Few things cause more confusion on export orders than copper pipe sizes. A buyer asks for “half-inch” copper, the supplier quotes 15 mm, the fittings arrive in a third size, and nothing fits on site. The root of the problem is that there are two main sizing systems in use around the world, and they work in different ways. This guide explains both and shows how to specify copper pipe clearly.
The metric system: size equals outside diameter
In Europe, the UK, the Middle East, India and many other markets, copper pipe is specified by its actual outside diameter in millimetres. A 15 mm pipe really is 15 mm across the outside. This makes the system very easy to understand. Standards such as EN 1057 for water and heating, and EN 13348 for medical gases, use this approach.
Common metric sizes include 6, 8, 10, 12, 15, 18, 22, 28, 35, 42, 54 mm and larger sizes such as 76.1 and 108 mm. For each outside diameter, several wall thicknesses may be available, which affects pressure rating, weight and price.
The American system: nominal size is not the outside diameter
In the United States and in markets that follow American standards, copper water tube is specified by a nominal size in inches, under ASTM B88. Here is the catch: the nominal size is not the actual outside diameter. For copper water tube, the outside diameter is always one-eighth of an inch larger than the nominal size. So “1/2 inch” copper tube actually measures 5/8 inch, or about 15.9 mm, on the outside.
ASTM B88 also defines wall thickness types:
- Type K: the thickest wall, used for underground and higher-pressure service.
- Type L: medium wall, the most common choice for general plumbing and medical gas.
- Type M: the thinnest wall, used for lower-pressure applications where codes allow.
Medical gas tube to ASTM B819 follows the same outside diameters as B88 Type K and L tube.
ACR tube: sized by outside diameter in inches
Air conditioning and refrigeration (ACR) tube to ASTM B280 is different again. It is designated by its actual outside diameter in inches, so a 1/2 inch ACR tube really is 1/2 inch across. This means a “1/2 inch” ACR tube and a “1/2 inch” water tube are not the same size, and their fittings are not interchangeable.
Why fittings must match the tube system
Capillary fittings, such as elbows, tees and couplings, rely on a very close fit between the tube and the socket. A 15 mm tube in a fitting made for 5/8 inch (15.9 mm) tube leaves a gap that is too large for a reliable brazed joint. That is why the tube system and the fitting system must always match.
How to order correctly
- State the standard: for example EN 1057, EN 13348, ASTM B88, B819 or B280.
- State the size the way that standard does: millimetres for metric, nominal inches for water tube, actual inches for ACR.
- Give the wall thickness or type: such as 1.0 mm wall or Type L.
- Give the form: straight lengths, coils, annealed or hard drawn.
- Order fittings to the same system, and list reducing fittings with both sizes.
- When in doubt, give the outside diameter in millimetres as well. It removes all ambiguity.
A quick reference
As a rough guide, metric 15 mm sits close to American 1/2 inch nominal, 22 mm close to 3/4 inch, 28 mm close to 1 inch, and 35 mm close to 1-1/4 inch. “Close” is the key word: these sizes are not identical, and they should not be mixed in one system without proper adaptors.
Need help with sizes?
Nefton Exports Industries supplies copper tubes and fittings in metric and imperial systems for medical gas, plumbing and HVAC. Send us your schedule and we will check sizes against your standard before we quote.
