Screw Diameters

Fasteners are sized in various ways to help ensure that the correct screw is used for each job. Screws are typically sized by driver type (flat, Phillips or hex), length and shank diameter. They can also be sized by threads per inch or fractional inches and metric millimeters.

Metric screw measurements usually list the diameter first, followed by the thread pitch — the space between the threads. Coarse threads have a larger diameter and are easier to find than fine pitch threads. The sizing system also includes sizes for single-start and double-start threadforms. Single-start threadforms have one “ridge” wrapped around the cylinder of the screw’s body each time the threads advance axially. A single-start thread’s lead is equal to the width of a single thread, while a double-start thread has two.

When identifying the size of a screw, the most important factor is the diameter. This measurement is the largest diameter of the screw’s threads along their peaks or “crests.” It can be measured with an accurate tool such as a vernier caliper, but we’ve simplified the process for convenience by creating a table that shows each screw’s diameter and number of threads per inch.

This information helps you match the correct screws with your materials, ensuring that the screw will fit well and hold securely. This is important because loose screws can weaken materials over time, causing them to come loose and fall out of place. By using the correct screw size, you’ll reduce the likelihood of this happening and help your project last longer. screw diameters