Common standard bolt diameters range from 1/4 inch to 1 inch, and sizes are often given on a scale of #0 to #10. Typically, sizes include a second and third number reflecting the characteristics of the threading and the length of the screw. Therefore, a common bolt size is #4-40 x 5. The Unified Thread Standard serves as a standard set of measurement for various fasteners.
The American Society of Mechanical Engineers maintains the Unified Thread Standard. The United States and Canada rely heavily on this standards to identify screw and bolt sizes for various applications. Bolt sizes cover a variety of dimensions including the diameter of the bolt itself, the distance between the threads and the length of the bolt.
Sizes are given in a variety of formats including fractions of inches, numbered denominations determined by the ASME on a scale of #0 to #10, or in millimeters in the metric system. Typically, bolt sizes are given in a set of three numbers. The first number indicates the largest diameter of the bolt. Some applications provide this measurement in terms of inches or millimeters, in which case the size of the bolt is simply a reflection of the actual diameter. When the size is given on a scale of #0 to #10, the number reflects the bolt diameter multiplied by 2, then added to 0.060 inches. For example, the #4 bolt has an actual diameter of 0.112 inches.
The second number in the set reflects the threads per inch in Unified Standard measurements or the distance between threads in metric measurements. The third number reflects the length of the bolt between the material surface and the end of the bolt.