, both with 14 threads per inch. US pipe thread of the same size is almost the same width, and BSP vs NPT shows how to tell them apart.
Imperial copper (BS 659)
Imperial copper tube to BS 659 is named in inches. Conex Bänninger and Kuterlite both say 3/8" imperial tube takes their 12mm compression fittings with no adaptor, 1/2" takes 15mm and 2" takes 54mm.
For 3/4" imperial tube, Conex Bänninger changes the 22mm fitting’s compression ring for a 3/4" ring, and Kuterlite fits its K978C adaptor ring. At 1" the makers differ. Kuterlite says to use its 28mm fittings, and Conex Bänninger changes the ring for a 1" one.
Turn the water off and open a tap to drain the pipe before you cut it or undo a fitting.