1/8 Copper Tubing Size Guide: OD, ID, and Nominal Dimensions Explained

Copper tube sizing is nominal, not measured, so a “1/8” callout is a trade label rather than a physical outside diameter.

Nominal size, outside diameter, and inside diameter are three different numbers, and only one stays fixed across tube types.

How Copper Tube Sizing Actually Works

Nominal size is a trade label assigned to a tube family; the outside diameter for that nominal size is fixed by standard, and the inside diameter changes with wall thickness. So you cannot measure a copper tube and read its nominal size off a tape measure. For any nominal size, every type of copper water tube shares the same OD — walls get thicker or thinner underneath, changing the bore.

Three designations dominate US plumbing copper tube:

• Type K — heaviest wall, smallest inside diameter, for buried lines and higher-pressure work.
• Type L — medium wall, the everyday choice for interior water distribution.
• Type M — thinnest wall, largest inside diameter, for lower-pressure drain, waste, and vent plus some residential supply lines.

Because OD is locked in, the same nominal size in Type K and Type M will not have the same flow capacity, even though both look identical from outside.

What A “1/8” Callout Actually Refers To

A true 1/8-inch nominal size does not appear in standard copper water tube dimension tables, which begin at 1/4-inch nominal. The ASTM B88-22 table opens at 1/4 in nominal with an actual outside diameter of 0.375 in.

The nominal figure is a naming convention inherited from an earlier era of pipe wall thicknesses, not a dimension verifiable with calipers.

Steel pipe is a separate system entirely. Copper tube dimension charts do not translate to steel pipe sizing, so never borrow a steel pipe table to identify a copper tube.

Reading The Dimensions: A Working Reference

Reading a copper tube chart means pulling the outside diameter for the nominal size first, then subtracting twice the wall thickness for each type to get the inside diameter.

The formula: inside diameter equals outside diameter minus two times the wall thickness. Type M is not listed for 1/4 in nominal in standard tables. The ASTM B88-22 copper water tube dimension tables carry the full nominal-by-type grid.

When buying, matching the tube to the right fittings matters more than matching a chart — our tested roundup of 1/8 copper tubing options worth buying covers connection compatibility and pressure ratings.

Mistakes That Cause Real Problems

Treating nominal size as a measured dimension is the most common error, cascading into wrong fittings, wrong flow assumptions, and failed pressure tests. Four others:

• Assuming every tube of the same nominal size shares one inside diameter. Type K, L, and M at 3/8 in nominal run roughly 0.402 in, 0.430 in, and 0.450 in inside diameter.
• Borrowing steel pipe size conventions. Copper tube charts are their own system.
• Converting to metric without checking the inch table.
• Ignoring temper. Hard, annealed, and drawn tube install differently and are not interchangeable in every application.

Before cutting into a potable water or pressure line, verify the exact product listing for NSF/ANSI 61 compliance and any stated maximum pressure at temperature — those ratings are set per product, not per nominal size. If a listing says 1/8, confirm whether that means outside diameter, a refrigeration or instrumentation tube, or vendor shorthand before stating any dimension.