Copper tube: two systems, one millimetre apart
US copper (ASTM B88) is named by nominal size but measures nominal + 1/8″ outside; wall thickness picks the type — K (thick), L (medium), M (thin) — while OD stays constant. European copper (EN 1057) is named by its actual OD in mm. The trap: a measured 15.0 mm tube is European; 15.88 mm is US ½″. One millimetre decides which fittings fit.
US — ASTM B88 (Type K / L / M)
| Nominal | OD (in) | OD (mm) | K wall | L wall | M wall |
|---|---|---|---|---|---|
| 1/4″ | 0.375″ | 9.52 | 0.035″ | 0.030″ | — |
| 3/8″ | 0.500″ | 12.70 | 0.049″ | 0.035″ | 0.025″ |
| 1/2″ | 0.625″ | 15.88 | 0.049″ | 0.040″ | 0.028″ |
| 5/8″ | 0.750″ | 19.05 | 0.049″ | 0.042″ | — |
| 3/4″ | 0.875″ | 22.22 | 0.065″ | 0.045″ | 0.032″ |
| 1″ | 1.125″ | 28.57 | 0.065″ | 0.050″ | 0.035″ |
| 1-1/4″ | 1.375″ | 34.92 | 0.065″ | 0.055″ | 0.042″ |
| 1-1/2″ | 1.625″ | 41.27 | 0.072″ | 0.060″ | 0.049″ |
| 2″ | 2.125″ | 53.97 | 0.083″ | 0.070″ | 0.058″ |
Europe — EN 1057
| OD (mm) | Common wall (mm) | Replaces old imperial |
|---|---|---|
| 6 | 0.5–1.0 | — |
| 8 | 0.5–1.0 | — |
| 10 | 0.5–1.0 | 3/8″ |
| 12 | 0.5–1.0 | — |
| 14 | 0.7–1.0 | — |
| 15 | 0.7–1.0 | 1/2″ |
| 16 | 0.8–1.0 | — |
| 18 | 0.8–1.0 | — |
| 22 | 0.9–1.2 | 3/4″ |
| 28 | 0.9–1.5 | 1″ |
| 35 | 1.0–1.5 | 1-1/4″ |
| 42 | 1.0–1.5 | 1-1/2″ |
| 54 | 1.2–2.0 | 2″ |
| 66.7 | 1.2 | — |
| 76.1 | 1.5–2.0 | — |
| 108 | 1.5–2.5 | — |
EN 1057 walls vary with tube type (X/Y/Z) and national practice — the OD is the constant to identify by. Full EN diameter family runs 6–267 mm; the table shows plumbing-common sizes.