How Screw Sizes Are Measured

Choosing the right screw size is crucial to ensure your project turns out as planned. A screw that’s too long can cause damage to the material it’s attached to and one that’s too short won’t hold the materials together properly. To avoid both problems it’s essential to have a good understanding of how screw sizes are measured and what they mean.

Screws are commonly sized in three figures – the gauge, threads per inch (TPI) and length, either in inches or millimeters. A typical measurement will look like this – 6 x 32 x 1 1/2″, meaning the screw has a #6 diameter with 32 threads per inch and is an inch and a half long. Screws may also be rated with a TPI and shaft length that’s different from the standard gauge, for example, masonry screws are designed to attach brick or concrete without needing a lag or plug, so have a slightly longer shaft and use a different thread pattern than wood screws.

In the case of the first number, the gauge, it refers to the diameter of the screw’s outer threaded portion – sometimes called the major diameter. Any gauge size less than a quarter inch is usually labeled from #0 to #12, while anything above that is labeled with the size in decimal form.

The second number, the TPI, indicates how many full or partial threads are in one inch of screwed length – the “pitch” of the screw’s thread. Typically, the TPI will be affixed to the major diameter. If you don’t have a TPI chart, you can determine the TPI of a screw by using a thread gauge, which consists of strips of metal with various threaded sizes cut into them. You systematically work your way through the different threaded strips until you find the one that fits into your screw’s major diameter. 3/8 lag bolt pilot hole

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