Screw Sizes and How They Are Rated

When screwing fasteners into wood or metal, it is important to have the right screw size for your project. Standard screw sizes were created to help ensure that the screws are compatible with the materials and can be anchored in properly. Screw sizing includes several factors: the driver type (flat, Phillips or hex), the length of the shaft and the number of threads per inch on the shank. In order to know which screw size to choose, it is necessary to understand how the standard screw sizes are rated.

Screw sizes are rated according to the Unified Thread Standard (UTS). The UTS includes a series of numbers that designate the head diameter, the shank diameter and the number of threads on the shaft. Each of these features is listed with a corresponding number on the screw chart, and these numbers are used to identify the proper screw size for any given job.

The first number on a screw chart is the screw gauge, which refers to the outer diameter of the screw. The screw gauges are designated from 0 to 14 and correspond with fractions of an inch. In addition to the gauge, a screw’s size is also specified in decimal form using a conversion table or calculator.

Threads per inch, or TPI, is the second number on a screw chart and indicates the amount of threads on the screw’s shaft. The screw’s TPI is determined by measuring the distance between two adjacent threads at their peaks, which can be done with a caliper. Screws with a higher TPI have more threads on their shaft than those with a lower TPI.

Screw length is the third number on a screw chart and specifies how long the shaft of the screw is. Screws with a pan head or flat head have their shaft measurement taken from the bottom of the head to the tip. Screws with a hexagonal head have their shaft measurement taken from the top of the head to the point where it begins to curve.

Most standard screw charts include a fourth number that indicates the tolerance class, which tells you how tight or loosely the screw fits into its mate. Screws with a tolerance class of 1 fit very loosely, while screws with a tolerance class of 5 fit tightly. Some screw charts may include a fifth number, which identifies whether the screw is left-handed. Screws with a left-handed designation have the letter L added to their TPI number. Left-handed screws are threaded in reverse to normal screws, so they will not fit into a standard threaded hole or nut. For this reason, it’s important to always check a screw chart to find the exact specification of any given screw. Using a screw size chart can save you time and effort when trying to find the correct screw for your job. 5/16 inch to mm