There are a lot of different types of screws to choose from, each suitable for a particular job. Screws are used in construction, carpentry and metalwork, but choosing the wrong screw size can have disastrous consequences – it may split wood or weaken a structure’s soundness. Understanding the three basic measurements used to determine screw size will help you make informed choices when buying collated screws online or at your local hardware store.
Screws are measured using a combination of a major diameter and threads per inch (TPI). The first number in a screw callout indicates the screw’s major diameter, and is usually expressed as an imperial measurement, such as 1/4 x 1. The second number is the screw’s threads per inch, which is normally given as an imperial measurement, such as 20 TPI. Screw threads are normally measured by counting the number of peaks on an inch-long segment of the screw’s thread. A screw with a coarse thread is thicker than one with a fine thread.
A third number will often be included in a screw callout. This is the screw’s tolerance class, which indicates how tight the screw fits into a hole or nut. Screws are rated in five classes, with class 1 being the loosest and class 5 being the tightest. You may also see a letter following the tolerance class, which indicates whether the screw is left-handed or not. This is important for some applications, since left-handed screws are threaded in reverse.
Finally, you may also see a head type indicated on screw packages. This is important, as a screw’s head shape determines how easily it can be driven into the material being fastened. There are several different head types, including Phillips, square and quadrex. Screws can also be classified as countersink or non-countersink, which indicates if they are designed to sit below the surface of the materials they are being driven into.
While measuring a screw’s length is straightforward enough, the width of the head can be confusing, especially when trying to convert between imperial and metric measurements. This can lead to people accidentally buying the wrong type of screw, which can waste time and money. To help, we’ve put together a handy chart that shows the measurements in both metric and imperial units. The chart also shows the conversion between a screw’s major diameter and its gauge. Simply find the measurement you need, and compare it to the table to find the appropriate screw. We hope this tool will help you buy the right screw for your project every time! screw sizes chart