When working with screws and bolts, you need to know how to read their callout numbers. The information on the callout is essential in determining the correct screw size for your project. The information includes the screw gauge, threads per inch and length. It’s important to understand these factors because you can then determine the drill bit sizes that are required for drilling into any material.
The numbering system for machine screws in the United States is called the Unified Thread Standard, or UTS. The gauge is the first part of the numbering system, and it’s what defines the diameter of a screw. The number is then followed by a decimal and a fraction, or a letter, which determines the screw’s thread pitch.
For instance, a screw with a #1 gauge has a diameter of 1/16 inch. A #2 screw has a diameter of 5/64 of an inch, and a #3 has a diameter of 3/10 of an inch. A #4 screw has a diameter of 3/16 of an inch, and a #5 screw has a diameter of 1/4 of an inch.
If the screw has a flat head, its diameter is measured from beneath the head. However, if the screw has a round or hexagonal head, it’s measured from the bottom of the head. Screw length is also measured in a different way depending on the type of screw. For example, a countersunk screw is measured from the end of the shank below the head to the tip. Other types of screws, like truss-, hex-, or button-head, are measured from the bottom of the head.
Screw length is determined by a factor of two, the gauge and the threads per inch. Some screws will include the threads per inch right on the packaging, while others will list it as a separate number. Screws with a coarse series will have more threads per inch than those with a fine series. For example, a screw with coarse threads has 32 threads per inch, while a screw with fine threads has 36 threads per inch.
In addition to the screw size, it’s important to understand what TPI means and how it’s measured. TPI is the number of threads in one inch of a screw, or more specifically, the distance between the center of two adjacent screw threads. This is important to know because a screw with more threads will have more strength than a screw with fewer threads, even if it’s the same diameter.
It can be tricky converting between the imperial and metric systems, especially when reading callouts on a box of screws. We’ve put together a helpful table below that shows the equivalant measurements of screw size in inches (fractions & decimals), millimeters and UNC and UNF threads. This will help you cross reference when using a digital caliper or when drilling a hole for the screw. The table also provides the diameter of the screw shank, or shaft, and the head diameter, and the overall length of the screw. machine screw size chart