How to Read Screw Sizes

The screw is used for a wide variety of tasks, from woodworking to metal roof installations. Choosing the wrong size can split a wood surface or weaken the integrity of a structure, so understanding how to read a screw’s measurements is essential for tradespeople. The three key values to know are screw gauge, threads per inch (TPI) and head type.

A screw’s gauge is an inch-based measurement that corresponds to a number in the range of 2 through 16. The lower the gauge number, the larger the screw’s diameter. For example, a Senco Duraspin washer-head screw with a #8 gauge has a major diameter of 0.17 inches.

Threads are a spiral shape that runs around the screw’s cylinder. They can be coarse, like a wood screw, or fine, as found on sheet metal screws and nuts. Screws have a TPI, or threads per inch, which indicates the number of thread peaks in a one-inch length. A screw with a fine TPI has a higher number of threads, while a screw with a coarse TPI has fewer threads.

Metric screw sizes are measured in millimeters and use a different system of measurement than inch-based UTS systems. To find a screw’s metric value, you need to determine its major diameter and thread pitch, which is the space between two adjacent thread peaks. Unlike TPI, this is more difficult to measure and requires a caliper or digital screw gauge. However, the metric system is slowly displacing the old-fashioned inch-based standards. 1/4 to mm