Thread Pitch Diameter Calculator
Calculate the pitch diameter and minor diameter for Unified (UN) and ACME threads from major diameter and TPI.
Results
What is it?
The pitch diameter is the theoretical diameter of an imaginary cylinder whose surface intersects the thread form where the thread width equals the space width. It is the most critical diameter for thread fit and function รขโฌโ class of fit tolerances (1A/1B through 3A/3B for UN threads) are applied to the pitch diameter. The minor diameter is the smallest diameter of the thread form (root diameter for external threads).
How to use
Enter the thread major diameter in inches, TPI, and select the thread standard. For UN/UNC/UNF threads, the pitch diameter formula subtracts 0.6495/TPI; for ACME threads, 0.5/TPI. Results are given in both inches and millimetres.
Example scenario
1/2-13 UNC thread: Major = 0.500", TPI = 13. Pitch Diameter = 0.500 รขหโ 0.6495/13 = 0.500 รขหโ 0.04996 = 0.4500". This matches the published value of 0.4500" for 1/2-13 UNC pitch diameter (nominal).
Pro tip
Go/No-Go thread gauges measure pitch diameter acceptance. The Go gauge must enter freely; the No-Go gauge must not enter more than 2 turns. For precision threads, use a 3-wire method with a micrometer to measure pitch diameter directly รขโฌโ the wire size should equal 0.57735 รโ pitch for best accuracy.