Tap Drill Size Chart (UNC, UNF & Metric)

Tap Drill Size Chart

Complete UNC, UNF, and metric ISO tap drill size chart with target drill diameters, nearest standard drill bits, clearance drill sizes, and adjustable thread engagement percentage. All values verified against Machinery’s Handbook, 29th Edition.

A tap drill size chart — also called a drill and tap chart — is a reference table that shows the correct drill bit diameter to use before tapping a threaded hole. The drill size is determined by the minor diameter tolerances for the specified thread class. This standard tap drill size chart covers all UNC, UNF, and metric ISO thread sizes with adjustable thread engagement percentage (default 75%). It also includes close-fit and free-fit clearance drill sizes for through-holes. UNC/UNF dimensions per ASME B1.1 Class 2B; metric dimensions per ANSI/ASME B1.13M-2005 Class 6H.

75%
Tap drill sizes for UNC threads, Class 2B internal

Source: Machinery’s Handbook 29th Ed. — Thread data: pp. 1817–1862 (ASME B1.1, Class 2B) & pp. 1893–1894 (ANSI/ASME B1.13M, Class 6H). Clearance holes (inch): p. 1609, Table 1a (ASME B18.2.8-1999). Clearance holes (metric): p. 1610, Table 2a (ASME B18.2.8-1999). Drill sizes: pp. 868–890 (ANSI/ASME B94.11M).

How to Use This Tap Drill Chart

Select UNC (coarse), UNF (fine) or METRIC using the toggle above, then find your thread size in the table. The Target Drill Dia column shows the ideal hole diameter — the midpoint of the ASME B1.1 Class 2B minor diameter tolerance range. The Nearest Std Drill column identifies the closest commercially available drill bit (number, letter, or fractional). Click any value to copy it to your clipboard. Click “Thread Dimensions + More” to reveal UNC and UNF thread dimensions including minor diameter tolerances, pitch diameter tolerances, and clearance drill sizes.

If you are thread milling instead of tapping, you will also need the Pitch Diameter Offset (PDO) — the radial distance your thread mill moves outward from the bore to cut the correct thread profile. Use our thread milling calculator to compute the PDO for Autodesk Fusion 360 and other CAM software.

For pipe threads, see our NPT pipe thread size chart.

Example: Drilling and Tapping a 3/8-16 Hole

You need to tap a 3/8-16 UNC thread in a steel bracket. From the chart above, the target drill diameter is 0.3140" and the nearest standard drill is 5/16" (0.3125"). Drill the hole with a 5/16" bit, then tap with a 3/8-16 tap. The 5/16" drill is 0.0015" under the ideal midpoint, which means slightly higher thread engagement — perfectly acceptable for steel.

For a smaller example: to tap a #10-24 UNC hole, the target drill is 0.1505" and the nearest standard drill is a #25 (0.1495"). In harder materials like steel or stainless, it is common to use 50% thread engagement to significantly reduce tapping torque and tap breakage risk — per Machinery’s Handbook, strength gains beyond 60% are minimal. Use the thread engagement slider above to adjust the chart and see the updated drill sizes for your target percentage. In soft materials like aluminum, stick with the standard 75% drill size or higher to maintain full thread engagement, since the lower shear strength of aluminum means you need more thread contact for joint strength.

Tap Drill & Thread Terminology

Minor Diameter (Class 2B)
The smallest diameter of an internal (female) thread, measured at the root. Class 2B is the standard ASME B1.1 tolerance class for most commercial internal threads. The Min and Max values in this chart define the acceptable range for the drilled hole before tapping. Drilling to the midpoint of this range maximizes your margin for tool wear and deflection.
Target Drill Diameter (Mid-Tolerance)
The ideal pre-drill hole diameter, calculated as (Minor Dia Min + Minor Dia Max) / 2. This midpoint provides the largest tolerance band on both sides, giving you the best chance of a good thread even if the drill wanders slightly or the material springs back.
Pitch Diameter
The diameter at which the thread tooth width and the thread space width are equal. Pitch diameter is the most critical dimension for thread fit and is what go/no-go thread gauges check. For thread milling in CAM software like Autodesk Fusion 360, the Pitch Diameter Offset (PDO) is the radial distance from your bore diameter to the pitch diameter — this value tells the CAM system how far to offset the thread mill cutter.
TPI (Threads Per Inch)
The number of thread crests per linear inch. A higher TPI means finer threads. UNC (coarse) has fewer TPI than UNF (fine) for the same nominal size. For example, a 1/2" bolt comes in 1/2-13 UNC (13 TPI) or 1/2-20 UNF (20 TPI).
75% Thread Engagement
The default thread depth shown in this chart. However, 75% is not always ideal. Per Machinery’s Handbook (29th Ed., p. 2020), “any increase in the percentage of full thread over 60% does not significantly increase the strength of the thread.” In steel and harder alloys, 50–65% engagement is common shop practice — it dramatically reduces tapping torque and tap breakage with minimal strength loss. In soft materials like aluminum and brass, 75–83% is recommended because the lower shear strength of the material requires deeper thread engagement to develop full joint strength. Use the thread engagement slider above to adjust the chart for your application.

