How to Measure Screw Diameter and Screw Length

When screwing things together, the size of the screw or bolt is crucial to ensuring that it will fit in the hole and hold fast. It is also important to make sure that the thread of the screw will work with the material it is being inserted into. Knowing how to accurately measure the diameter of a screw or bolt, as well as the length and head size, is an essential skill for any DIYer, mechanic, or professional.

In the imperial system, screw diameter is typically indicated by a single number or a letter (like #8 wood screws or hex nuts). Screws with rounded heads, sometimes called oval countersunk screws, are numbered from where the top of the rounded head and the flat underside meet in the middle.

Measuring screw diameter requires a high-precision caliper to avoid catching the threads as you are measuring. The major diameter is the widest point of the screw’s thread, which corresponds to the first number in the gauge number, while the minor diameter is the distance from the crest of one side of the screw’s thread to the root of the opposite side.

Screw length is determined by measuring from the very tip of the screw to where it would naturally sit flush on the surface once inserted. The length measurement can be a little tricky to determine for screw heads that protrude from the surface, like those on wood screws. The easiest way to figure out the length of a screw with a round headed or oval countersunk head is by measuring from where the arrow on the bottom of the screw head meets the top of the head, as shown in this image. #6 screw diameter

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