Machine screws, or threaded fasteners, come in a wide variety of sizes. Choosing the right size for your application is vital to ensure compatibility with existing threaded holes or to prevent the screw from becoming loose. Screw dimensions are usually listed with two numbers separated by a dash. The first number indicates the screw’s diameter and the second indicates the thread pitch. A screw labeled “3/8 – 16” has a diameter of 3/8 inches and a thread pitch of 16 threads per inch.
In addition to size, machine screws are also available in different head types, materials, and thread gradings. A machine screw can be either coarse or fine threaded, and a number of head styles are available including flat heads, hex heads, and slotted heads. Most machine screws are designed to be drilled into the workpiece, but some self-tapping varieties can drill their own holes.
While there are many different standards for measuring screw and bolt sizes, the metric system is used by most of the world. However, most American machine screws follow the standard measurements of the Unified Thread Standard (UTS) system and are measured in inches.
Metric screw threads are designed in the form of a symmetric V-shaped profile. The flanks of the V have a 60deg angle to each other, and the height H of the V is defined by the pitch, or distance between thread pitches. The major diameter Dmaj and the minor diameter Dmin are the maximum dimensions of a metric thread. The thread profile must be flat at Dmaj and rounded out below Dmin, but may have additional facets above Dmin depending on the application.
A major difference between UTS and ISO metric screw threads is that the major diameter and stress area are defined by Dmaj, whereas the pitch is determined by the distance between the threads, or P. As a result, a metric screw with a larger pitch will have a smaller stress area than an ISO screw with the same pitch, and vice versa.
Screws sold with imperial systems often include the gauge, or diameter, and length on their packaging, but don’t typically list the threads per inch. To measure the thread pitch, you’ll need a tool called a thread gauge. A thread gauge resembles a spark plug feeler gauge and will have multiple metal leaves with serrated edges that match the thread profile of common screw sizes. The gauge should fit snugly over the screw and be held against it by hand so there is no light between the screw and the serrated edge of the gauge.
Screw nomenclature can be confusing. Manufacturers often use different terminology to describe the same fastener size, making it difficult to cross-reference. For example, some manufacturers will say a panel fastener fits a 1/4″ hole while others will list it as fitting a 6.35mm hole. A machine screw size chart makes it easier to find the correct fastener for your project. The following chart lists the equivalant measurements in inches, fractions, decimals, and millimeters for several popular machine screw sizes. machine screw size chart