What Size Is 1/4 Copper Pipe? | Real OD and ID Numbers

A 1/4 copper tube measures 3/8 inch (0.375 in) on the outside — the “1/4” is nominal size, not the actual OD or ID.

Grab a tape measure and a length of 1/4 copper tubing and the numbers won’t line up: the outside reads 0.375 in, not 0.25 in. That gap trips up plumbers, HVAC techs, and DIYers constantly, and it matters most at the fitting counter, where the wrong ferrule means a leak. The full dimension set, the common mistakes, and the weight per foot are below.

Why 1/4 Copper Pipe Isn’t 1/4 Inch

Copper tube sizing in the US uses nominal trade size, meaning the number is a label, not a measurement. The actual outside diameter for 1/4 nominal copper tube is 0.375 in (9.53 mm), and that OD stays identical across wall thicknesses — only the inside diameter changes.

The ASTM B88 standard governs these dimensions, and it’s the reference plumbers and supply houses use for US and Canadian imperial copper tube. This is copper tube sizing specifically; pipe language and tube standards don’t always align.

Inside diameter depends on which type you buy. Type M isn’t listed at this size in the standard dimension charts.

1/4 Copper Tube Dimensions At A Glance

Those dimensional tables come from the ASTM B88 copper tube dimension charts used across plumbing and HVAC material selection.

If you’re shopping for a coil or a cut length and want the practical buying breakdown, our tested guide to 1/4 inch copper pipe options covers what’s actually on the shelf.

The Fitting Mistake That Causes Leaks

Here’s the costly part: because a 1/4 nominal tube is 3/8 in on the outside, every fitting must match 3/8 in OD copper tubing — not “1/4 in OD.” Asking for a 1/4 in compression nut gets you the wrong part.

The failure chain is predictable. A homeowner reads “1/4 copper” on an old line, buys a 1/4 in OD compression ferrule, threads it on, and it never seats. The nut tightens down on a tube that’s too large for it, the ferrule crushes unevenly, and the joint weeps under pressure. Same story with flare fittings — the flare cone has to match 0.375 in OD or it won’t seal.

What to check before you buy:

• Outside diameter first. Measure across the tube with calipers or a good tape; if it reads 0.375 in, you have 1/4 nominal copper tube.
• Fitting callout language. Look for “3/8 in OD” or “3/8 in compression” on the package, not “1/4 in.”
• Wall type matters for ID. Type K runs tighter inside than Type L, which can affect flow in tight-radius coils.
• Tolerance drift. ASTM excerpts show OD tolerances around ±0.002 in in one edition and ±0.001 in in another, so verify against the edition your supplier cites if the job is exacting.

Heavier wall, tighter ID, slightly less flow — that’s the trade.

Quick Reference: Fitting And Type Cheat Sheet

• OD for every 1/4 nominal type: 0.375 in — the fitting size never changes.
• ID difference: Type K ≈ 0.305 in; Type L ≈ 0.315 in.
• Type M at 1/4? Not listed in the standard dimension charts.
• Ferrules and nuts: specify 3/8 in OD copper, every time.