When replacing fasteners or purchasing new ones for a project, it's important to have an accurate measurement of the screw/bolt you already have. This will help ensure that you buy the correct size replacements and that they will fit together well. Generally, screws are sized by two different numbers; the first being the Gauge and the second the Threads Per Inch (also referred to as TPI). The gauge number indicates how coarse the screw's thread is, while the TPI number determines how many threads are on one inch of screw length.

Screws are numbered in both imperial (inch-based) and metric units, depending on when and where they were made. The numbering system differs slightly between the two, so it's best to use a screw size chart to reference when measuring a fastener or when purchasing new screws/bolts.

A screw size chart lists the equivalant measurements in both inches (fractions and decimals) and millimeters for various screw sizes. This is a helpful resource for those who aren't familiar with both systems or need to convert from one to the other.

The metric system uses a formula to calculate the major diameter of a screw/bolt - the first number listed on the screw size chart. The major diameter is calculated by multiplying the screw gauge (from zero to 10) by 0.013 in and adding 0.060 in. For example, a #8 screw has a major diameter of 0.164 inches. If you're unsure how to determine the diameter of a screw/bolt, a quick online search will provide several helpful guides and calculators.    screw size chart