Industry Guide
Plumbing Pipe Sizes: Inches and Millimeters
Understanding pipe size designations, nominal vs actual dimensions, and inch-to-metric conversions for plumbing.
Last updated: 2026-03-15
NPS to DN (Metric) Conversion
| NPS (inches) | DN (metric) | Actual OD (in) | Actual OD (mm) |
|---|---|---|---|
| 1/4" | DN8 | 0.540 | 13.72 |
| 3/8" | DN10 | 0.675 | 17.15 |
| 1/2" | DN15 | 0.840 | 21.34 |
| 3/4" | DN20 | 1.050 | 26.67 |
| 1" | DN25 | 1.315 | 33.40 |
| 1-1/4" | DN32 | 1.660 | 42.16 |
| 1-1/2" | DN40 | 1.900 | 48.26 |
| 2" | DN50 | 2.375 | 60.33 |
| 3" | DN80 | 3.500 | 88.90 |
| 4" | DN100 | 4.500 | 114.30 |
Copper Pipe Sizes (Type M, L, K)
| Nominal | OD (in) | ID Type L (in) | ID Type M (in) |
|---|---|---|---|
| 1/2" | 0.625 | 0.545 | 0.569 |
| 3/4" | 0.875 | 0.785 | 0.811 |
| 1" | 1.125 | 1.025 | 1.055 |
| 1-1/2" | 1.625 | 1.505 | 1.527 |
| 2" | 2.125 | 1.985 | 2.009 |
Residential Pipe Applications
| Application | Typical Size | Material |
|---|---|---|
| Main water supply | 3/4" - 1" | Copper, PEX, CPVC |
| Branch supply lines | 1/2" | Copper, PEX, CPVC |
| Sink drain | 1-1/2" | PVC, ABS |
| Shower drain | 2" | PVC, ABS |
| Toilet drain | 3" - 4" | PVC, ABS, cast iron |
| Main sewer line | 4" | PVC, cast iron |
| Vent stack | 2" - 4" | PVC, ABS |
Why Nominal Size Does Not Equal Actual Size
Pipe sizing conventions date back to the early 1900s when pipes were described by their approximate inside diameter. As manufacturing improved and wall thicknesses changed, the outside diameters became standardized while inside diameters varied by wall thickness (schedule). The nominal size remained for compatibility but no longer matches any actual measurement. Always verify actual dimensions when mixing pipe materials or connecting to fittings.
Flow Rate by Pipe Size
| Pipe Size | Max Flow (GPM) | Max Flow (LPM) |
|---|---|---|
| 1/2" | 4-5 | 15-19 |
| 3/4" | 8-12 | 30-45 |
| 1" | 15-22 | 57-83 |
| 1-1/2" | 30-40 | 114-151 |
| 2" | 50-70 | 189-265 |
Convert pipe dimensions with our length converter.