How this calculation works
A cut-tap drill removes material before a cutting tap forms the internal thread. A smaller drill leaves more material and raises the nominal thread percentage in this conventional approximation. That percentage is not the length of thread engagement in the joint.
Approximate metric drill = major diameter − percentage × pitch/76.98; implied percentage = 76.98 × (major − actual drill)/pitchA worked example
For a nominal 10 mm thread with 1.5 mm pitch, the seventy-five-percent approximation gives 10 minus 112.5/76.98, or about 8.53858 mm. An entered 8.5 mm drill corresponds to 76.98 percent in the same relation. The simple major-minus-pitch reference is also 8.5 mm, but its numerical agreement here does not establish a universal drill-selection rule.
Good to know
Conventional sixty-degree cut-tap approximation only; no forming-tap sizing, ISO tolerance, engagement length, torque or strength prediction. Use the actual tap manufacturer’s drill and tolerance guidance. This is an educational comparison of formula conventions, not an instruction to use the computed diameter.
Reference: Tapmatic — cutting tap formulas (opens in a new tab) ↗
What this model answers
A cut-tap drill removes material before a cutting tap forms the internal thread. A smaller drill leaves more material and raises the nominal thread percentage in this conventional approximation. That percentage is not the length of thread engagement in the joint.
This page reverses the same approximate relation to compare an actual drill diameter. The comparison helps show why two nearby drill sizes imply different nominal percentages without pretending that the formula establishes a complete thread fit.
The Thread pitch TPI and helix calculator converts the axial pitch convention. Use its pitch in millimetres when starting from a TPI specification; multi-start lead is not a substitute for axial pitch in this tap-drill relation.
Measure the right inputs
Enter nominal major diameter and axial thread pitch in millimetres. Percentage uses the whole-number convention: seventy-five percent is entered as 75, not 0.75.
The actual drill field is a comparison dimension and must be smaller than the major diameter. A thread-forming tap needs a different preparation model because it displaces material rather than cutting it away; this formula must not be transferred to that process.
Read the result in context
The major-diameter-minus-pitch value is shown as a simple reference, not as a selected drill recommendation. The approximation’s coefficient and rounding can differ from a full standard-based tolerance calculation.
An implied percentage above one hundred is kept visible as outside the nominal convention. It is not an approval of that drill size. Material, hole tolerance, tap design and maker guidance determine a usable process.
Work through the numbers
For a nominal 10 mm thread with 1.5 mm pitch, the seventy-five-percent approximation gives 10 minus 112.5/76.98, or about 8.53858 mm.
An entered 8.5 mm drill corresponds to 76.98 percent in the same relation. The simple major-minus-pitch reference is also 8.5 mm, but its numerical agreement here does not establish a universal drill-selection rule.
Where the model stops
Conventional sixty-degree cut-tap approximation only; no forming-tap sizing, ISO tolerance, engagement length, torque or strength prediction.
Use the actual tap manufacturer’s drill and tolerance guidance. This is an educational comparison of formula conventions, not an instruction to use the computed diameter.
A few questions, answered
Is thread percentage the same as engagement length?
No. This nominal profile percentage is different from how much axial thread length is engaged in a joint.
Can this size a forming-tap hole?
No. Forming taps require their own material and manufacturer-specific preparation dimensions.