Measuring Screws and Bolts
Knowing how to find the diameter and length of the fasteners you need is an important first step in acquiring the correct parts.
See Also
Understanding Metric FastenersUnderstanding Metric Fasteners - Fastener MartIs 1/4 inch the same as 6mm?What is the difference between 10-32, 12-24, and M6 rack screws?Diameters of Screws / Bolts
To measure the diameter of screws and bolts, you measure the distance from the outer thread on one side to the outer thread on the other side. This is called the major diameter and will usually be the proper size of the bolt. You can also measure from the bottom cavity of the threads from one side to the other to find the minor diameter.
For sizes of 1/4" or lower, a number is commonly used to designate the size. This is called a nominal measurement. Here are those numbers and their decimal equivalent.
