Pipe Size Reference Chart: NPS, OD, ID & Wall Thickness | Apex Flow

Nominal Pipe Size (NPS) is the single most misunderstood dimension in piping: a "1-inch" pipe is neither 1 inch on the outside nor 1 inch on the inside. The outside diameter is fixed for a given NPS regardless of wall thickness, while the inside diameter shrinks as the schedule (wall thickness) increases. This master reference gives the actual OD, ID, and wall thickness for Schedule 40 and Schedule 80 pipe so you can spec fittings, valves, and flow capacity correctly.

Apex Flow Solutions supplies pipe fittings, nipples, and valves sized to these standards. Bookmark this chart — nearly every fitting and valve decision traces back to NPS.

In This Guide

• What NPS and Schedule Mean
• NPS to Outside Diameter Chart
• Schedule 40 & 80 Wall and ID Chart
• Why OD Stays Fixed
• Identifying Pipe Size by Measurement
• Standards & References
• Frequently Asked Questions

What NPS and Schedule Mean

NPS (Nominal Pipe Size) is a North American designation that loosely refers to the bore of the pipe — historically the approximate inside diameter. For NPS 1/8" through 12", the actual outside diameter does not equal the nominal size; for NPS 14" and above, the OD equals the nominal number in inches. "Schedule" is a wall-thickness designation: Schedule 40 is the standard wall, Schedule 80 is extra-heavy. Because the OD is held constant for a given NPS, a thicker schedule consumes inside diameter — Schedule 80 pipe has the same OD as Schedule 40 but a smaller bore.

The metric equivalent of NPS is DN (Diameter Nominal), where DN 25 corresponds to NPS 1", DN 50 to NPS 2", and so on. The schedule system is mirrored in metric as the same wall designations on the fixed metric OD.

NPS to Outside Diameter Chart

The outside diameter is fixed for each NPS across all schedules. This is the number to measure when identifying an unknown pipe.

NPS	DN (metric)	Actual OD (in)	Actual OD (mm)
1/8"	DN 6	0.405	10.3
1/4"	DN 8	0.540	13.7
3/8"	DN 10	0.675	17.1
1/2"	DN 15	0.840	21.3
3/4"	DN 20	1.050	26.7
1"	DN 25	1.315	33.4
1-1/4"	DN 32	1.660	42.2
1-1/2"	DN 40	1.900	48.3
2"	DN 50	2.375	60.3
2-1/2"	DN 65	2.875	73.0
3"	DN 80	3.500	88.9
4"	DN 100	4.500	114.3

Schedule 40 & 80 Wall and ID Chart

For each NPS, the wall thickness and resulting inside diameter for the two most common schedules. Note how Schedule 80 always yields a smaller ID than Schedule 40 at the same OD.

NPS	OD (in)	Sch 40 Wall	Sch 40 ID	Sch 80 Wall	Sch 80 ID
1/4"	0.540	0.088	0.364	0.119	0.302
1/2"	0.840	0.109	0.622	0.147	0.546
3/4"	1.050	0.113	0.824	0.154	0.742
1"	1.315	0.133	1.049	0.179	0.957
1-1/2"	1.900	0.145	1.610	0.200	1.500
2"	2.375	0.154	2.067	0.218	1.939
3"	3.500	0.216	3.068	0.300	2.900
4"	4.500	0.237	4.026	0.337	3.826

All dimensions in inches. Going from Schedule 40 to Schedule 80 on 2" pipe drops the bore from 2.067" to 1.939" — roughly a 12% loss of flow area for the same nominal size. Account for this when sizing for flow.