As the use of wood screws continues to increase in the manufacturing world, it is important that all woodworkers understand how screw sizes are determined. Sizing is based on driver type, length, shank diameter and threads per inch. The sizing of fasteners is also important to ensure that they will fit into the threaded holes of other hardware such as nuts and bolts.
Screw sizes are typically referred to according to the standard screw threads of ASME/ANSI, called the Unified Thread Standard (UTS). A UTS screw chart will typically list three things: the screw gauge, the number of threads per inch, and the shaft length in inches. The shaft length is often found in the middle of the sizing and it is typically indicated with an “x” or a fraction such as 6-32 x 1 1/2″.
The first measurement is the screw gauge, which is usually an inch-based number. The larger the gauge number, the larger the screw size. Screws with a major diameter less than 1/4″ are normally labeled from #0 to #14. The gauge number is followed by the number of threads in one inch of a screw, which is often noted as “threads per inch,” or TPI. A screw with a coarse thread will have a higher TPI than a fine threaded screw.
For example, a screw with a major diameter of 1/2″ will have 32 threads in a one-inch length and the screw is therefore designated as a coarse screw. There are also a variety of finer threaded wood screws available, but the numbers don’t match with the standard coarse and fine screw gauges.
It is also possible for a screw to have no major diameter and instead be designated simply by the shaft length. This is often the case with finish screws, which have smaller heads than standard wood screws and are designed to be driven just under the surface of the wood, leaving a tiny hole that can easily be filled with wood putty.
The last two measurements are not as common, but they are important to note. Some manufacturers will indicate the head style of a screw, such as flat or Phillips, and some may also include the drive type or symbol (LH or RH). This is usually not important to the average woodworker as most manufacturers offer both types of heads and neither are especially difficult to work with. For those who will be using metric screws, most suppliers will include the diameter, threads per inch and shaft length in mm on their charts. It is important to know these differences if you will be working with international standards or converting from imperial measurements. For more information on screw sizes, check out our article, Screw Sizes Explained. Then you can start using them with confidence. 3/8 in to mm