Screw Sizes – How to Measure Screw Diameters and Lengths
Screws come in a wide variety of sizes to suit many different construction tasks. Knowing how to measure screw diameter and length can help you choose the right screws for your needs and avoid mistakes that could cost time, money or damage the integrity of your project.
There are three main measurements that indicate a screw’s size: gauge, length and threads per inch (TPI). The gauge is the screw’s diameter; the pitch is the distance between adjacent screw threads; and the length is the overall screw length. Measuring these dimensions can be done by hand, with a ruler or tape measure, or by using a tool called a thread gauge. Most of the time, screw sizes are given in inches. Some are also given in the metric system, which uses millimeters. Screws are often referred to by their major diameter and thread count, for example, a size 6 screw has a major diameter of 6 mm and a thread count of 20 threads per inch.
The majority of screw sizing is determined by the Unified Thread Standard or UTS, an inch-based system that divides screws into different thread series like coarse threads and fine threads. This division allows for different combinations of head types, shaft materials and threads to be used with different applications. For example, a coarse thread screw can be used with wood or metal while a fine thread screw can only be used with metal.
Screws are usually listed with their gauge size and length on their packaging, with the thread count and a symbol following. The UTS chart lists the tolerance class, which indicates how tight or loose the screw fits into holes and nuts, the left-handed symbol (LH) and the length of the screw in a one-inch section. Screws are rated from class 1 to class 5, with 1 being the loosest fit and 5 being the tightest.
In addition to the UTS measurement system, there are a number of other screw sizing systems that are in use, including metric measurements and imperial system measurements. Screws sold with imperial system measurements list the gauge size first and then the length, for example, 10 x 2″. If they include the thread count, it will appear between the two numbers, for example, 10 – 35 x 2.
Metric screw measurements begin with the major diameter of the screw, which is usually represented in decimals, then the thread pitch, or how many thread peaks are in a one-inch section, followed by the length of the screw in mm. Unlike the UTS system, a thread pitch is not measured by counting the number of peak peaks but rather by measuring the distance between the two adjacent peaks with a caliper. Having an understanding of these measurement systems can be helpful when shopping for fasteners online or at your local hardware store. It can also help you determine which drill bit sizes are needed to drive the fasteners into your materials. 1/4 to mm