Tapping Drill Size Calculator
Drill diameter for a metric thread at a chosen engagement, plus the clearance and pitch diameters.
Results
Tapping drill size
6.985
mm
Tapping drill size
6.750
mm
Diameter
8.50
mm
Diameter
7.188
mm
What this tool does
The rule of thumb “diameter minus pitch” gives about 75 % thread engagement, which is what almost every workshop chart uses. This works it out properly for any engagement you choose, and also gives the clearance drill for the bolt to pass through.
Formula
drill ≈ D − pitch (metric thread, 100 % profile ≈ D − 1.0825 × pitch)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
d | Diameter | mm |
p | Thread pitch | mm |
eg | Percentage | % |
DR | Tapping drill size | mm |
SR | Tapping drill size | mm |
CL | Diameter | mm |
MD | Diameter | mm |
Worked example
- Diameter8 mm
- Thread pitch1.25 mm
- Percentage75 %
- Tapping drill size6.985 mm
- Tapping drill size6.750 mm
- Diameter8.50 mm
- Diameter7.188 mm
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
Why not always aim for 100 % thread engagement?
Because the last quarter of the thread adds very little strength and a great deal of difficulty. Going from 75 % to 100 % engagement raises the strength of the joint by only a few percent while roughly doubling the torque needed to cut it, which is how taps get broken. Seventy-five percent is the workshop standard for good reason.