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.

Matching fittings to an existing pipe?

Measure the pipe OD and use the chart below to identify the NPS, then order fittings to that nominal size. Contact our team if you are unsure of the schedule or material.

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.

Cross-section comparing Schedule 40 and Schedule 80 pipe of the same nominal size

Same NPS, same OD — but Schedule 80's thicker wall reduces the inside diameter and flow area compared to Schedule 40.

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.

Why OD Stays Fixed

The outside diameter is held constant across schedules so that the same threading dies, fittings, and tooling work regardless of wall thickness. A 1" Schedule 40 and a 1" Schedule 80 pipe both have a 1.315" OD, so a 1" NPT fitting threads onto either. This is the practical reason fittings are ordered by NPS, not by ID — and why you identify an unknown pipe by measuring its OD, not its bore.

Identifying Pipe Size by Measurement

To identify an unknown pipe, measure the outside diameter with a caliper (or measure the circumference with a flexible tape and divide by π = 3.1416), then match the OD to the chart above. For example, a measured OD of 1.315" is NPS 1", and 2.375" is NPS 2". Do not measure the bore — two pipes of the same NPS can have very different IDs depending on schedule. Once the NPS is known, measure wall thickness or compare the ID against the schedule chart to determine whether you have Schedule 40, 80, or another wall.

Stack of steel pipe sections of increasing nominal size

Outside diameter is fixed per nominal size — the basis for interchangeable fittings and threads.

Standards & References

Carbon and alloy steel pipe dimensions are defined by ASME B36.10M; stainless steel pipe by ASME B36.19M. PVC and CPVC pipe follow the same NPS/OD system under ASTM D1785 (Schedule 40/80 PVC) and ASTM F441 (CPVC). NPT pipe threads are governed by ASME B1.20.1. The DN metric designation is defined in ISO 6708. These standards fix the OD and wall thickness; pressure ratings depend additionally on material, temperature, and joint type.

Frequently Asked Questions

Why is the OD of 1 inch pipe 1.315 inches and not 1 inch?

Nominal Pipe Size refers loosely to the historical bore, not the outside diameter. The OD is standardized at a fixed value (1.315" for NPS 1") so that fittings and threads are interchangeable across all wall schedules.

What is the difference between Schedule 40 and Schedule 80 pipe?

Both share the same outside diameter for a given NPS, but Schedule 80 has a thicker wall and therefore a smaller inside diameter and higher pressure rating. Schedule 80 reduces flow area — about 12% on 2" pipe — compared to Schedule 40.

How do I measure pipe size?

Measure the outside diameter with a caliper, or measure the circumference and divide by 3.1416, then match the OD to a pipe chart to find the NPS. Do not measure the inside diameter — it varies with schedule.

Is NPS the same as the actual inside diameter?

No. NPS only approximates the historical bore. The real inside diameter depends on the schedule (wall thickness) and is always different from the nominal number.