Screw Sizes – How to Choose the Right One For Your Project
The right screws and bolts can make all the difference in a project. They draw materials together and create a tight, secure joint that can last for years. But understanding screw sizes can be confusing. The measurements on a package can be listed in either imperial or metric, and the screw gauge and threads per inch (TPI) are different. And some projects require a specific type of head. This article covers the basics of screw size and how to choose the right one for a job.
The first number of a screw is the gauge — the basic size, or major diameter — and the second number is the threads per inch (TPI) or “tooth count.” This number tells you how many threads are grouped together in an inch of length. The threads can be coarse or fine, tapered or parallel, and full or partial. The third number is the screw length, normally in inches or mm.
In the US, screws are typically rated in three numbers: the gauge, the TPI, and the shaft length in inches or mm. The last two numbers are usually paired: the head diameter and the shank diameter. The head diameter is the largest surface at the top of the screw’s cylinder and the shank diameter is the lower extreme. The diameters can be measured with a screw gauge or a calibrated dial indicator.
Metric screws are becoming increasingly popular in Australia and are often denoted with a M prefix. A table can be found below that shows the metric equivalent of US fractions and decimals to screw sizes up to 4″. pilot hole for 1/4 lag screw