Choosing Screw Sizes and Lengths

Screws and other fasteners are typically listed with their gauge and length on their packaging. Choosing the right screws for the job is critical to ensure they’ll hold and anchor properly and not cause damage to the materials they’re screwed into.

Standardization of screw thread dimensions has evolved over time. There are many systems still in use, but ISO metric thread preferred series have largely displaced older ones.
Diameter

The diameter of a screw is the distance between the top of one thread (crest) and the bottom of another (root). The root of a thread has the self-locking property that allows the fastener to be tightened by turning it without the need to apply a significant amount of force. This is the primary reason that screws are so popular as fasteners for woodworking and sheet metal work.

In a single-threaded screw, the lead is equal to the pitch; in multiple-start screws, which have several intertwined threads, the lead is defined by the product of the pitch and number of starts. In a screw with a uniform helix, the lead is also equal to the thread diameter.

Screws sold in imperial measurements on their packaging will usually list the gauge first, followed by a length and then a thread count. The thread count is a measure of the number of thread peaks that are found in a one-inch section of the screw, and will often be listed as TPI, or Threads per inch.
Length

When choosing a screw, the length is an important factor. If the fastener is too long it can cause damage and if it’s too short, it won’t be able to fully penetrate the material.

The length of a screw is measured from the bottom of the head to the tip. This applies to flat heads, including hex, pan, button, and truss screws, and oval-head screws (non-countersinking). Nylon-tip grub screws have an exception; they are measured from the bottom of the head to the end of the shaft.

Screws that have their measurements listed on their packaging will typically list the diameter first and then the length. For example, a package of wood screws that say “#6 – 32 x 1 1/2” means the screws have a #6 diameter and are an inch and a half long. This is called nominal sizing. The other number represents the threads per inch. The shaft length is usually not included in this measurement.
Threads per inch

The threads on a screw are measured in terms of their number per inch, which tells you how many thread peaks exist along one-inch length. The threads also have a pitch, which is the distance between the crests of each thread. The diameter and pitch of a screw are related, with larger threads having a lower pitch, and smaller threads having a higher pitch.

Thread standards typically define class of fit for a range of diameters, with class 1 being loosest and class 3 being tightest. This allows for wrench-assisted assembly, while minimizing the possibility of thread interference and galling.

While ISO metric threads are the most commonly used worldwide, older systems such as the American Standard Whitworth and British Association are still widely used in some niche applications. However, globalization is putting pressure on these systems to converge with ISO standards. Even so, some hardware manufacturers will continue to mark their products as USS and SAE to convey the fact that they are not metric.
Gauge

The gauge of a screw refers to its diameter. A screw with a smaller diameter will have a higher gauge, while a larger one has a lower gauge. For imperial system screws, this number is usually shown first on the packaging, followed by the length.

A screw’s gauge can also be determined by measuring from the tip of the head to the point where its shaft starts. However, this is not as reliable for metric system screws.

In addition, screw sizes may be listed with a tolerance class, an LH symbol (if they’re left-handed) and the threads per inch. The latter is specified right after the gauge and can help you determine whether a screw is coarse or fine. For example, a screw with a #10 gauge has 32 threads in one inch. This is a coarse screw, while a screw with a #06 gauge has 36 threads per inch and is fine. These are both UNC screw standards.5/16 inch to mm