How do I interpret the numbers on a bolt?

For threads labeled under the imperial convention, the first number is the major diameter, the second number is threads per inch, and the last number is the length. For example, a ¾”-10 x 1.5” bolt has a diameter of ¾” with 10 threads per inch (and therefore a pitch of 1/10) and is 1.5” long. Its depth would be ¾” x 1/10, or 0.075”.

In some cases, the first number will not be the diameter, but rather a letter or number that corresponds with a set diameter. For example, a #10 bolt will have a major diameter of 0.190”, and a Z tap has a diameter of 0.413”. These can be found by consulting a chart like this one. Here are more resources for understanding and visualizing different properties of drills.