How Screw Sizes Are Measured

For woodworkers, determining the correct screw size for a project is an important step. Using an incorrectly sized screw can damage the material and make it difficult to secure or remove. The good news is that understanding how screw sizes are measured can help you avoid this problem.

Screws are sized in several ways, including driver type (flat, Phillips, square and hex), length, shank diameter and threads per inch. Each of these measurements has a specific purpose and impact on how a screw performs. The most important dimension to understand is the diameter of the screw threads, which is called the major or nominal diameter. This is measured along the crest or ‘peak’ of each thread, and not on the base or ‘root’ of the threads.

The next most common measurement is the number of threads per inch, which is usually listed on the package or in the catalogue. This number tells you how many threads are on a inch of the screw, and is a general indication of the screw strength and ease of installation. Screws with coarser thread pitches have more threads per inch, while those with finer threads have fewer threads per inch.

The last, and sometimes most confusing, measurement is the shaft length of the screw. Depending on the country of origin, manufacturers may measure this in mm or inches. If the screws are sold with imperial system measurements on the packaging, the gauge and length will be listed together, for example, 6-32 x 2″. Screws sold with metric system measurements often list the diameter first, followed by the length in mm. pilot hole for 1/4 lag screw

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