What is the actual diameter of 1 inch pipe?

The actual outside diameter of 1 inch nominal pipe (NPS 1) is 1.315 inches (33.4 mm). The inside diameter depends on the schedule (wall thickness): Schedule 40 has an ID of 1.049 inches (26.6 mm), while Schedule 80 has a thicker wall giving an ID of 0.957 inches (24.3 mm). Nominal pipe size is a historical trade size that no longer corresponds directly to any measured dimension.

What is DN 25 in NPS?

DN 25 (Diameter Nominal 25 mm) is the metric equivalent of NPS 1 (1 inch nominal pipe size). Both designate the same pipe with an actual outside diameter of 33.4 mm (1.315 inches). DN is the ISO/European system for nominal pipe sizes; NPS is the North American system. DN numbers approximate the pipe's inside diameter in millimeters for older schedules, though the actual ID varies by schedule.

What does schedule 40 mean for pipe?

Schedule 40 refers to the wall thickness of the pipe. For any given nominal pipe size, the outside diameter is fixed, but the schedule number determines how thick the wall is. Schedule 40 is the lightest common schedule — used for most residential water supply and drain applications. Higher schedule numbers (Schedule 80, Schedule 160) have thicker walls, smaller inside diameters, and higher pressure ratings. Schedule 40 steel pipe is rated for 150–300 PSI depending on size; PVC Schedule 40 is rated for 220–480 PSI cold water.

Pipe Sizing Guide: NPS vs Actual OD, DN Metric, and Schedule 40 Wall Thickness

Why Pipe Sizes Are Confusing

Nominal pipe size (NPS) is a North American trade designation that originated in the 1800s when the nominal size roughly corresponded to the inside diameter. As pipe schedules evolved with thinner or thicker walls, the outside diameter was standardized — but the nominal name stuck. Today, a “1 inch pipe” has an outside diameter of 1.315 inches and an inside diameter that varies depending on the schedule.

The metric equivalent, DN (Diameter Nominal), is defined by ISO standards and follows the same naming convention — DN numbers do not exactly equal any measured dimension on modern pipe.

NPS to Actual OD Reference Table (Schedule 40)

NPS (in)	DN Metric	Actual OD (in)	Actual OD (mm)	Sch 40 Wall (in)	Sch 40 ID (in)
½	DN 15	0.840	21.3	0.109	0.622
¾	DN 20	1.050	26.7	0.113	0.824
1	DN 25	1.315	33.4	0.133	1.049
1¼	DN 32	1.660	42.2	0.140	1.380
1½	DN 40	1.900	48.3	0.145	1.610
2	DN 50	2.375	60.3	0.154	2.067
2½	DN 65	2.875	73.0	0.203	2.469
3	DN 80	3.500	88.9	0.216	3.068
4	DN 100	4.500	114.3	0.237	4.026

Common Residential Pipe Uses by Size

NPS Size	Typical Application	Common Material
½ in	Branch supply lines to fixtures (faucets, toilets)	PEX, copper, CPVC
¾ in	Main supply lines within the home	PEX, copper
1 in	Service entry line from meter to house	Copper, PVC, PEX
1½ in	Sink and laundry drain lines	PVC, ABS
2 in	Shower and tub drains, vent stacks	PVC, ABS
3 in	Toilet drain, branch drain lines	PVC, ABS, cast iron
4 in	Main drain / sewer line to street	PVC, cast iron

Understanding Schedule Numbers

The schedule number is not wall thickness in inches — it is a dimensionless number that, when multiplied by the allowable stress and divided by the service pressure, can be used to back-calculate wall thickness. In practice, simply remember:

Schedule 40 — standard wall; residential water supply, drain, waste, and vent.
Schedule 80 — extra-heavy wall; higher pressure or corrosive applications, exposed exterior.
Schedule 160 — very heavy wall; high-pressure industrial steam and hydraulics.
XXH (Double Extra Heavy) — heaviest commercial schedule; specialty industrial.

Use our length converter to convert pipe dimensions between inches and mm when working from metric engineering drawings or international supplier specifications.