When working with screws, getting the right size for the job is essential. A screw that is too loose won’t hold, while one that is too tight can cause damage to the material and create a hazardous situation. The best way to avoid these problems is by learning three important screw measurements: gauge, threads per inch (TPI) and length.
Screw sizes are usually listed on packaging using either the imperial or metric system. Screws listed using the imperial system will list the gauge (or diameter) first and then the length, while screws that use the metric system will list the thread count after the diameter. Screws that use the metric system generally have a single number for the diameter, while those in the imperial system have a numbered system that starts at 1 and goes up to 12 (with a number of zeroes in between, making it easy to remember).
There are a few different ways to measure screw sizes, but the most accurate method is with a digital caliper. Calipers are able to provide measurement accuracy down to the thousandths of an inch or millimeter, so you can be sure that you’re getting exactly the correct size screw for your needs. A standard ruler or tape measure can also work in a pinch, although the results will be less precise. 1/4 lag screw pilot hole