Screw Diameter – How to Determine the Size of a Screw

When you’re screwing into wood, it can be difficult to know which size of screw you need. The length and type of screw is fairly self explanatory but the diameter can be a little more confusing. The basic screw thread design is based on a standard created in 1841 by Joseph Whitworth, which was commonly used throughout the British Empire and United States and is still used today in many types of hardware.

While most manufacturers use this thread standard, there are other designs that can be used as well including the metric system. Metric screw sizes are generally stepped up in a minimum of 0.5mm.

Screws are typically labelled with three figures – the first being the gauge of the screw which is the basic outer diameter of the thread. The second is the threads per inch (TPI) and the third is the shaft length in inches or millimeters if you’re using metric screws.

A screw’s shear strength is the amount of force a fastener can support in a straight line, while its tensile strength is how much stress it can hold when placed under tension in a perpendicular direction. Screws with a coarser thread are more suited for shear strength, while finer threads have greater tensile strengths.

Screws are usually labeled with a number followed by the hyphen and then a series of fractions of an inch. This means a screw labeled as #6-32 is actually 6 mm in diameter and has 32 threads per inch on the shaft. The diameter of a screw can be determined by measuring one side to the other and matching it to a screw gauge guide or a go/no-go gauge. #8 screw diameter

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