Screws are the backbone of many DIY projects, from wall framing to wood furniture. It’s no wonder the screw aisle at your local hardware store is always packed with a seemingly endless number of options! Different types of screws are used for different purposes, and each type requires a special fastener. For example, wood screws are designed to hold things in wooden materials, while metal screws are more useful for connecting items to a wooden base. To make sure you’re using the right screw for each project, it’s important to familiarize yourself with the intricacies of screw size, head type and length.
Screw size is determined by the major diameter and the thread pitch. The major diameter is how wide the screw is, while the thread pitch is how close together are the individual threads. Both of these numbers are found on a screw’s label, typically at the top, as well as on the box itself. You can also see this info by examining the screw with a magnifying glass.
Knowing how to read a screw size chart is essential to understanding the screw sizes on your box of screws. A good screw size chart will also tell you what head type and length of screw is needed for your particular application. Head type refers to the shape of the screw’s head, with the most common being a slotted or Phillips head. Other heads include bugle, fillister, hex, pan and socket and low socket head screws. Screw length is the overall length of the screw, from underneath the head to the end of the shank.
When choosing a screw, it’s also important to take into account its material and coating. Indoor screws can be made from less expensive materials like zinc, while outdoor screws need to be durable and protected against moisture and temperature change.
While determining screw size is relatively simple, there are some important differences between imperial and metric systems that you need to be aware of. The table below gives a rough comparison between imperial and metric screw size.
You’ll also find a screw length chart on this page that explains how to measure screw length in inches and metric units, as well as the recommended pilot hole sizes for various wood screw applications. This chart also shows the recommended tolerance class for each wood screw diameter, which can be helpful if you’re planning on using your screws with a specific nut or bolt.
The first column of the screw length chart shows you what each screw size is in inches, followed by the corresponding number in the metric system. The second column provides the recommended thread pitch in percent of the major diameter. This makes it easy to convert metric screw sizes to imperial and vice versa.
The final column of the screw length chart is an imperial and metric size conversion table that allows you to quickly and easily convert between standard screw sizes, including the popular wood screw diameters. The last column of the chart lists the general guidelines for each diameter and the recommended application for each. #4 screw diameter