Frequently Asked Questions

What size drill bit do I use for a 1/4-20 tap?

For a 1/4-20 UNC tap (Class 2B), the target drill diameter is 0.2015" (the midpoint of the 0.1960"-0.2070" minor diameter tolerance range). The nearest standard drill is a #7 (0.2010"). A 13/64" (0.2031") fractional drill also works. These sizes produce approximately 75% thread engagement.

How do you calculate tap drill size?

Tap drill size is calculated from the ASME B1.1 minor diameter tolerances for your thread class. For Class 2B (the most common): Target Drill Diameter = (Minor Dia Min + Minor Dia Max) / 2. This midpoint gives you the largest margin for error from tool wear or deflection. The simplified rule of thumb (Major Diameter minus Pitch) gives roughly 75% thread engagement but is less precise than using the actual ASME tolerance data.

What is 75% thread engagement?

75% thread engagement means the internal thread depth is 75% of the theoretical maximum (a full sharp-V thread). While 75% is the default on most tap drill charts, it is often more thread than you need. Per Machinery’s Handbook (29th Ed., p. 2020), increasing thread depth beyond 60% does not significantly increase strength. A 100% thread is only about 5% stronger than 75%, but requires roughly 3× the tapping torque. In steel and stainless steel, 50–65% engagement is common shop practice — it reduces tapping torque and tap breakage dramatically. For soft materials like aluminum and brass, 75–83% is recommended because the lower shear strength of the parent material requires deeper engagement to develop full joint strength. For engagement lengths greater than 1.5× the nominal bolt diameter, 50–55% is generally sufficient per Machinery’s Handbook.

What is the difference between UNC and UNF threads?

UNC (Unified National Coarse) has fewer threads per inch and is the default for general-purpose fastening. UNF (Unified National Fine) has more threads per inch, which provides higher tensile strength, better vibration resistance, and finer adjustment. UNC is preferred when tapping into soft materials, when assembly speed matters, or when cross-threading risk is a concern. UNF is preferred for precision assemblies, thin-wall parts, and applications subject to vibration. For example, a 1/4" bolt comes in 1/4-20 UNC (20 TPI) or 1/4-28 UNF (28 TPI).

Do I need to use the exact tap drill size on the chart?

No. Just use the nearest standard drill size shown in the chart. Standard drills are ground to tight tolerances, so the hole will be very close to nominal. The nearest standard drill listed here will produce a good thread. If your tap is cutting hard or breaking, step up one drill size to reduce thread engagement percentage and tapping torque.

Where can I find UNC and UNF thread dimensions?

This chart includes full UNC, UNF, and metric thread dimensions. Click “Thread Dimensions + More” above the table to reveal major diameter, minor diameter (min/max), and pitch diameter (min/max) tolerances per ASME B1.1 Class 2B, plus close-fit, normal-fit, and loose-fit clearance drill sizes per ASME B18.2.8. For pipe threads (NPT), see our NPT thread size chart.

What are the dimensions of a 1/4-20 UNC thread?

A 1/4-20 UNC thread (Class 2B internal) has a major diameter of 0.2500", 20 threads per inch, a minor diameter tolerance range of 0.1960"–0.2070", and a pitch diameter tolerance range of 0.2127"–0.2175" per ASME B1.1. The recommended tap drill is a #7 (0.2010") for approximately 75% thread engagement. Click “Thread Dimensions + More” to see the full tolerance data for all UNC and UNF sizes.

Related Calculators

References

  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012, pp. 1817–1862. Unified Screw Thread tables (ASME B1.1 Class 2B minor and pitch diameter tolerances for UNC/UNF).
  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, pp. 1893–1894. Internal Metric Thread M Profile limiting dimensions (ANSI/ASME B1.13M-2005, Class 6H tolerances).
  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, pp. 868–890. ANSI Straight Shank Twist Drill sizes (number, letter, and fractional drill diameters per ANSI/ASME B94.11M-1993).
  • ASME B1.1 — Unified Inch Screw Threads. ANSI/ASME B1.13M — Metric Screw Threads, M Profile. Clearance holes per ASME B18.2.8 (inch) and ISO 273 (metric).

Data last verified: March 2026

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