Measuring Screws and Bolts, fasteners you should know

Knowing how to find the diameter and length of the fasteners you need is an important first step in acquiring the correct parts.

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.

Screw SizeDecimal

It is important to measure the length of a screw correctly for it to fit your application. An incorrectly measured screw can end up hindering a projects timeline.

If the fastener is measured incorrectly, you may end up with fasteners that are too long or too short for your needs. To that end, we have created these diagrams to show you how to measure a fastener based on its type. These simple rules should help keep things straight!

Flat head, countersunk screws are flat on the top of the head. They are measured from the top of the head to the tip.

Hex and hex washer head screws and bolts, although flat on the top and the bottom, are measured from the bottom of the head.

Oval head screws are an exception to the rule. The starting point for their length is at the widest point of the head. From that point to the tip is the length of the screw.

Measuring Screw Weight

Due to minuscule fluctuations in the structure, manufacturing process, and even chemical composition of most fasteners, it is often inaccurate to weigh one screw and use the weight of a single screw as a representation of the batch as a whole.

It is more reliable to take a weight measurement of multiple screws and calculate an average based on this value.