How it works
A tapped hole only needs to contain part of the full thread height. The fundamental (sharp V) profile is cut back to 0.6495 times the pitch, and the tap drill is the major diameter minus twice the thread height you want to cut, which is 1.299 times the pitch at 100% engagement.
At 75% engagement the constant works out to about 0.974 divided by threads per inch, close to the 0.975 published in many shop references. The result is a starting drill size; round to the nearest available drill, and use a slightly larger one for tougher material or smaller taps.
Worked example
A 1/4-20 tap at 75% thread:
- Pitch = 1 ÷ 20 TPI = 0.05 in
- Drill = 0.25 − 1.299 × 75% ÷ 100 × 0.05 = 0.2013 in
- In millimeters = 0.2013 × 25.4 = 5.11 mm; nearest 1/64: 13/64 in
| Input | Value |
|---|---|
| Major (nominal) diameter | 0.25 in |
| Pitch entered as | Threads per inch |
| Threads per inch (if chosen) | 20 TPI |
| Pitch length (if chosen) | 0.05 in |
| Percent of thread | 75 % |
| Result | Value |
|---|---|
| Tap drill diameter | 0.2013 in |
| Tap drill diameter in millimeters | 5.11 mm |
| Nearest fractional drill | 13 /64 in |
| Thread depth at your percent | 0.0244 in |
| Pitch used | 0.05 in |
Assumptions and limits
- UN and ISO metric 60° threads only. Pipe threads (NPT) use taper reamers and different drills.
- The result is a theoretical diameter; use the closest standard drill and confirm against a tap drill chart for your tap and material.
- Real holes drill slightly oversized; the percent of thread you get depends on the drill, the material, and the tap.
Common questions
What is the tap drill for 1/4-20?
About 0.2013 in at 75%, which is a #7 drill (0.2010 in).
What is the tap drill for M8×1.25?
About 6.78 mm; a 6.8 mm drill is the standard choice.
What does percent of thread mean?
How much of the theoretical thread height the tapped hole contains. 75% is standard; higher takes more torque and rarely adds strength.
Do I use the same formula for metric?
Yes. Enter the diameter and pitch in millimeters on the Metric setting.
Sources
- Machinist’s tap drill formula: drill = major diameter − 0.975 ÷ TPI at 75% (American Machinist, Newman Tools), from the UN/ISO thread height of 0.6495 × pitch.
- Checks against standard tap drill charts: 1/4-20 → #7 (0.2010), 3/8-16 → 5/16, 1/2-13 → 27/64, M6×1 → 5.0 mm, M8×1.25 → 6.8 mm, M10×1.5 → 8.5 mm.
- Read them: Wikipedia: List of drill and tap sizes
Updated 2026-09-30