Nominal pipe size, CTS and IPS

Entry in Sizes, codes and marks · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

Nominal pipe size is the trade name for a pipe's size, such as "1/2 inch" or "2 inch", and it is not a direct measurement of the pipe. Two sizing systems are common in US homes: copper tube size (CTS) and iron pipe size (IPS).

Each system fixes the outside diameter for every nominal size. Wall thickness then varies with the material, type or schedule, so the inside diameter changes while the outside stays put. That is what lets one fitting socket accept every wall weight of pipe in the same system.

1 Copper tube size (CTS)

CTS is the outside-diameter system used by copper water tube and by the plastics made to match it. One maker sells its CPVC pipe made to ASTM D2846 as "copper tube size" pipe, with an average outside diameter of 0.625 in at 1/2 in nominal, 0.875 in at 3/4 in and 1.125 in at 1 in^[2]. PEX tubing uses the same figures: a plumbing industry design guide lists 1/2 in PEX at 0.625 in actual outside diameter and 1 in PEX at 1.125 in^[4].

In CTS, then, the outside diameter is the nominal size plus 1/8 inch^[2][4]. The same guide notes that PEX can generally replace copper size for size^[5], and its glossary keeps three abbreviations apart: CTS (copper tube size), NPS (nominal pipe size) and NTS (nominal tubing size)^[5]. See copper pipe types K, L and M, CPVC pipe and PEX pipe.

2 Iron pipe size (IPS)

IPS is the second outside-diameter system. Schedule 40 PVC pressure pipe is made in iron pipe size to ASTM D1785^[3], and CPVC Schedule 80 pipe is made in IPS to ASTM F441^[6]. For PVC Schedule 40 DWV pipe, one maker lists 1-1/2 in pipe at 1.900 in outside diameter, 2 in at 2.375 in, 3 in at 3.500 in and 4 in at 4.500 in^[1].

IPS outside diameters do not follow a fixed offset from the nominal size. A pipe's schedule (40, 80 and so on) sets its wall thickness within the same outside diameter; see PVC Schedule 40 and PVC Schedule 80.

3 Why it matters

A 1-1/2 in CTS pipe and a 1-1/2 in IPS pipe share a name but differ by more than a quarter inch on the outside^[1][2]. Fittings, couplings and repair parts are made for one system or the other, so a CTS fitting will not seal on IPS pipe of the same nominal size. Where two systems meet, codes call for an approved adapter or transition fitting; the 2024 International Residential Code, Section P2906.18.3, requires one between different plastics or between plastic and another material^[7].

The print line on the pipe usually names the system and standard, for example "CTS" with ASTM D2846 or "IPS" with ASTM D1785; see reading pipe markings. Measuring the outside diameter with calipers and comparing it with a maker's table is the other check.