Screw diameter refers to the width of a fastener’s threads, as measured along their peaks or crests. It is an important measurement when choosing a screw for a specific application because different diameters have distinct effects on the screw’s ability to hold or anchor.

Measuring screw thread diameter is simple enough, although you must take into account the type of screw head. For example, flat-headed countersunk screws rest flush against a material’s surface; for non-countersunk round-headed screws that are countersunk or have a rounded head, start the measuring point where the rounded top would rest on the surface.

You can also measure a screw’s threads using a thread gauge, which shows you the number of threads per inch, or TPI. This measurement is usually printed on the screw’s callout. For instance, a wood screw with a #4 diameter and 40 threads per inch has a major diameter of about 0.120 in (0.028 cm).

Some screw manufacturers use the UTS system to determine screw size, while others use the metric system. The UTS system typically lists the screw size, threads per inch and shaft length in inches together. For example, a wood screw with a #4 major diameter and 32 threads per inch has a total of 36 in (0.914 cm) when all the measurements are combined. The metric system will list the screw size, major diameter and shaft length in millimeters, preceded by the letter M. For the most accurate measurement, you should always use a precision measuring tool like a vernier caliper to get your screw sizes. #8 screw diameter

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