Needle gauges explained

A gauge number is a legacy size designation for the tube's outside diameter only. It fixes nothing about the bore: ISO 9626 requires the metric outer diameter, the gauge and the wall thickness together to designate a tube.

Why a higher gauge number means a thinner needle

The Birmingham (Stubs) gauge was never derived from measurement. In 1912, the Bureau of Standards described its basis as "the reductions of size made in practice by drawing wire from rolled rod. Thus, rod was called No. 0, first drawing No. 1, and so on." One mil is one thousandth of an inch. Each drawing pass thinned the wire. Higher numbers mean thinner wire. The Bureau contrasted that "utter irregularity" with the American Wire Gage, "the only wire gage now in use whose successive sizes are determined by a simple mathematical law." There is no conversion formula. Use the table for the diameter.

The first table compares the 1912 wire diameters with a current tubing supplier's nominal outside diameters. They agree at 20 of 22 sizes (10G–31G). The differences are 18G (49 vs 50 mils) and 20G (35 vs 35.5 mils). They reflect the wire and tubing specifications, not errors. Vita Needle Company says hypodermic tubing "is sized according to the Stubbs Iron Wire gauge (also known as the Birmingham Wire Gauge)." The scale was inherited, not redefined. A size-by-size listing is set out in our needle gauge chart.

ISO 9626:2016 calls gauge a "legacy size designation." Clause 3.2 says a gauge size corresponds to a designated metric size defining limits for outer diameters. Clause 5.5 requires three things to designate a tube: nominal outer diameter in millimetres, the corresponding gauge size (e.g. G31 or 31G), and wall thickness. A gauge number alone is incomplete.

The wall-class table for 21G shows the mechanism. The outside diameter is pinned between OD_MIN 0.800 mm and OD_MAX 0.830 mm. Minimum inner diameter spans 0.490 mm (RW) to 0.645 mm (UTW). American Tube Technology lists an OD of 0.050 in for all three wall classes at 18G. Wall thickness rises from 0.004 in to 0.0085 in, while the bore falls from 0.042 in to 0.033 in. At 25G regular wall, the bore is half the outside diameter.

ISO 6009:2016 says needle colour codes "shall indicate the nominal outer diameter of the tube of the needle." The colour code applies equally to regular-walled, thin-walled, extra-thin and ultra-thin needles. One colour therefore covers up to four possible bores at a single gauge — four at 21G and 30G, two at 25G. The standard does not specify that needles are to be colour-coded or where on the needle or packaging the colour is applied. The hub end of the needle is a separate fitting question, covered in luer lock vs luer slip.

Where to buy

No legitimate retail listing exists for the tubing this page describes, so none is linked.

Sources

• Circular of the Bureau of Standards No. 31 — Copper Wire Tables (1912) — Department of Commerce and Labor, Bureau of Standards
• ISO 9626:2016 — Stainless steel needle tubing for the manufacture of medical devices — International Organization for Standardization (preview via iTeh)
• ISO 6009:2016 — Hypodermic needles for single use, colour coding for identification — International Organization for Standardization (preview via iTeh)
• Vita Needle Company — "What is Hypodermic Tubing?" (blog, 2021-06-01) — Vita Needle Company
• American Tube Technology Inc. — Small Diameter Tubing Gauge Chart — American Tube Technology Inc.

Specifications last verified: 2026-07-31