How to Measure a Screw Diameter

Getting the right screw for your project can mean the difference between a sturdy, long-lasting connection and a screw that falls out of place or breaks. Measuring a screw’s diameter can help you confirm that it will fit into the hole in your material without forcing it, and determine whether it has the right thread pitch for the type of materials it will encounter. The length of the screw can also influence the strength of your attachment.

Measuring the diameter of a screw involves measuring across the crest or peak of its threads, so you will need precise tooling to get accurate results. Using a vernier caliper is preferable to a standard ruler or tape measure for this task, as it will give you a more exact measurement.

Screws typically have three different measurements on their packaging: the gauge (or shaft diameter), threads per inch, and shaft length in inches. For example, a package labeled 6-32 x 1 1/2″ shows that the screw has a diameter of 6 mm, 32 threads per inch, and is an inch and a half long.

In the imperial system, the first number on a screw’s measurement refers to its diameter, which is also called its major diameter. Screws with a major diameter smaller than 1/4″ are often marked with numbers from 0 to 14, with the larger sizes designated by their decimal equivalent. For small imperial screws, the diameter is indicated by a series of zeros, with bigger zeroes signifying smaller diameters (for example, “#12″ equals 0.0783”). This naming convention gets trickier when working with very small screws: As the size increases, the number of zeros doubles, resulting in decimal equivalents like #0000 (which is super tiny). For metric screw sizes, they are typically listed by their nominal diameter, with the leading numbers representing the major diameter and the following number representing the length in millimeters. #8 screw diameter

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