PlayInfoBox

Home /Converter /US Machine Screw Sizes: Gauge Number to Millimeters

US Machine Screw Sizes: Gauge Number to Millimeters

US screws under 1/4 inch use a numbered gauge system instead of a fractional or metric measurement β€” here is how to translate a gauge number into an actual size.

Screw numbers are not metric or directly fractional

The US numbered system runs #0 through #12 (then switches to fractional inches like 1/4" for larger sizes), and each number corresponds to a specific, standardized diameter that has to be looked up rather than calculated from the number alone.

A few commonly used gauge sizes

#4 = 0.112 in (2.8 mm), #6 = 0.138 in (3.5 mm), #8 = 0.164 in (4.2 mm), #10 = 0.190 in (4.8 mm). These four sizes cover the large majority of small electronics, furniture, and appliance screws.

A rough estimation formula, not exact

Diameter in inches is approximately (gauge number Γ— 0.013) + 0.060. This gets close enough for a ballpark estimate, but for anything that needs to actually fit, checking a published chart against the exact gauge is more reliable.

A full screw callout adds thread count

A callout like "#6-32" means gauge #6 diameter with 32 threads per inch. The second number (threads per inch) is a separate spec from diameter and must match on both the screw and the nut or tapped hole for it to thread properly.

Why the US still uses a numbered system at all

The numbered screw gauge system predates widespread metric adoption in manufacturing and has stuck around because so much existing US hardware, tooling, and replacement-part inventory is built around it. Metric screws (M2, M3, M4, etc.) are common in imported electronics and increasingly in US manufacturing, but numbered gauges remain standard for many US-made small parts.

Why matching gauge alone is not enough

Two screws can share the same gauge number but differ in thread pitch (coarse vs. fine threads per inch), length, drive type, and head style. Matching only the gauge number when buying a replacement screw is a common mistake β€” the full callout, including thread count and length, is needed to guarantee a proper fit.

Frequently Asked Questions

Is a #8 screw bigger or smaller than a #6 screw?

Bigger. In the numbered gauge system, a higher number means a larger diameter, the opposite of some other gauge systems (like wire gauge) where higher numbers mean smaller. #8 (4.2 mm) is thicker than #6 (3.5 mm).

Can I substitute a metric screw for a numbered-gauge screw?

Sometimes, if the diameters are close enough and thread pitch is compatible, but they are generally not interchangeable by design β€” mismatched thread pitch between metric and US numbered screws will not thread properly even if the outer diameter looks similar.