Whether you are a novice or an experienced builder, choosing the correct screws for your project can be confusing. There are many different sizes and types of screws available, each with its own special purpose. Knowing how to read a screw size chart is essential for making sure that you get the right type and length of fastener for your needs.
The first time you see a screw callout, it may seem intimidating, but once you understand what each number stands for, deciphering a screw size chart is actually pretty easy. The first number in a screw callout is usually the diameter of the threads, and the second number indicates the number of threads per inch. For example, a #4-40 screw has 40 threads per inch and a diameter of 0.112 in (0.28 cm).
Screws are usually labeled in either imperial or metric units, but most screw packages will use only metric measurements. If you come across a box of screws that are labeled in imperial, don’t worry – you can easily convert them to metric by using a conversion table.
In addition to the thread diameter and number of threads, a screw callout also contains other important information, such as the tolerance class, head style, and screw length. The tolerance class is a number that indicates how tight or loose the screw’s threads are. Screws with higher tolerance classes have finer threads, and are more likely to resist damage caused by vibration. Screws with lower tolerance classes have coarser threads, and are less likely to hold tightly.
The head style of a screw is the shape that sits above the screw’s shaft. Screws with a flathead are flush with the surface of the material, while screw with a Phillips head have a star-shaped cross-section. Screws with a countersunk head have an oval top that is recessed into the surface of the material, and are usually used for decorative purposes.
Screw length is the distance from the end of the screw to the tip of its threads. Screws of a particular length are usually designed to fit into a specific thickness of material, or to reach a certain point in the structure being built. The length of a screw can be adjusted by changing the threaded end or by cutting off excess screw.
When reading a screw size chart, you will also need to know if the numbers in the callout are imperial or metric. The imperial system uses fractions, while the metric system uses millimeters. You can find a conversion chart online to help you determine which measurement is being used in the callout for a given screw. In most cases, the imperial number will be followed by a letter to indicate what kind of thread standard it follows, such as UNC or UNF. The most common screw sizes are UNC, but there is also an extra-fine thread standard (UNEF) that is becoming more popular. You will also often see a code that indicates if the screw is left-handed, or LH. screw size chart