Screw Sizes and Types

Screws come in a variety of sizes to fit any project. Sizing is determined by the driver type – flat, Phillips or square head – length and shaft diameter. Screws are also rated by threads-per-inch.

Measuring threads per inch is simple; you simply count the number of peaks along one-inch of your fastener. But measuring major diameter and thread pitch is a bit more complicated.
Diameter

The diameter of a screw is the width of the threaded part. It’s measured from one end of the thread to the other and is also known as the major diameter. For imperial parts, the diameter is shown as a gauge number and fraction of an inch; for metric parts, it’s listed in millimeters preceded by “M.”

The second number is the thread pitch, which indicates how many threads are spaced one-inch apart. Screws with lower pitches have coarser threads, and screws with higher pitches have finer threads. The pitch can be more or less important depending on the material you’re screwing into and how strong you need your fastener to be.

The pitch can also affect how fast the screw turns, as well as how easy it is to remove once you’re done with it. A screw with a shorter pitch is easier to undo, but a longer pitch means the threads are more exposed and therefore can be more difficult to clean out.
Length

The length of a screw refers to the total distance from its head to the point where it’s fully embedded in the substrate. This can vary from one screw type to another. Screws that are too short run the risk of stripping out of the material when an external force is applied.

Similarly, screws that are too long may be at risk of extending beyond the edge of the substrate, potentially damaging it. In order to avoid this, it’s important to understand how to determine the correct length of a screw for your particular project.

If you’re using a metric screw gauge, its first number indicates the diameter of the threads, also known as the major diameter. The second number indicates the pitch, which is a ratio between the major diameter and the length of the screw. To help you choose the right screw size for your project, we have a handy chart that converts imperial screw sizes to metric mm and US fractions to metric.
Pitch

The length of one complete turn of a screw’s body around its axis. It is measured along the threads themselves and specified as “Treads Per Inch,” or TPI. It is closely related to the lead of a screw.

The distance from the crest of one thread to the crest of the next. With most single-start threadforms, the pitch and lead are equal; this is why it’s important to distinguish between the two.

Coarse and fine threads are a result of the thread form rather than the tolerances used or quality of the fastener. However, the terms often get confused as many companies offer both coarse and fine thread versions of the same screw size. This makes it difficult to track down the right screw for a job. Fortunately, there are several ways to measure thread pitch, including using a thread pitch gage. For metric screws, it can also be calculated by measuring the distance between thread peaks with a caliper.
Material

Screws come in a wide variety of materials, types and sizes designed for specific purposes. Using the wrong type for a project can cause the screws to bend, break or work themselves loose over time. Choosing the right type for your project ensures that it will have a firm hold and can carry whatever load is required.

Screw packaging typically lists the gauge (major diameter) first, followed by the threads per inch and then the length in inches or millimeters. Metric screws are usually appended with a multiplication sign (e.g. M8 = 8 mm).

Screws for thicker material often require longer screws to get a solid grip. However, the length of a screw should not be greater than half the material thickness to avoid damaging the materials or creating excessive friction that can weaken them. It’s also important to choose a screw with a head type that suits the material, such as hex, slotted or button heads.