Free Engineering Calculator
Tap Drill Size Calculator
For a cutting tap, the tap drill is roughly the major diameter minus the pitch, about 75% thread: a #7 drill for 1/4-20, 6.8 mm for M8 x 1.25. Pick an inch or metric thread below for the drill and the thread percentage.
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The Mechanics of Cut Tapping
Traditional tapping creates internal threads by cutting away material to form the thread profile. Unlike forming, this process produces metal chips that must be cleared from the hole. The drill size you choose determines the percentage of thread engagement. If your drill is too small, the tap will work too hard and likely snap. If the drill is too large, the threads will be shallow and strip easily under load.
Establishing the correct balance between tap life and thread strength is critical for high-volume production. Most engineering specs target a specific thread height to ensure the bolt has enough material to grip without overloading the machining equipment.
The 75% Standard
A 75% thread depth is the industry standard for most general-purpose industrial applications, providing ample strength while maintaining reasonable tap life.
Interactive Tap Drill Calculator
Select your nominal thread size and desired thread percentage to find the ideal drill diameter. These calculations are designed for standard cutting taps in various materials.
Tap Drill Size Calculator
Inch (UNC and UNF) or metric threads: the drill to use and the thread percentage it gives.
| Drill | Size | Thread % |
|---|
Cutting taps: about 75% thread suits most steel; 50 to 65% eases tapping in hard or tough material. Forming taps need a larger hole (about D - 0.45 x P). Inch data from ASME B1.1 and standard drill tables, checked against two sources (30 Sep 2026).
How to Use the Tap Drill Size Calculator
- Choose inch (UNC or UNF) or metric.
- For inch, pick the thread size, for example 1/4-20 UNC. For metric, enter the diameter and pitch in millimeters (M8 x 1.25 is 8 and 1.25).
- Read the recommended tap drill and the thread percentage it gives.
- Use the table to compare nearby standard drills: a larger drill gives less thread and easier tapping, a smaller one more thread.
The Formula and a Worked Example
Hole size for a given thread percentage: Drill = D – (%thread x 1.299 x P), where D is the major diameter and P the pitch in millimeters. At 75% thread this is close to D – P, the shop rule of thumb. For an inch thread the same rule is Drill = D – (0.01299 x %thread / TPI).
Example: M8 x 1.25. D – P = 8 – 1.25 = 6.75 mm, so use a standard 6.8 mm drill. For exactly 75% thread the hole is 8 – (0.75 x 1.299 x 1.25) = 6.78 mm; for 65% it is 6.94 mm, and for 50% it is 7.19 mm.
Inch example: 1/4-20 UNC. 0.250 – (0.01299 x 75 / 20) = 0.201 in, which is a #7 drill.
Why not always drill for 100%? Going from 75% to 100% thread adds very little strip strength but sharply raises tapping torque, which breaks taps. Most shops run 65 to 75% in steel and up to 75 to 85% in soft aluminum.
Tapping Process Considerations
| Design Factor | Engineering Impact | Machining Goal |
|---|---|---|
| Chip Clearance | Larger drill sizes provide more room for chips to escape the flutes. | Prevents tap jamming. |
| Engagement Area | The contact area between the screw and the internal thread. | Maximizes pull-out strength. |
| Material Hardness | Tougher materials often require a lower thread percentage to save tooling. | Reduces tool wear. |
| Minor Diameter | The hole diameter left after the tapping process is complete. | Ensures ANSI/ISO compliance. |
Frequently Asked Questions
What size drill for an M8 tap?
A 6.8 mm drill for M8 x 1.25 coarse thread, about 75% thread. For M8 x 1.0 fine thread use a 7.0 mm drill.
What is the formula for tap drill size?
Tap drill = major diameter minus pitch for about 75% thread. For another percentage: drill = D – (% x 1.299 x P) in millimeters, or D – (0.01299 x % / TPI) in inches.
What thread percentage should I use?
65 to 75% for most steel parts, 75 to 85% for soft aluminum and brass, and 50 to 60% for hard or tough materials such as stainless and titanium, where tapping torque is the limit.
Is the drill size different for a forming tap?
Yes. A forming (roll) tap displaces material instead of cutting it, so it needs a larger hole, roughly D – 0.45 x P. The calculator shows this as the forming tap reference.
What drill do I use for 1/4-20?
A #7 drill (0.201 in) gives about 75% thread for 1/4-20 UNC. For 1/4-28 UNF use a #3 drill (0.213 in).
Next Steps
- Tap Drill Size Chart: every UNC, UNF and metric size in one table
- Thread Engagement Calculator: how deep the tapped thread needs to be
- Thread inserts: repairing or reinforcing tapped holes
- Request a quote for the fasteners in your joint.
Related Calculators
More free tools for the same job:
- Pilot Hole Design for Metric Thread Formers: Size the pilot hole for metric thread-forming screws.
- Metric Thread Profile Calculator: Work out metric thread dimensions for correct fit.
- Thread Engagement Length Calculator: Find the minimum thread engagement for blind and through holes.
- ISO Fit Calculator: Look up ISO 286 tolerance classes for shafts and holes.
Experiencing Tap Failure?
Our engineering team can analyze your application and recommend the exact drill size and tap geometry for your specific material and depth requirements.