How to Calculate Minor Diameter

The minor diameter is an engineering reference to the diameter of a screw without the threads, as opposed to the major diameter, which is the diameter of a screw with the threads. There are a couple of other factors involved that determine what the minor diameter will be, such as the pitch or taper of the screw, and the amount of threads per inch. But for all practical purposes, a good ruler will give anyone the basic calculation for the minor diameter of any screw.

You can calculate the minor diameter of any screw.
  1. Place the screw onto its head so that the tip is pointing upwards.

  2. Using an engineering ruler for the most precise results, measure the screw at the top between the threads.

  3. Use the engineering ruler to get a precise decimal point measurement. For example, a 3/8-inch screw with 24 threads per square inch will have a .32 minor diameter. Using a regular ruler, that calculates to approximately 1/3 inch.