Screws are an essential construction tool for a wide range of tasks from woodworking to metal roofing. But choosing the wrong type or size can split wood, damage surfaces and undermine a structure’s soundness. Screws come in a huge selection of types, sizes and styles to suit virtually any task. But understanding the three fundamental qualities of screws — their gauge, thread spacing and length — can make the difference between success and failure.
Most woodworkers are familiar with a variety of screw sizes from plans, instructions and specs for their projects. But what do those numbers really mean? This article is an attempt to clear up some of the confusion.
Screw values can be listed in either metric or imperial measurements, but the most common values are gauge (diameter), threads per inch and shaft length in inches. The gauge number corresponds to the screw’s major diameter and may appear as “#0,” “#2,” “#4,” “#6” or “#8.” For example, a #8 “Duraspin” screw has a major diameter of 0.164″.
A screw’s thread count is the number of threads that cover its outer diameter. A screw’s thread may be tapered, parallel or both, and it may be fine or coarse. The higher the thread count, the tighter the screw’s fit in the hole.
If a screw’s head is flat or oval, it’s called a countersunk screw. Its head is cut back into the screw shank so that when installed, it sits below the surface of a wood or other material. The head may also be crowned or round.
When a screw’s head is rounded or cupped, it’s called a hex screw. The cupping is intended to grip a hex socket wrench, which can be used to turn the screw and fasten materials in place.
Screw shaft length may seem self-explanatory, but it’s not always easy to understand when you see a measurement like 6-32 x 1 1/2″. This indicates a #6 screw with 32 threads per inch and a shaft that’s an inch and a half long.
Using a tape measure or digital caliper, you can easily determine the decimal equivalent for any screw measurement. To help with this, we have attached a chart that converts inch fractions to screw sizes up to 4″. We also have a US Screw Size Chart that shows the screw sizes in metric, which can be handy for users of either system. It’s especially useful to compare metric sizes, as they are sometimes different from imperial. In general, though, one pitch — or distance between threads — is assigned to each diameter size. This is similar to the way that wires and nails are measured, but it’s not identical. This is to allow for the wide variety of head shapes and drive types that are available. This is also true of hex and allen wrenches. These are often interchangeable, but other types of wrenches have very specific head and thread fits that are not compatible with all other sizes of heads and drives. 3/8 in to mm