Screw Sizes – What You Need to Know

Screws come in a huge selection for a multitude of tasks. Choosing the right ones ensures they will hold a load and stay securely fastened to material.

The size of a screw is determined by its gauge, length and threads per inch. These are three essential measurements every tradesperson should know.
Length

One of the most important things to know about screw sizes is length. Generally speaking, screws must be long enough to pass through the material they are fastening and firmly engage the internal thread of the mating piece. However, they also must be short enough to prevent damage and allow for easy driving and removal. For this reason, many types of screws have a specified length that is not necessarily consistent with their shaft length. These are known as ‘nominal’ lengths and can be found in product names or catalogue listings.

For example, a wood screw might be marked as 6 x 1. This indicates that the screw has a #6 diameter and 32 threads per inch, and is an inch and a half long. It should be noted that the equivalent stress of the bone-screw interface decreases with increasing screw length and diameter. Using longer screws can be more expensive than shorter ones but can provide a stronger, more stable hold.
Diameter

The diameter of a screw is the largest dimension that delimits the thread profile. It is measured on a cross-sectional view taken along the peaks or ‘crests’ of the thread.

Screw diameters are generally standardized to avoid confusion and allow for easy identification of the correct screws for various projects. Accu recommends using a high-quality vernier caliper for these measurements.

The second number on the chart, which is hyphenated and followed by the major diameter number, identifies the thread size. It is calculated by counting the number of thread crests per inch on one side of the screw and multiplying that measurement by the major diameter number.

This measurement is often converted to metric by simply dividing by 10. For example, a 6-32 screw would be described as having a 3.2mm major diameter and 32 threads per inch. This sizing system was originally developed to ensure interchangeability between American, British and Canadian screw threads during World War II.
Threads

The threads of a screw affect several characteristics, including how easily it can be tightened or loosened. They also have a direct effect on the strength of the fastener by influencing how much force is required to create friction between them. These factors can be important when finding the right screws for specific applications, and understanding how they are measured can make the process easier.

The most common aspect of a screw’s thread is its diameter, which can be determined using go/no-go gauges or an optical comparator. The resulting value is known as the major diameter, or screw gauge.

Another significant factor is the spacing between threads, known as their pitch. This can be expressed as either “threads per inch” or “threads per millimeter,” and both indicate the same thing: how densely a screw’s threads are spaced. A higher thread density is typically easier to tighten, but may require more torque to accomplish. The precise geometry of a screw thread is often specified, describing its form, size, and finish.
Material

The material of a screw is essential in determining its strength and durability. It also affects how the screw fits into a specific material. Screws can be made from various metals, including steel, nickel, aluminum, titanium and copper. These metals are often alloyed to increase their strength or corrosion resistance. Screws can be coated in materials such as zinc or chrome for added protection and aesthetics.

The thread type of a screw can also have an impact on its performance. Coarse-thread screws are less prone to galling, thread crossing and seizing than fine-thread screws. This is important for applications where the screws will be subject to a lot of jostling and movement.

The most common material used in screw manufacturing is stainless steel, which has good chemical and corrosion resistance. However, there are a number of other options available depending on the needs of the application. For example, high-temperature grade alloys offer exceptional strength, while low carbon steel is ideal for machine manufacturing and medical device manufacturing.screw size chart