Copper pipe is sized by nominal diameter, but the outside diameter is the nominal size plus 1/8 inch. Type M and Type L differ in wall thickness. This chart gives the key dimensions.
| Nominal | OD (in) | Type M ID | Type L ID |
|---|---|---|---|
| 1/2 in | 0.625 | 0.569 | 0.545 |
| 3/4 in | 0.875 | 0.811 | 0.785 |
| 1 in | 1.125 | 1.055 | 1.025 |
| 1-1/4 in | 1.375 | 1.291 | 1.265 |
| 1-1/2 in | 1.625 | 1.527 | 1.505 |
M vs L: Type M has thinner walls and is standard for residential water lines; Type L is thicker for underground and commercial work. Both share the same outside diameter, so fittings interchange.
See the PEX Pipe Dimensions and PVC Pipe Dimensions.