Screws provide a more secure hold than nails and are often preferable for heavy materials and projects that require a bit of extra strength. While there are a variety of options, choosing the right screw size can be confusing. Luckily, there are a few key points to remember when selecting a screw size. The primary factors include the screw gauge, head type and screwdriver type. The final factor is the material on which the screw will be used. These guidelines will help you select the correct screw size for your project and ensure that it is firmly anchored to your material.
Most screws are sized using the UTS system, which has different thread series for coarse and fine threads. When a screw is labeled with the UTS system, the diameter of its major thread and the number of threads per inch (TPI) are listed on the package.
The screw's major diameter is usually given as a single or two-digit number, and the number of threads per inch is typically represented by a decimal or fraction, with larger numbers indicating larger screw sizes. The screw length is sometimes specified as well, although this is less common in the United States. Screws are also sometimes categorized by their tolerance class, which indicates how tightly they fit into nuts or threaded holes. The tolerance class is typically written in a number of 1 through 5, with higher numbers indicating tighter fits. In addition, some screws are marked with an L or R to indicate left-handed or right-handed threading.
In the U.S., a screw's diameter is designated by its gauge, with a lower number indicating smaller sizes. For example, a #0 wood screw has a very small diameter, while a #8 does not. The length of a screw is not always indicated, but it is important to choose a screw that will extend through the material by about half its width. A screw that extends much longer may cause the material to warp or buckle over time.
Screws sold in the U.S. are typically labeled with an imperial system measurement, but some suppliers also offer metric screw sizes. Imperial screws have a major diameter and a thread pitch that is represented by a letter or a number, such as 10 x 2 inches or M6 x 1. Metric screw sizes are based on the diameter of the major thread and the distance between threads, and can be measured with a metric threading tool. Engineering Toolbox provides a handy chart that converts imperial system measurements to metric sizing. It also converts metric screw diameters to imperial gauges and vice versa. For more details on screw sizing, check out our Screw Sizes Guide.3/8 in to mm