When you're matching a grate, repair clamp, coupling, or tapping sleeve to an existing main, the outside diameter is the number that matters — and it is not the nominal size. Sizes now run 3" through 64". This chart covers the three OD systems you'll meet in waterworks: ductile iron pipe (AWWA C151), PVC C900/C905 (which shares the cast iron OD system), and IPS-sized steel, PVC, and HDPE.
| Nominal size | Ductile iron OD (in) — AWWA C151 | PVC C900 / C905 OD (in) — CIOD | IPS OD (in) — steel / PVC / HDPE | DI OD (mm) |
|---|---|---|---|---|
| 3" | 3.96 | — | 3.500 | 101 |
| 4" | 4.80 | 4.80 | 4.500 | 122 |
| 6" | 6.90 | 6.90 | 6.625 | 175 |
| 8" | 9.05 | 9.05 | 8.625 | 230 |
| 10" | 11.10 | 11.10 | 10.750 | 282 |
| 12" | 13.20 | 13.20 | 12.750 | 335 |
| 14" | 15.30 | 15.30 | 14.000 | 389 |
| 16" | 17.40 | 17.40 | 16.000 | 442 |
| 18" | 19.50 | 19.50 | 18.000 | 495 |
| 20" | 21.60 | 21.60 | 20.000 | 549 |
| 24" | 25.80 | 25.80 | 24.000 | 655 |
| 30" | 32.00 | 32.00 | 30.000 | 813 |
| 36" | 38.30 | 38.30 | 36.000 | 973 |
| 42" | 44.50 | 44.50 | 42.000 | 1130 |
| 48" | 50.80 | 50.80 | 48.000 | 1290 |
| 54" | 57.56 | — | 54.000 | 1462 |
| 60" | 61.61 | — | 60.000 | 1565 |
| 64" | 65.67 | — | 64.000 | 1668 |
Older grey cast iron mains vary by class and era: measure the OD in the field before ordering fittings for them. PVC C900 covers 4"–12" and C905 14"–48", both on the cast-iron OD system, which is why those columns match ductile iron.
Copper & CTS service tubing
Water service lines use copper tube size (CTS) — the OD your curb stop, main stop, and service fittings must match.
| Nominal (CTS) | OD (in) | OD (mm) |
|---|---|---|
| 3/4" | 0.875 | 22 |
| 1" | 1.125 | 29 |
| 1 1/4" | 1.375 | 35 |
| 1 1/2" | 1.625 | 41 |
| 2" | 2.125 | 54 |
Older grey cast iron mains
Grey cast iron pipe was made in several wall classes (historically Class A through D and beyond), and outside diameters changed across eras and foundries — a 6" cast iron main from 1935 and one from 1960 can carry different ODs. Two cautions worth taping to the truck: always field-measure the OD of an existing grey iron main before ordering a repair clamp or sleeve, and watch the 54" ductile trap — pipe made before the late 1980s used a 57.10" OD, while modern AWWA C151 54" pipe is 57.56".
Why nominal size and OD differ
Nominal size describes the approximate inside diameter a pipe family was designed around. Wall thickness varies by material and pressure class, so the outside surface — the one your fitting has to grip — lands on a different, standardized number. Two 8" pipes can differ by almost half an inch of OD depending on the system.
References: AWWA C151/A21.51 (ductile iron pipe), AWWA C900/C905 (PVC pressure pipe), ASME B36.10 (IPS). Verify against the governing standard for your project.