Pitch Diameter Calculator
Free Pitch diameter Calculator for materials specifications projects. Enter dimensions to get material lists and cost estimates.
Reviewed by Abdullah, Technical Content Specialist
Formula
Pitch Diameter = Major Diameter - 0.6495 x Pitch (metric)
For ISO metric threads with a 60-degree included angle, the pitch diameter equals the major (nominal) diameter minus 0.6495 times the thread pitch. This factor comes from the thread geometry: 2 x (3/8) x H where H = 0.866025 x P is the height of the fundamental triangle. For unified threads, substitute pitch = 1/TPI.
Worked Examples
Example 1: M10 x 1.5 Metric Thread
Problem:Calculate the pitch diameter for an M10 x 1.5 metric thread.
Solution:Pitch Diameter = D - 0.6495 x P\nPitch Diameter = 10 - 0.6495 x 1.5\nPitch Diameter = 10 - 0.9743 = 9.0257 mm\nMinor Diameter = 10 - 1.2269 x 1.5 = 8.1596 mm
Result:Pitch Diameter = 9.0257 mm, Minor Diameter = 8.1596 mm
Example 2: 1/2-13 UNC Thread
Problem:Find the pitch diameter for a 1/2-13 UNC thread (0.500 inch major diameter, 13 TPI).
Solution:Pitch = 1/13 = 0.07692 in\nPitch Diameter = 0.500 - 0.6495 x 0.07692\nPitch Diameter = 0.500 - 0.04997 = 0.4500 in
Result:Pitch Diameter = 0.4500 inches
Frequently Asked Questions
What is the pitch diameter of a thread?
The pitch diameter (also called effective diameter) is the diameter of an imaginary cylinder where the thread groove width equals the thread ridge width. It is the most critical dimension for thread fit because it determines whether a bolt and nut will assemble properly. The pitch diameter lies between the major diameter (outer crest) and the minor diameter (inner root) of the thread. For ISO metric threads, it equals the major diameter minus 0.6495 times the pitch.
How is the pitch diameter different from major and minor diameters?
The major diameter is the largest diameter measured across the thread crests of an external thread (or the roots of an internal thread). The minor diameter is the smallest diameter measured across the roots of an external thread. The pitch diameter falls between these two and represents the theoretical diameter where the thread tooth and groove are equal in width. Thread gauges measure pitch diameter because it controls the fit quality.
What is the formula for pitch diameter of metric threads?
For ISO metric threads (60-degree thread angle), the pitch diameter equals the major diameter minus 0.6495 times the pitch. The full formula derives from the thread geometry: Pitch Diameter = D - 2 x (3/8) x H, where H = 0.866025 x P is the fundamental triangle height. This simplifies to D - 0.6495P. For example, an M10 x 1.5 thread has a pitch diameter of 10 - 0.6495 x 1.5 = 9.0258 mm.
How do I measure pitch diameter?
Pitch diameter can be measured using thread micrometers with anvils shaped to contact the thread flanks at the pitch line. The three-wire method is more accurate: place three precision wires of known diameter in the thread grooves and measure over the wires with a standard micrometer. The pitch diameter is calculated from the over-wire measurement using published formulas. Thread plug and ring gauges provide go/no-go verification of pitch diameter tolerances.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy