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.
Matching a grate to a pipe end
We supply cast iron grates for corrugated steel, concrete, and HDPE pipe ends; see sewer grates & drain covers or the grate catalogue. This chart gives standard outside diameters. It doesn't confirm that a particular grate fits your pipe, so we check fit at quote. Send us:
- Pipe material (corrugated steel, concrete, HDPE, ductile iron, or PVC)
- Measured outside diameter at the pipe end
- Required grate pattern or drawing number, if your standard names one
Copper & CTS service tubing
Water service lines use copper tube size (CTS). That is 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.
Common size questions
What is the outside diameter of 4 inch ductile iron pipe?
4 inch ductile iron pipe (AWWA C151) has an outside diameter of 4.80 inches (122 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for a 4 inch main is sized to 4.80, not 4. PVC C900/C905 shares the same OD; IPS pipe does not.
What is the outside diameter of 6 inch ductile iron pipe?
6 inch ductile iron pipe (AWWA C151) has an outside diameter of 6.90 inches (175 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for a 6 inch main is sized to 6.90, not 6. PVC C900/C905 shares the same OD; IPS pipe does not.
What is the outside diameter of 8 inch ductile iron pipe?
8 inch ductile iron pipe (AWWA C151) has an outside diameter of 9.05 inches (230 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for an 8 inch main is sized to 9.05, not 8. PVC C900/C905 shares the same OD; IPS pipe does not.
What is the outside diameter of 10 inch ductile iron pipe?
10 inch ductile iron pipe (AWWA C151) has an outside diameter of 11.10 inches (282 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for a 10 inch main is sized to 11.10, not 10. PVC C900/C905 shares the same OD; IPS pipe does not.
What is the outside diameter of 12 inch ductile iron pipe?
12 inch ductile iron pipe (AWWA C151) has an outside diameter of 13.20 inches (335 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for a 12 inch main is sized to 13.20, not 12. PVC C900/C905 shares the same OD; IPS pipe does not.
What is the outside diameter of 16 inch ductile iron pipe?
16 inch ductile iron pipe (AWWA C151) has an outside diameter of 17.40 inches (442 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for a 16 inch main is sized to 17.40, not 16. PVC C900/C905 shares the same OD; IPS pipe does not.
What is the outside diameter of 24 inch ductile iron pipe?
24 inch ductile iron pipe (AWWA C151) has an outside diameter of 25.80 inches (655 mm). Nominal size is close to the inside diameter, so a grate, clamp or fitting for a 24 inch main is sized to 25.80, not 24. PVC C900/C905 shares the same OD; IPS pipe does not.
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.