A screw is a fastener that’s threaded and used to anchor items to wood or another material. Screws come in many shapes and sizes and are used for a wide variety of projects and tasks, from hanging pictures to fastening furniture together. The best screw for a specific project depends on the materials being used and how much torque will be applied. Knowing how to read a screw callout number is important, since it’ll help you determine what drill bit size to use and other important dimensions of the screw.
Screws are typically labeled with three numbers, the gauge, threads per inch and shaft length in inches. For example, a wood screw with the label 6-32 x 1 1/2″ has a head diameter of 32 threads per inch (almost double the normal thread count for that size), and is an inch and a half long.
Knowing how to read screw measurements is essential for any home or professional handyman. Screws are typically sized in either the imperial or metric systems, with the imperial measurement being more common. Screws that are labeled with a number in the imperial system have a gauge number that corresponds to a fraction of an inch. To figure out what size screw is being used, you can use a tool called a screw gauge, which will allow you to match up the gauge number with the proper fraction of an inch.
Screws that are sized in the metric system will have their diameter and shaft length measured in millimeters. The millimeter measurement is usually displayed as a whole number, with the decimal point being optional. A metric screw will also have its pitch (how far one thread is from the next) specified, which can be determined by counting the number of peaks on a one-inch section of the screw’s thread.
The last number on a screw callout is its tolerance class, which indicates how tightly the screw fits into holes and nuts. This number will be listed right after the screw’s major thread diameter and is based on a standard chart of the most common screw sizes, from class 1 all the way to class 5. You may also see a letter designation, such as LH, following the tolerance class to indicate that the screw is left-handed. This means that the screw will fit into holes or nuts in reverse, which can be useful in some cases. Screws are also sometimes marked with an asterisk to designate if they’re coated or uncoated, which can impact how well the screw holds in certain materials. Uncoated screws are more likely to rust, but coated screws will last longer in harsh environments. #5 screw diameter