Tools reference
Tap drill size chart for common Metric, UNC, and UNF threads
Common tap-drill sizes with their actual diameters, nearby Metric or Inch alternatives, and the reason a tap drill is not a clearance hole.
Reviewed against GaugeFox data and the sources below · Updated August 29, 2026
Common tap-drill sizes
The primary drill is the standard catalog label nearest the approximate 75% cutting-thread target used by GaugeFox. The last column is a close drill from the other measurement system, not a claim that every nearby drill is interchangeable for every material or tap.
| Thread | Primary tap drill | Actual diameter | Nearby other-system drill |
|---|---|---|---|
| M3 × 0.5 | 2.5 mm | 2.500 mm | #39 · 2.527 mm |
| M4 × 0.7 | 3.3 mm | 3.300 mm | #30 · 3.264 mm |
| M5 × 0.8 | 4.2 mm | 4.200 mm | #19 · 4.216 mm |
| M6 × 1.0 | 5.0 mm | 5.000 mm | #8 · 5.055 mm |
| M8 × 1.25 | 6.8 mm | 6.800 mm | H · 6.756 mm |
| M10 × 1.5 | 8.5 mm | 8.500 mm | R · 8.611 mm |
| 1/4-20 UNC | #7 | 0.2010 in · 5.105 mm | 5.1 mm |
| 1/4-28 UNF | #3 | 0.2130 in · 5.410 mm | 5.5 mm |
| 5/16-18 UNC | F | 0.2570 in · 6.528 mm | 6.6 mm |
| 3/8-16 UNC | 5/16 in | 0.3125 in · 7.938 mm | 8.0 mm |
| 1/2-13 UNC | 27/64 in | 0.4219 in · 10.716 mm | 10.8 mm |
Tap drill and clearance hole are different jobs
A tap cuts the internal thread into the wall left by the smaller tap-drill hole. A clearance hole is intentionally larger than the bolt so the bolt can pass through without engaging the workpiece.
For example, M6 × 1.0 commonly starts with a 5.0 mm tap drill. The verified ISO 273 M6 clearance choices are 6.4, 6.6, or 7.0 mm depending on the fit. Substituting one for the other either leaves too little material to tap or prevents the bolt from passing through.
Why one chart cannot settle every tapping job
- Forming taps and cutting taps require different hole guidance.
- Hard or brittle materials may call for a lower thread percentage than a general starting chart.
- Coatings, worn drills, runout, and machine rigidity change the hole actually produced.
- For production or critical work, use the tap maker's chart for that tap and material.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standard- ISO 261:1998 — ISO general purpose metric screw threadsInternational Organization for Standardization (opens in a new tab)
- ASME B1.1-2024 — Unified inch screw threadsAmerican Society of Mechanical Engineers (opens in a new tab)
- Tap Drill Sizes — inch and metric, approximate 75% threadGuhring, Inc. (opens in a new tab)
- Precision Tools and Gages Catalog 71 — drill-size tablesL.S. Starrett Company (opens in a new tab)
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GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.