How to Determine Screw Size & Thread Count

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You will be able to determine the size and thread count of screws in a few easy steps.

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.


Michael Jones

Michael Jones reported campus news stories for The University of Southern California's student newspaper, "The Daily Trojan," for four years before graduating Summa Cum Laude with a Bachelor of Science in journalism. He has since gone on to write for several publications both in America and abroad and has an idiosyncratic knack for translating the most intricate tasks into layman speak.