Often when you buy screws they will list the gauge number first then the length. This is usually the case for imperial system screw sizes. The gauge number corresponds to a fraction of an inch and the thread count corresponds to a decimal. This can be confusing for those unfamiliar with this measuring system. Length When looking at a screw size chart it is easy to get confused by the various different sizing systems that are used. In the metric system this is made a little easier by using a letter followed by two numbers – for example M3x40mm indicates that the screw has a major diameter of 3 mm and overall length of 40 mm. The Imperial system uses a number (gauge) followed by the head size and length of the screw. It is a nice coincidence that from screw gauges 6 upwards, the diameter of the head is approximately twice the diameter of the shank of the screw. We have included a handy table that enables you to convert from imperial sizes into metric sizes – simply select the appropriate column for your screw and a table will appear showing both the screw size in metric and the approximate imperial fraction equivalent – just click on a cell of each screw size to see a detailed drawing of the specific dimensions. Diameter The diameter of a screw is usually measured from the tip to the head. This can vary for different kinds of screws, but is important because it indicates how much material the screw will need to penetrate into the workpiece. A screw that is too long may protrude through the material and cause damage, while a screw that is too short will not penetrate enough and be ineffective. The metric system typically uses a “Gauge” figure to indicate the diameter, while the imperial system usually lists both the diameter and length in inches. The first number of a metric screw size, for example M6 x 1, gives the diameter of the threads in millimetres. The second number, for wood screws with a countersunk head, indicates the head diameter. It is also helpful to know the diameter of a screw when choosing which type of driver to use. This is because a slotted screw requires a Philips driver while a cross head screw needs a pozidrive or supadrive driver. Thread Pitch When a screw has more than one thread, the space between these threads is known as the thread pitch. The closer together the threads are, the shorter the lead (/’li:d/) of the screw and the less distance it covers in one revolution. Pitch and thread size are two different things, although they’re often confused with each other. For Unified and imperial threads, pitch is the number of threads per inch while for metric threads it’s the distance between two adjacent thread crests. Screw threads are almost never made perfectly sharp (no truncation at the crest or root), instead they’re truncated to achieve certain classes of fit. The standard for class of fit is defined by standards governing dimension, form and surface finish. These standards are used to ensure predictable mating between male and female threads and assured interchangeability of parts. Achieving these classes requires tight tolerances for both dimension and surface finish. To meet these tolerances, the process of creating a thread must include both subtractive processes, such as cutting (single-pointing and taps and dies) and forming; and additive processes, such as molding and casting. Material A screw’s material is a crucial factor in how it performs. Screws made of harder materials need a larger pilot hole to prevent them from stripping or snapping. Similarly, screws with coarser threads need a larger thread pitch to hold them. Knowing how to read a screw size chart will help you get the right fastener for your project. Whether you’re looking for standard or metric sizes, the full name of a screw size is listed in the chart by its major diameter and thread pitch. You can calculate the thread pitch by counting the number of thread peaks on a one-inch length, or you can use a caliper for more accuracy.
