CNC / machinist calculator
Thread Dimension Calculator (Pitch and Minor Diameter)
Get the basic dimensions of a 60-degree thread from its major diameter and pitch. Enter an inch thread by its threads per inch or a metric thread by its pitch in millimetres and this calculator returns the pitch diameter, the basic minor diameter and the single-point cutting depth. These are the numbers behind a thread callout, useful for inspection, single-point turning and modeling, and they sit alongside the tap drill tool, which answers the separate practical drilling question.
- Pitch diameter
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- Basic minor diameter
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- Single-point cut depth
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How it works
A 60-degree thread is defined by its major diameter and its pitch, and the rest of the basic profile follows from the geometry. The pitch diameter, the effective diameter where the thread thickness equals the groove width, is the major diameter minus 0.6495 times the pitch. The basic minor diameter, the bottom of the internal thread, is the major diameter minus 1.0825 times the pitch.
These basic dimensions describe the ideal profile of the standard, ISO 724 for metric and the Unified standard for inch. They are not the same as the tap drill size, which deliberately leaves a partial thread, around seventy-five percent, to cut tapping torque. So the minor diameter here is the basic full-thread minor, while the tap drill tool gives the practical hole to drill.
For single-point turning of an external thread, the cutting depth is about 0.6134 times the pitch, the standard infeed for a 60-degree form. The single-point threading tool turns that total depth into a pass-by-pass infeed schedule.
Worked example
A 1/4-20 thread (major 0.25 in, pitch 0.05 in) has a pitch diameter of 0.25 - 0.6495 x 0.05 = 0.2175 in, and a basic minor near 0.196 in.
Frequently asked questions
What is the pitch diameter of a thread?
The pitch diameter is the effective diameter where the width of the thread ridge equals the width of the groove. For a 60-degree thread it is the major diameter minus 0.6495 times the pitch, and it is the diameter that governs fit.
How is the minor diameter different from the tap drill?
The basic minor diameter is the bottom of a full standard thread, the major diameter minus 1.0825 times the pitch. The tap drill is deliberately a little larger to leave about a seventy-five percent thread, which cuts tapping torque, so the two numbers differ.
What is the cutting depth for single-point threading?
For a 60-degree external thread the standard single-point cutting depth is about 0.6134 times the pitch. The single-point threading tool splits that total depth into decreasing passes so each pass removes a similar chip area.
Do these formulas work for metric threads?
Yes. The 60-degree geometry is the same for ISO metric and Unified inch threads, so the factors 0.6495 and 1.0825 apply to both. Just enter the pitch in millimetres for metric or as threads per inch for inch threads.
What is the pitch diameter used for?
Pitch diameter is the dimension that actually controls how a thread fits and is what thread gauges and the three-wire method measure. Major and minor diameters set clearance, but the pitch diameter sets the class of fit.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.