Screw Sizes – How to Measure a Screw

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Screws are used to fasten a wide range of materials including wood, drywall, metal roofing and concrete. Using the proper screw size makes your job easier and can improve the longevity of your screws. Knowing how to measure a screw correctly is an essential skill for any tradesman. There are three basic screw measurements: gauge, length and threads per inch. This article explains how to measure a screw for these measurements and also helps you choose the correct screw for your project.

The most important thing to understand about Screw Sizes is that you cannot simply look at the screw and determine its size. You must know how to read the measurements on the screw packaging. Screws with imperial system measurements typically list the gauge first and then the shaft length in inches. They do not include the thread count on the label. If the thread count is included, it usually comes between the two numbers and indicates either a coarse or fine thread. Screws with coarse threads are known as UNC screws, while those with fine threads are called UNF.

If you are not familiar with the imperial system, Engineering Toolbox has a great table that translates screw gauges into decimal equivalents. The table also includes the number of threads per inch that each screw has.

Metric screw measurements are different from imperial and are used primarily outside of the U.S. Metric screw measurements are determined by diameter and pitch. When selecting a metric screw, you will need to know the material you will be using it in, because the thickness of the material will affect the strength of the hold. A general rule of thumb is that the screw’s length should enter at least half of the material’s thickness to ensure a strong hold. 1/4 in to mm

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