Screw Sizes – How to Measure Screws For Proper Sizing

Screws have become the fasteners of choice over nails for many woodworking projects thanks to power screwdrivers and new types of screws that offer more holding power. But the wide variety of screw types and sizes makes it easy to get the wrong one for a job, or even worse, damage something by using the wrong size. This article will help you avoid making these mistakes by teaching you how to measure your screws for proper sizing. It will also show you how to determine which screw head type is right for your project.

There are three basic factors to consider when selecting a screw: the diameter (called the major diameter) of the shaft, the number of threads per inch on the shaft and the length of the shaft in inches. Generally, the longer the screw, the more threads there are per inch. Most screws are listed by their major diameter and length, with the number of threads per inch a close second. Screws may also be labeled according to their head type, such as flat, square or Phillips heads. The head type can affect how hard it is to drive a screw and whether it can be removed from the shaft, but usually doesn’t affect length.

The first number on a screw’s sizing chart indicates the diameter of the shaft, which is measured from just under the screw head to the tip. Screws come in a range of sizes from 0 through 24. The larger the diameter, the longer the screw. The number of threads per inch on a screw’s shaft is listed as the second number in the sizing chart, followed by a hyphen. Screws with round heads measure the length of the shaft from the bottom of the head to the tip; flat-head screws are typically measured from the edge of the head to the tip.

If the screw’s length is not clearly indicated on the sizing chart, you can usually figure out its length by dividing the shaft diameter by its thread count. For example, a screw with a major diameter of 1/4 inch and threads of 20 threads per inch is measured as 1/4″ x 20″.

You can use a thread gauge to check the pitch of a screw’s threads. These tools resemble a ruler, with strips of metal that have different sizes of thread cut into them. A thread gauge lets you systematically work your way through the various thread sizes to find the one that matches the pitch of the screw’s threads. This is important when selecting a screw for a specific application because the pitch of a thread can affect how easily it drives into or out of a material. For example, a coarse-thread screw will require a rough surface while a fine-thread screw will not. A coarse-thread screw will also need a bigger hole in the material than a fine-thread screw. 1/4″ to mm

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