Whether you’re working in imperial units or the newer metric systems there are a few things to consider when choosing screw sizes. Imperial screw sizes are normally presented as a gauge by length, the gauge being the diameter of the screw thread (not the head). For example, a size 6 screw has a threaded section that is 6mm wide.
Metric screw sizes are usually stepped up in a number of sizes to make it easier to work out the correct diameter. This system allows for a much quicker identification of the screw size than using the Imperial ‘Gauge’ and length method. For example, a metric screw will be designated M6 x 40mm. This identifies that the screw has a major diameter of 6 mm and a total length from the head to the shank of 40mm.
The length is classed as the amount of screw that will be buried or exposed in the material. It is important that you know the length of the screw that you need as incorrect lengths can cause problems, such as protruding from the surface which could damage or cause injury.
The screw head is also very important, especially when working with wood. There are two main types of screw head – slotted and cross head, each requires a different type of spanner or driver. If you don’t have the right tool, it is a good idea to purchase a set of spanners that will cover all common screw sizes. screw size chart