Pitch Diameter Calculator

Your details

Choose whether you are working with a screw thread, a spur gear, or a three-wire measurement setup.
ISO metric threads use pitch in mm. Unified threads use threads-per-inch (TPI).
The outermost diameter of the screw thread (the nominal bolt size).
Distance between adjacent thread crests. For metric threads enter pitch in mm; for unified threads enter pitch in mm as well (1/TPI * 25.4).
mm
Number of thread crests per inch. Used for unified (UN) threads.
TPI
ISO 965-1 tolerance grade. Grade 6 is the standard commercial grade for external threads.
Fundamental deviation letter for the external thread (bolt). "g" is the most common commercial choice.
Basic pitch diameter
18.3763mm

The theoretical mid-flank diameter (d2 / D2)

Minor diameter (external)16.9328mm
Max pitch diameter18.3337mm
Min pitch diameter17.633mm
Tolerance band (Td2)0.7008mm
Basic pitch dia.18.3763
Minor dia. (ext)16.9328
Max pitch dia.18.3337
Min pitch dia.17.633

Basic pitch diameter: 18.3763 mm

  • The basic pitch diameter is 18.3763 mm. This is the theoretical mid-flank diameter where thread engagement is most effective.
  • With ISO tolerance class 6g, the allowable range is 17.6330 mm to 18.3337 mm, a band of 0.7008 mm.
  • Pitch diameter determines thread fit quality more than major or minor diameter. A bolt that gauges within the tolerance band will assemble with its mating nut without binding.

Next stepUse the three-wire mode to verify your machined thread against this value with a micrometer and precision wires.

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