Thread Per Inch (TPI)
Threads per inch, commonly abbreviated as TPI, is a term frequently used when talking about fasteners.
TPI is a term used to help identify how many threads are in an inch. To determine treads per inch an inch of the bolt is measured and then the peaks on the fastener are counted. In the picture, you can see there are 5 peaks in the inch measurement. This means the TPI of this fastener will be 5. When looking at an example bolt measurement:
1/4″-20 x 2″
The 1/4″ refers to the bolt diameter (in inches), the -20 refers to the threads per inch meaning that there are 20 threads per inch on this bolt, and the 2″ refers to the length (also in inches).
Thread Pitch
Thread pitch is the term used to calculate the difference between two threads. For metric fasteners, the thread pitch is used in place of TPI. The distance is also measured in millimeters.
To measure thread pitch, use a caliper and measure from the peak of one thread to the next.
M2 x .4 x 5M
The M2 refers to the diameter of the bolt (in millimeters), the .4 refers to the thread pitch in millimeters meaning that there are .4 millimeters between each thread peak and the 5M refers to the length of the bolt it millimeters.
Why It Matters
Thread Pitch and Threads Per Inch are used to measure the threading of a bolt or nut to ensure that they will couple together properly. If the threading of a bolt and nut are different they will either seize or strip the threading resulting in an unusable connection.
A Word About Fine And Coarse Threading
Fine thread fasteners have a tighter helical structure and are usually less pronounced. A coarse thread fastener has larger deeper and more forgiving threads (meaning if the threading gets lightly damaged they may still work). Most standard fasteners and metric fasteners have a fine and a coarse thread version. You can identify each of them by using the TPI or Thread Pitch.
For US Fasteners, you might see 1/4″-20 and 1/4″-28. To determine which of these is coarse thread and which is fine, simply take the TPI number (the 20 and the 28) and compare. Remember a coarse thread means the threads are bigger so less will be able to fit within an inch so the 20 means the fastener is coarse thread and the 28 means the fastener is a fine thread.
*The TPI and Thread Pitch will change based on the diameter of the fastener so you will not always see 20 and 28.
For metric fasteners, you will see a M8 x 1.25 or an M8 x 1. For thread pitch, the distance between two points is the second number meaning the higher the number the fewer threads there are. This means the M8 x 1.25 is the coarse threading and the M8 x 1 is the fine thread.
If you have a problem identifying thread type and size, please use our Thread Identification Charts.
For more information about fasteners check out Fasteners 101.
For information on Thread Pitch & Threads Per Inch consider checking out our video on the subject below.
