How to Read a Machine Screw Size Chart

When working with fasteners, it’s essential to know how to read a machine screw size chart. This guide will help you decipher the callouts on a box of screws, find the right screw for your project, and understand key specifications like diameter and threads per inch. We’ve also included a US fraction to metric mm conversion chart to make it easier for you to work with international machine screw sizes.

There are two major screw thread systems that determine the size of a machine screw: the Unified Screw Thread System (based on inches) and the ISO Metric Screw Thread System (based on millimeters). This article covers both, so you’ll have everything you need to identify fasteners anywhere in the world.

How to Read a Screw Size Chart

Screws are typically measured and sold by three factors: diameter, pitch, and length. Diameter refers to the width of a screw’s threading, which is usually expressed in decimal form (ranging from #12 all the way down to #0000—super tiny). Pitch is the distance between threads and can be measured by measuring a fixed length of a screw and counting its threads. Length is the total length of the screw, which normally does not include its head (except for countersunk screws).

Most machine screws are specified by a combination of these factors. For example, a screw with a flat head (pan head, hex head, button head, or truss head) and a particular length is labeled as #6-321/2″. The major diameter of this screw is 0.138″, its threads are 32 threads per inch, and it’s 1/2 inch long.

When reading a machine screw size chart, you’ll want to pay special attention to the first number in the screw size designation. This is the screw’s diameter, and it will be indicated in either decimal or hex format. A screw with a larger diameter is more “fat” than one with a smaller diameter. If you’re using a hex tool, the major diameter will be displayed with a number followed by a hex wrench icon.

When a screw is labeled with a second number, this is the screw’s thread pitch. You can measure thread pitch in several ways, but most often by putting a thread gage or countersunk screw into your workpiece and then counting the number of threads that fit within one inch of its length. This method is quick and easy, but it may not be accurate for certain applications. You can also use a screw size calculator to find out how many threads are in an inch of a screw’s length. The calculator will also display the stress area for the screw. This information can help you decide whether a coarse screw or fine thread will be better for your application. This is a great place to start, but there are many other factors to consider when selecting the right fastener for your job. If you need more guidance, contact our team of experienced salespeople for recommendations. machine screw size chart

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