Copper Pipe Sizes

The Same Tube, Two Different Names

Where the American and Australian series both carry a diameter, they frequently call it something different — because one is naming the nominal and the other is naming the diameter itself.

One trade size apart, on an identical tube. Ordering by number across the two systems gets you the wrong diameter, and it will not announce itself — both tubes exist and both are correct where they were made.

ASTM B88 — the American Series

Four wall series on one set of diameters. K, L and M share an outside diameter at every size and differ only in wall, so they take the same fittings; DWV is drain, waste and vent tube and is designated on its outside diameter rather than on a nominal.

Type K

Type L

Type M

Type DWV

Showing ASTM B88, from the Copper Development Association handbook. 60 of 60 rows verified against m = pi * (OD - t) * t * rho, rho(Cu) = 8.94 g/cm3 = 0.323 lb/in3, against ID = OD − 2t, and against the nominal-plus-an-eighth rule — three independent checks, all of which must pass. How this data was verified.

The Eighth-Inch Rule

American copper tube has something steel pipe conspicuously lacks: a rule. The outside diameter of a B88 tube is its nominal size plus exactly one eighth of an inch, at every size in the series, with no drift and no exceptions. A 1-inch tube is 1.125 in across; a 2-inch tube is 2.125 in.

That makes copper the easy case for identifying a tube you already have: measure the outside, subtract an eighth, and you have its trade name. Steel offers no such shortcut, which is why how to measure pipe size has to work from a lookup table instead, and why nominal pipe size explained is a history rather than a formula.

What We Could Not Verify

Three copper standards that belong on this page are not on it, and each is absent for a stated reason rather than an oversight.

EN 1057

OD x wall matrix deliberately not compiled: the only free copy was rate-limited and its cell grid collapsed on extraction. Do not invent it.

JIS H3300

Not parsed in this pass.

ASTM B280

ACR tube not parsed in this pass; it is sized on the outside diameter, unlike B88.

4 of the 116 rows parsed for this page failed a gate and are excluded — 33 Australian and 19 New Zealand rows survived, out of 35 and 21 attempted. The failures are published masses that do not reconcile with the diameters printed beside them, which is a fault in the catalogue rather than in the standard. Everything found that way is listed at errors found in published pipe data.

Copper Against the Other Materials

Copper is the outlier here in every direction. Its diameters are on a series of their own, its size label follows a rule no other material has, and the rule changes at the border. For the materials that do share the steel series, see steel pipe sizes, stainless steel pipe sizes and galvanized pipe sizes. For the other material that runs its own diameters, ductile iron pipe sizes.

The pattern of one label meaning several things is not unique to copper, only sharpest in it — what DN actually measures follows the same problem through the steel standards.

Frequently Asked Questions

What is the outside diameter of 1/2-inch copper pipe?

Under ASTM B88, the American standard, 15.88 mm — 0.625 inches. That is the nominal size plus an eighth of an inch, which is how every B88 size is built. It is not 1/2 inch across anything: the label is a name, not a measurement.

Why is copper pipe measured differently from steel pipe?

Copper tube is designated on a rule of its own. ASTM B88 fixes the outside diameter at the nominal size plus 1/8 inch exactly, at every size, with no exceptions — we check all 52 of the rows here against that rule and the page refuses to build if one fails. Steel pipe has no such rule; its nominal size drifted away from both the bore and the outside diameter for historical reasons instead.

Is UK or Australian 15 mm copper the same as US 1/2 inch?

No, and they are not the same as each other either. The three countries do not merely round differently, they label different things: ASTM B88 sizes by nominal, AS 1432 sizes by outside diameter, NZS 3501 sizes by bore. A tube ordered by number in one country and supplied to another country's standard is a different tube, not a tolerance away.

What are Type K, L and M copper?

Three wall thicknesses on one set of outside diameters. K is the heaviest, L is the usual choice for water supply and M is the lightest, and all three share a diameter at any given size — so they take the same fittings and only the bore changes. DWV is a fourth, thinner still, and it is sized on the outside diameter directly rather than on the nominal.

Do copper and steel pipe of the same size use the same fittings?

No. A copper tube labelled 1/2 inch is 15.88 mm across; a steel pipe labelled 1/2 inch is 21.3 mm. They are different products on different diameter series, and nothing that locates on the outside will cross between them.