How To Determine Screw Size & Thread Count

Thread count is also known as thread pitch or threads per inch (TPI) and is used to determine how fine the threads on a screw are. This number will help you determine whether a screw will thread into a certain bolt properly. Screw size is measured in diameter. For example a 1/4-20 screw has 20 threads per inch and a diameter of 1/4 inch. All you need is a steel rule to determine the size and thread count of any screw. Once you learn how to do so, household projects will be much easier.

Step 1

Lay the screw down on a flat surface. Let the head of the screw hang off the edge of the surface so the screw threads are lying flat.

Step 2

Place a steel rule down the axis of the screw. The first thread on the screw is counted as zero, not one. Starting a thread count at one will throw results off by one point.

Step 3

Count the number of thread gaps within 1 inch of the screw.

Step 4

Divide the count of thread gaps into the length. If there are five threads gaps in a 1-inch length, the thread pitch is .200 inch. For example, 1 inch / 5 thread gaps = .200 inch. There is .200 inch between each thread, and the screw has a TPI of 5.

Step 5

Place the screw on a flat surface.

Step 6

Lay a steel rule between two screw threads and measure across the diameter. Make sure to measure from the outermost ends of the threads. Also be sure not to measure any worn-out or rolled-up screw threads. This will give an inaccurate diameter reading.