Screws are an essential tool for a variety of construction projects. They come in a wide range of lengths and diameters to suit different needs. The callout on a screw box often includes three things: the screw gauge, its length and the threads per inch.
Each gauge number corresponds to a fraction of an inch in diameter. The other important detail is the shaft length, which refers to the part of the screw that drives beneath a surface.
Length
It’s important to select the correct length of wood screw for your project. There are a variety of factors that influence screw length: head-bore diameter, threads per inch and shaft-length in inches. In some cases, screw sizes are marked with all three measurements on the head-bore or shank, such as 6-32 x 1 1/2″. This indicates the screw has a #6 diameter, 32 threads per inch and is an inch and a half long.
To find out the correct screw size, look at the callout printed on the box of screws. This will include the diameter, threads per inch, and number of turns needed for a proper fit. Alternatively, you can use a ruler or caliper to measure the screw’s head-bore diameter and shaft-length. Screws also have additional width measurements beneath the heads and in the unthreaded sections (or shank) of the screws if they are not fully threaded. These are called root diameters or minor diameters.
Diameter
In the screw size world, there are several different measurements that can be used to specify a specific screw. These include the diameter of the shank, the number of threads per inch (TPI) and the overall length of the screw.
The diameter of a screw is usually written as the first number on the callout. The number is preceded by either a # sign or an mm symbol. For example, a #8 screw has a diameter of approximately 5/16ths of an inch.
Length is often specified at the end of the screw callout. This measurement is taken from underneath the head of the screw to the tip. Most screws have flat heads, which means they can be countersunk. Screws that can only be partially countersunk are referred to as non-countersinking screws. Examples of these types include hex-, pan-, button- and truss head screws.
In addition to the diameter, it is important to know how long a screw is before purchasing it for a project. Screw length can significantly affect the strength of a joint.
Threads
Screw thread dimensions are usually expressed in the form “major diameter x number of threads per inch.” If you’re unsure what size screw you need, check out our screw sizes chart for help.
The major diameter, or screw gauge, is indicated by the first number on the chart. Each successive number on the chart indicates a smaller screw major diameter. The chart also lists the threads per inch, or TPI, which is the number of threads per one-inch length of the screw.
The next part of the numbering system is the tolerance class, which is used to indicate if the screw fits tightly or loosely. Finally, the hex or square head is identified by the letter or symbol. If you have a lag screw application, we recommend choosing a hex head with a sharp point and spaced threads to allow self starting without the need for pre-drilled pilot holes. In addition, lubricating the screw’s threads frequently eliminates thread galling.
Material
The material that a screw is made from plays an important role in its strength and effectiveness. Most commonly, screws are categorized by their diameter and length. Screws that mate with nuts and washers are usually denominated based on their diameter, while those that only require an outer thread are often denominated by their length.
Diameter measurements are most reliable when they are taken with a caliper. A ruler can also be used, although it is not as accurate.
It is also helpful to understand the different factors that can affect a screw’s performance. For example, wood screws are prone to splitting when they penetrate hard materials. Adding a pilot hole can help reduce this tendency. The type of drill bit that is used will also affect the grip of the screw. In addition, the presence of fibers can add to or subtract from a screw’s holding power. Having a basic understanding of these factors can help you select the right screw for your project.1/4 screw diameter