Screw Sizes – What You Need to Know

Screws are a great alternative to nails for many applications as they tend to hold much stronger and longer than their counterpart. However, picking the right screw size can be difficult if you don’t know what to look for. Screw sizes are usually listed with three figures – the gauge (also known as the major diameter), threads per inch, and length either in inches or millimeters.

The gauge number, or major diameter, is a measure of the outside threads on the screw’s cylinder and is used to determine the size of the threads. The higher the gauge number, the finer or tighter the threads. The second number, the threads per inch, is a measurement of how many thread peaks are on an inch of screw. Screws with coarser threads have lower thread counts and are designated as Unified Coarse Thread (UNC). Screws with a finer thread have a higher thread count and are designated as Unified Fine Thread (UNF).

In North America, screw sizes are typically represented in inches. These numbers, ranging from #0 to #14, are based on the gauge of the major diameter and indicate how many threads per inch is available on the screw. Engineering Toolbox has a handy chart that lists the screw gauges and decimal equivalents.

Screws in Europe and Australia are normally listed using a metric system, which is denoted by the letter M. This is usually followed by the screw length which can be in either millimeters or inches, depending on the supplier.  1/4 in to mm