PEX is flexible plastic tubing sized like copper — the outside diameter is the nominal size plus 1/8 inch. This chart lists the common sizes and inside diameters.
| Nominal | OD (in) | ID (in) |
|---|---|---|
| 3/8 in | 0.500 | 0.350 |
| 1/2 in | 0.625 | 0.475 |
| 3/4 in | 0.875 | 0.671 |
| 1 in | 1.125 | 0.863 |
Color is convention, not rule. Red PEX is used for hot and blue for cold to keep runs organized, but the tubing is identical. 1/2 in serves most fixtures; 3/4 in feeds branches and manifolds.
FAQ
Is PEX pipe sized the same way as copper pipe? No. PEX is typically sized by nominal size which corresponds to a different actual inside diameter than copper of the same nominal size, so PEX and copper fittings and flow capacities aren’t directly interchangeable without checking the specific size conversion. Always check the manufacturer’s actual ID specification rather than assuming nominal sizes translate directly across pipe materials.
See the Copper Pipe Dimensions and Fixture Flow Rate Chart.