As a means of accurately describing threaded fasteners, such as bolts and screws, a measurement of threads per inch (tpi) is often used. This measurement is sometimes referred to as pitch and assists you when purchasing screws, bolts and nuts that match the required specification. Threads per inch can also be measured as the distance between threads, expressed in millimeters. Because inches and millimeters are both measurements of distance, conversion is possible with simple mathematics.
Acquire the measurement of threads per inch. This can be described in specifications or you can simply count the number within an inch's space.
Invert your measurement to calculate the thread spacing, expressed in inches. As an example, convert 26 tpi from threads per inch to inches per thread by dividing 26 into 1, resulting in 0.0385 inches between threads.
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Multiply this number by 25.4 millimeters, which is the number of millimeters in an inch, to convert the spacing into millimeters. In the example, 0.0385 times 25.4 gives you a thread spacing of 0.978 millimeters. This number would be rounded up to just 1 millimeter.