Standard Tap Drill Chart for Inch Threads (Imperial)
This standard tap drill chart provides recommended drill sizes for inch threads, including UNC and UNF thread series, helping ensure proper thread engagement, accuracy, and consistent results in machining and maintenance applications.
Use the chart below to find the correct drill size for common UNC and UNF thread sizes.
| Drill Bit Size Category | Drill Bit Size | Decimal Inc Size | Metric mm Size | Tap Or Screw Size | Tap Or Screw Size Category | Thread Standard |
|---|---|---|---|---|---|---|
| Inch | 1/8 | 0.1250 | 3.18 | 5/32-32 | Inch | UNS |
| Number | 30 | 0.1285 | 3.26 | 5/32-36 | Inch | UNS |
| Number | 26 | 0.1470 | 3.73 | 3/16-24 | Inch | UNS |
| Number | 22 | 0.1570 | 3.99 | 3/16-32 | Inch | UNS |
| Number | 12 | 0.1890 | 4.80 | 7/32-32 | Inch | UNS |
| Number | 7 | 0.2010 | 5.11 | 1/4-20 | Inch | UNC |
| Number | 4 | 0.2090 | 5.31 | 1/4-24 | Inch | UNF |
| Number | 3 | 0.2130 | 5.41 | 1/4-28 | Inch | UNEF |
| Letter | F | 0.2570 | 6.53 | 5/16-18 | Inch | UNC |
| Letter | I | 0.2720 | 6.91 | 5/16-24 | Inch | UNF |
| Inch | 9/32 | 0.2812 | 7.14 | 5/16-32 | Inch | UNEF |
| Inch | 5/16 | 0.3125 | 7.94 | 3/8-16 | Inch | UNC |
| Letter | Q | 0.3320 | 8.43 | 3/8-24 | Inch | UNF |
| Letter | U | 0.3680 | 9.35 | 7/16-14 | Inch | UNC |
| Inch | 25/64 | 0.3906 | 9.92 | 7/16-20 | Inch | UNF |
| Inch | 27/64 | 0.4219 | 10.72 | 1/2-13 | Inch | UNC |
| Inch | 29/64 | 0.4531 | 11.51 |
1/2-20 |
Inch | UNF |
| Inch | 31/64 | 0.4844 | 12.30 | 9/16-12 | Inch | UNC |
| Inch | 33/64 | 0.5156 | 13.10 | 9/16-18 | Inch | UNF |
| Inch | 17/32 | 0.5312 | 13.49 | 5/8-11 | Inch | UNC |
| Inch | 37/64 | 0.5781 | 14.68 | 5/8-18 | Inch | UNF |
| Inch | 5/8 | 0.6250 | 15.88 | 11/16-16 | Inch | UNS |
| Inch | 21/32 | 0.6562 | 16.67 | 3/4-10 | Inch | UNC |
| Inch | 11/16 | 0.6875 | 17.46 | 3/4-16 | Inch | UNF |
| Inch | 49/64 | 0.7656 | 19.45 | 7/8-9 | Inch | UNC |
| Inch | 13/16 | 0.8125 | 20.64 | 7/8-14 | Inch | UNF |
| Inch | 7/8 | 0.8750 | 22.23 | 1-8 | Inch | UNC |
| Inch | 59/64 | 0.9219 | 23.42 | 1-12 | Inch | UNF |
| Inch | 15/16 | 0.9375 | 23.81 | 1-14 | Inch | UNEF |
| Inch | 63/64 | 0.9844 | 25.00 | 1-1/8-7 | Inch | UNC |
| Inch | 1-3/64 | 1.0469 | 26.59 | 1-1/8-12 | Inch | UNF |
| Inch | 1-7/64 | 1.1094 | 28.18 | 1-1/4-7 | Inch | UNC |
| Inch | 1-11/64 | 1.1719 | 29.77 | 1-1/4-12 | Inch | UNF |
Understanding Standard Tap Drill Sizes
Standard tap drill sizes are used to determine the correct hole diameter before tapping inch-based threads. These include Unified National Coarse (UNC) and Unified National Fine (UNF) thread series, which are commonly used in industrial, manufacturing, and maintenance applications.
Selecting the correct drill size ensures the proper balance between thread engagement and tapping efficiency. Incorrect sizing can lead to poor thread quality, excessive tool wear, or tap breakage.
How to Use a Standard Tap Drill Chart
- Identify the thread size (for example, 1/4-20 UNC or 3/8-24 UNF).
- Locate the corresponding tap drill size in the chart. From this chart, the preferred drill bit size to tap 1/4-20 threads is a #7 drill size.
- Hover over the dark gold dill bit sizes and they are linked to pages where you can purchase a variety of drill bits in that size.
- Drill the hole to the recommended size before tapping.
- Use appropriate cutting fluid and tapping technique for best results.
Why Drill Size Matters for Inch Threads
Proper drill sizing is essential to achieve the correct percentage of thread engagement. A hole that is too small increases tapping torque and the risk of tool breakage, while a hole that is too large reduces thread strength and holding capability.
Most standard tap drill sizes provide approximately 70–75% thread engagement, which is suitable for general machining operations.
UNC vs UNF Thread Differences
UNC (Unified National Coarse) threads have fewer threads per inch and are commonly used for general-purpose fastening. UNF (Unified National Fine) threads have more threads per inch and are used when greater strength, precision, or vibration resistance is required.
Because thread pitch differs between UNC and UNF, the recommended tap drill sizes will also vary for the same nominal diameter.
For example, a 1/4-20 UNC thread typically uses a #7 drill, while a 1/4-28 UNF thread typically uses a #3 drill.
For example, a 1/2-13 UNC thread typically uses a 27/64 drill size, while a 1/2-20 UNF thread typically uses a 29/64 drill size.
Common Inch Thread Sizes
This chart includes common standard sizes used across industrial and maintenance applications, including UNC and UNF thread series for fasteners, components, and threaded holes.
Printable Standard Tap Drill Chart
Download a printable PDF version of this standard tap drill chart for convenient reference in the shop or on the job site.
Related Tap Drill Charts
Frequently Asked Questions
What is a standard tap drill chart?
A standard tap drill chart shows recommended drill sizes for inch threads, including UNC and UNF thread series. It helps match thread size to the correct hole diameter before tapping.
What is the difference between UNC and UNF threads?
UNC threads have fewer threads per inch and are used for general applications, while UNF threads have more threads per inch and provide greater strength and precision.
How do I find the drill size for an inch tap?
Identify the thread size (such as 3/8-16 or 3/8-24), then use a tap drill chart to find the recommended drill size before tapping.
What percentage of thread engagement do standard tap drill sizes provide?
Most standard tap drill sizes provide approximately 70–75% thread engagement, which balances strength and ease of tapping for most applications.
Is there a printable version of this standard tap drill chart?
Yes. A printable PDF version is available for use in machining, maintenance, and fabrication environments.
