Screws come in a wide array of sizes and types to meet almost any job or construction need. But choosing the wrong kind or size can split wood, damage an interior wall and compromise the soundness of a structure. So, if you’re planning to work with screws, it pays to understand what all that “#8 x 1-1/4″” business means.
Screw sizes are measured using three essential numbers: gauge, threads per inch and shaft length in inches. The first number is the screw’s diameter or “gauge,” and the second relates to how many threads fit into an inch of one-inch space, or its “twist rate.” The third number describes the length from the head to the shank.
For imperial-system screws, the major diameter is usually listed in inches (or fractions of an inch) with the number of threads per inch being displayed on the other side of the head. For example, a #6 screw is 7/64 inches in diameter and typically available in lengths from 3/8 inch to 1 1/2 inch. This screw is suitable for woodworking and other light construction projects.
The metric system uses a similar system, but the major diameter is measured in millimeters with the threads per millimeter listed on the other side of the head. For a quick reference, here’s a table that shows the metric equivalence of each screw size and type. pilot hole for 1/4 lag screw