When screwing into materials, selecting the right size is crucial for a number of reasons. Screws that are too large may crack or damage the material, while screws that are too small might not anchor adequately.
Knowing how to measure screw size accurately can save time and money, as well as help you avoid costly mistakes. The basics of screw size are major diameter, thread count and length.
Diameter
The diameter of a screw is the measure of its outermost threads. It is usually determined by the first number in the screw size system, which is called a gauge. For example, a wood screw with a gauge of #6 has a diameter of approximately 0.164 inches.
The metric system measures screws with a number followed by a unit of measurement such as mm or thou (mm). Screws are often marked with three numbers on their packaging, starting with the gauge and then listing the threads per inch and length.
When selecting a screw for a project, it is recommended that you select one with a length equal to half the thickness of the material it will be going into. A screw too long will protrude past the material and may cause damage. It is also possible that the screw will not anchor properly in the material. The right screw length will ensure proper installation and longevity of your project.
Length
Depending on the screw head type, length is either measured from the flat bottom surface of the screw or the tip of the threads. Hex-, pan-, button-, round-, and truss-head screws are all measured from the bottom of their heads; however, flat head, countersunk screws (and other types with a countersunk head) are measured from the tip or blunt end of the head.
The first number in a screw gauge measurement is the diameter of the outside threads, which is also known as the major diameter. It’s usually measured in inches, and the number scale increases with each increasing gauge size.
The second number is the pitch of the screw threads, which is the distance between the peaks of adjacent threads. The pitch of a screw is a reciprocal of the thread diameter, so the higher the diameter, the closer together the threads are and vice versa. The third number is the shaft length of a screw, which is typically in inches.
Pitch
Pitch is the distance between a screw’s threads. Screws with wider threads will have a higher pitch than those with narrower ones.
The pitch of a screw can also be described as its “starts per mm” or “threads per inch.” Threads have a certain number of ridges that wrap around the cylinder of the screw body each time it rotates one turn (360 degrees). The number of crests that make up one complete thread is the count known as the thread pitch.
If the screw’s thread pitch is not compatible with the thread of the nut that it will be fastened into, the screw won’t be secure in the nut. This can lead to loosening and potentially damage the nut or the threaded hole. You can determine the screw’s thread pitch by using a steel rule or caliper. It is also a common practice to use a thread gauge, which consists of strips of metal with various sizes of thread cut into them, to measure the pitch of a screw.
Material
The material used to make a screw determines its strength and ability to resist corrosion. Most screws are made of steel, but they can also be made from other metals like bronze or brass. Steel screws are most commonly used because they are strong and inexpensive. They are often plated with chromium to improve their wear resistance and corrosion resistance.
Screws can be made from low-carbon steel or medium-carbon steel. Medium-carbon steel is a stronger option that can be hardened to increase its strength. Other metals are available as well, including stainless steel, which offers superior strength and corrosion resistance.
Some screws have self-drilling tips to eliminate the need for pre-drilling. This is useful for applications where the screw will be located within a wall or other hard-to-reach place. The length of a screw is important to consider, as too short a screw may not provide a secure hold in the material it is being installed into, while too long of a screw can cause structural problems.#6 screw size