Short definition

In plumbing, pipe is sized by its inside diameter (nominal pipe size, NPS) and tube is sized by its outside diameter (CTS for copper tube size, OD on PEX). The distinction matters because fittings are sized to one convention or the other — a “1/2-inch” copper tube has a 5/8-inch OD; a “1/2-inch” galvanized pipe has an OD around 0.84 inches. They don’t interchange.

What it is

Two parallel sizing systems coexist in residential plumbing:

  • Pipe — galvanized iron, black iron, brass, and PVC schedule pipes. Sized by NPS, which is roughly the inside diameter (the actual ID varies a bit with wall thickness). Fittings are threaded or socket-welded to the NPS dimension.
  • Tube — copper (Type K, L, M), PEX, chrome supply lines. Sized by OD, with wall thickness specified separately. Fittings are sweat, compression, push-fit, crimp, or expansion to the tube OD.

A “1/2-inch” copper Type L tube has an OD of 5/8 inch and an ID around 0.545 inch. A “1/2-inch” galvanized pipe has an OD of about 0.84 inch and an ID around 0.62 inch. Same nominal label, different actual dimensions, different fittings.

Why it matters to a homeowner

The hardware store organizes by this distinction even if the signage doesn’t say so. The “pipe and fittings” aisle (galvanized, black iron, brass, PVC schedule) speaks NPS. The “supply tubing” aisle (copper, PEX, chrome supply) speaks OD or CTS. A push-fit coupling marked “1/2-inch CTS” fits 1/2-inch copper or PEX; the same nominal size in NPS fits something different.

When a quote says “1/2-inch copper” or “1/2-inch galvanized,” each is unambiguous. “1/2-inch pipe” alone is ambiguous — ask which.

Pipe fitting · Pipe schedule · Type L copper · Type M copper · PEX · Galvanized iron pipe · Flexible copper tubing · Pinched / undersized supply pipe