Thread Pitch Calculator

Thread pitch is the axial distance between corresponding points on adjacent thread forms, measured in millimeters for metric threads. It is distinct from lead, the axial travel produced by one full turn, which equals the pitch only on a single-start thread and equals pitch times the number of starts otherwise. Imperial threads invert the same quantity and count threads per inch, so the two systems convert exactly through the defined inch of 25.4 millimeters: TPI = 25.4 / pitch. A pitch of 1.5 mm is therefore 16.93 TPI, a value that appears on no imperial chart, which is exactly why metric and inch fasteners must never be forced together. The standard coarse metric series runs 0.5 mm for M3, 0.7 for M4, 0.8 for M5, 1.0 for M6, 1.25 for M8, 1.5 for M10, 1.75 for M12, 2.0 for M16, 2.5 for M20, and 3.0 for M24, with fine pitches usually 60 to 75 percent of those values. Measuring an unknown thread by counting crests over a span works well provided at least ten threads are included, because the division averages out the error in locating the first and last crest: over a 15 mm span, a one tenth millimeter reading error becomes only 0.01 mm of pitch. The method cannot detect flank angle, so it will not separate a 60 degree ISO thread from a 55 degree Whitworth or BSP form, nor will it reveal the taper of an NPT thread. A pitch gauge blade set or a direct form comparison is needed for that.

Uniform pitch became possible only after Henry Maudslay built accurate screw cutting lathes with lead screws and change gears around 1800, which allowed one master screw to reproduce its pitch in every part made from it. Before that, each nut was hand fitted to its own bolt and nothing interchanged. Joseph Whitworth turned that new capability into a system in 1841 by assigning one standard pitch to each diameter, and the resulting threads-per-inch tables became the working vocabulary of British engineering for more than a century. The exact conversion between millimeters of pitch and threads per inch rests on a twentieth century definition. The inch was originally derived from national yard bars that differed slightly between countries, so metric and inch thread dimensions could not be reconciled exactly. Industrial bodies moved first, with the British Standards Institution accepting an inch of 25.4 mm in 1930 and American standards bodies following in the early 1930s for industrial use. The International Yard and Pound Agreement, adopted by the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa and effective from 1959, then made 25.4 millimeters the legal definition of the inch. Since then the relation TPI = 25.4 / pitch has been exact rather than approximate. Metric pitch is now the primary specification through ISO 261, while the inch series UNC and UNF remain in wide use in North America.