Your thread plug gauge has a tendency to damage or wear out and requires that you check if it is still working within acceptable tolerances. The gauge can be calibrated with a variety of methods, but if high accuracy is required then using the Universal Length Measuring Machine (ULM) is the most appropriate practice. However, if you do not need very high accuracy, then the Three-Wire Method is your calibration method of choice.This method not only saves your money but it is accepted by the international standards too.

Calculating the Pitch Diameter The pitch diameter can be found using the following formula.
• The thread pitch for a 60° American Metric Unified National thread

E = M-3d+0.866025P (1)

• The thread pitch for a 55° Imperial Unified National thread

E = M-3.16568d+0.960491P (2)


P: Thread pitch (metric unit only)
d: Wire diameter
E: Pitch diameter
M: Measurement over wire



Anusorn Tonmueanwai
Chief of Laboratory, Acting Chief – Dimensional Metrology section National Institute of Metrology,
Ministry of Science and Technology, Thailand

With over 20 years of professional experience in metrology and an invited lecturer at the Faculty
of Engineering, King Mongkut’s University of Technology Thonburi