Tap Drill Size
Calculator
Results
- Tap drill diameter (mm)
- 7.0625
- Tap drill diameter (inches)
- 0.278051
- Drill for a full thread (mm)
- 6.75
- Clearance hole diameter (mm)
- 8.56
- Thread depth (mm)
- 0.766749
Results
| Tap drill diameter (mm) | 7.0625 |
| Tap drill diameter (inches) | 0.278051 |
| Drill for a full thread (mm) | 6.75 |
| Clearance hole diameter (mm) | 8.56 |
| Thread depth (mm) | 0.766749 |
formula-map diagram
- Tap drill diameter (mm)
- 7.0625
- Tap drill diameter (inches)
- 0.278051
- Drill for a full thread (mm)
- 6.75
- Clearance hole diameter (mm)
- 8.56
- Thread depth (mm)
- 0.766749
Formula map
Formula
Tap drill Ø = D − (P × engagement ÷ 100)= 7.0625
Note
This result is a simplified model; verify against your shop's own tolerances and material data before cutting.
More in Woodworking and metalwork
See all →Frequently asked questions
What is a tap drill size and why isn't it the same as the thread's major diameter?+
The tap drill size is the diameter of the hole drilled before threading, and it must be smaller than the thread's outer (major) diameter so that material remains for the tap to cut into and form the thread grooves. Drilling at the major diameter would leave nothing for the tap to engage.
How is tap drill size calculated from thread specifications?+
It's derived from the thread's major diameter, its pitch (or threads per inch), and a target percentage of thread engagement, commonly 75%. A common approximate formula for standard threads is major diameter minus pitch.
Why is 75% thread engagement the usual target rather than 100%?+
Full 100% thread engagement dramatically increases the torque needed to tap the hole and the risk of breaking the tap, while providing only a small strength increase over lower engagement percentages. About 75% is widely used because it balances thread strength against tapping ease and tool life.
Does material type affect the recommended thread engagement percentage?+
Yes. Softer or more brittle materials, or thin-walled parts, may call for a different engagement percentage than the standard 75%, and some material and application combinations specify their own recommended tap drill sizes in reference charts rather than relying on the general formula.
What happens if I use the wrong tap drill size?+
A hole drilled too small forces the tap to remove excessive material, dramatically increasing the chance of the tap breaking off in the hole, while a hole drilled too large produces weak, shallow threads that can strip under normal fastener torque.