KEVOS® MAKEOS · Fasteners
Tap Drill for Percentage of Thread
What drill size gives the required percentage of thread for a cutting or forming tap?
Theoretical tap drill or hole size for a chosen percentage of thread, for cutting taps and cold-forming (roll) taps, in metric or unified threads.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread system | choice | Metric threads in millimetres, or unified threads in inches. |
| Basic major diameter | mm or in | Nominal size: millimetres for metric, inches for unified. |
| Pitch (metric) | mm | Metric thread pitch (ignored for unified). |
| Threads per inch (unified) | TPI | Threads per inch (ignored for metric). |
| Percentage of thread | % | About 75% is usual for cutting taps (60–65% in tough materials or deep holes); 65% is usual for forming taps. |
| Tap type | choice | Cutting taps remove metal; forming taps displace it and need a larger hole. |
Method and formulas
- Cutting tap: drill = D − 0.01299 × % × P (or D − 0.01299 × % ÷ n in inches).
- Forming tap: hole = D − 0.0068 × % × P (or ÷ n in inches).
- Basic minor diameter of the internal thread = D − 1.082532P.
- Example: M10 × 1.5 at 75% with a cutting tap needs a 8.5386 mm drill.
Limitations
- Theoretical sizes; drills cut oversize, so check the actual hole and choose the nearest available drill.
- The resulting minor diameter must stay within the thread class limits.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Pick a tap drill for tough materials or deep holes
- Find the hole size for a forming tap
- Check an existing drill against thread engagement
Who it is for
- Machinists
- CNC programmers
- Manufacturing engineers
What you get
- Theoretical drill or hole size
- Drill size in both units
- Basic minor diameter for comparison
Interpreting the result
- Choose the nearest standard drill and check the minor diameter against the thread limits table.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2029–2036. Topic reference; no source tables or recommended settings are reproduced.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
