Whether you are an aerospace machinist or a dedicated DIY hobbyist, creating flawless internal threads requires absolute accuracy. One of the most common pitfalls in metalworking and manufacturing is selecting the wrong hole size before tapping.
Using a precise Precision Tap Drill Size reference ensures that your threads have the ideal depth—typically around 75% thread engagement—preventing stripped threads or, worse, a broken tap lodged inside your workpiece.
In this guide, we provide a definitive Precision Tap Drill Size reference table and explain how to calculate these dimensions on the fly.
When you use a tap to cut internal threads, the drill bit creates the initial hole (the minor diameter). If the drill bit is too small, the tap will experience excessive torque and snap. If the drill bit is too large, the thread engagement will be too shallow, resulting in weak threads that strip under load.
To achieve the industry-standard 75% thread percentage, engineers rely on specific formulas, but a quick-reference chart is the most efficient tool on the shop floor.
Below is the standard reference data for the most common Unified National Coarse (UNC), Unified National Fine (UNF), and Metric threads.
| Tap Size & Thread Pitch | Drill Bit Size (75% Thread) | Decimal Equivalent (Inches) |
|---|---|---|
| #4-40 UNC | #43 | 0.0890″ |
| #6-32 UNC | #36 | 0.1065″ |
| #8-32 UNC | #29 | 0.1360″ |
| #10-24 UNC | #25 | 0.1495″ |
| #10-32 UNF | #21 | 0.1590″ |
| 1/4″-20 UNC | #7 | 0.2010″ |
| 1/4″-28 UNF | #3 | 0.2130″ |
| 5/16″-18 UNC | F | 0.2570″ |
| 3/8″-16 UNC | 5/16″ | 0.3125″ |
| 1/2″-13 UNC | 27/64″ | 0.4219″ |
| Tap Size x Pitch | Drill Bit Size (mm) | Inch Decimal Equivalent |
|---|---|---|
| M3 x 0.5 | 2.50 mm | 0.0984″ |
| M4 x 0.7 | 3.30 mm | 0.1300″ |
| M5 x 0.8 | 4.20 mm | 0.1654″ |
| M6 x 1.0 | 5.00 mm | 0.1969″ |
| M8 x 1.25 | 6.80 mm | 0.2677″ |
| M10 x 1.5 | 8.50 mm | 0.3346″ |
| M12 x 1.75 | 10.20 mm | 0.4016″ |
If you don’t have a Precision Tap Drill Size chart on hand, you can easily calculate the required drill size using these standard engineering formulas:
Drill Size=Major Diameter of Tap−Threads Per Inch (TPI)1
Drill Size=Major Diameter (mm)−Thread Pitch (mm)
Example: For an M6 x 1.0 thread, the math is simply 6−1.0=5.0 mm.
If the regular drill bit size matches the decimal equivalent required by the Precision Tap Drill Size chart, it is perfectly fine. However, fractional drill sets often lack the precise dimensions found in lettered or numbered drill bit sets (e.g., using a #7 drill instead of a standard fractional bit for a 1/4″-20 tap).
A 75% thread depth provides maximum holding strength without placing unnecessary stress on the tap. Increasing the engagement to 100% only adds about 5% more strength but significantly increases the risk of breaking the tap.
Yes, practically speaking. For harder materials like stainless steel, machinists often use a slightly larger drill bit (yielding around 60% to 65% thread engagement) to prevent the tap from binding and breaking, as stainless steel is highly prone to work-hardening.
Fiona Huang is a Sales Engineer with 16+ years of experience in the mechanical and precision manufacturing industry. She specializes in CNC machining components, custom fasteners, and engineering-driven manufacturing solutions. With strong expertise in international trade and technical project support, she focuses on transforming complex engineering requirements into high-precision, production-ready solutions for global clients.
