---
type: chart
title: "Tap Drill Chart"
tables: 2
url: https://manufacturingprocesses.org/charts/tap-drill-chart
---

# Tap Drill Chart

Tap drill sizes for UNC and UNF threads from #0 to 1/2 in at ~75% thread engagement, and for metric coarse threads M1.6 to M12 by the D−P rule.

A tap drill is sized to leave a chosen percentage of full thread depth in
the hole. The inch column below uses the *Machinery's Handbook* / ASME B1.1
standard of approximately **75% thread engagement**; the metric column uses
the ISO 2306 recommended drills, which follow the **major diameter minus
pitch** rule and land at roughly 76–77% engagement.

**Why 75% and not 100%.** Going from 75% to 100% engagement buys only about
5% more thread strength but roughly triples tapping torque, and is the
leading cause of broken taps. In tough or gummy materials — stainless,
titanium, Inconel — drop to 60–65% engagement by moving up one drill size.

Drill decimals here are the true drill diameters from the ASME B94.11M
drill chart, not the three-place approximations most tap charts print.

## UNC / UNF tap drills, #0 to 1/2 in (~75% thread engagement)

| Size | Major dia (in) | UNC TPI | UNC tap drill | UNC drill (in) | UNC drill (mm) | UNF TPI | UNF tap drill | UNF drill (in) | UNF drill (mm) |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| #0 | 0.06 | — | — | — | — | 80 | 3/64 | 0.046875 | 1.191 |
| #1 | 0.073 | 64 | #53 | 0.0595 | 1.511 | 72 | #53 | 0.0595 | 1.511 |
| #2 | 0.086 | 56 | #50 | 0.07 | 1.778 | 64 | #50 | 0.07 | 1.778 |
| #3 | 0.099 | 48 | #47 | 0.0785 | 1.994 | 56 | #45 | 0.082 | 2.083 |
| #4 | 0.112 | 40 | #43 | 0.089 | 2.261 | 48 | #42 | 0.0935 | 2.375 |
| #5 | 0.125 | 40 | #38 | 0.1015 | 2.578 | 44 | #37 | 0.104 | 2.642 |
| #6 | 0.138 | 32 | #36 | 0.1065 | 2.705 | 40 | #33 | 0.113 | 2.87 |
| #8 | 0.164 | 32 | #29 | 0.136 | 3.454 | 36 | #29 | 0.136 | 3.454 |
| #10 | 0.19 | 24 | #25 | 0.1495 | 3.797 | 32 | #21 | 0.159 | 4.039 |
| #12 | 0.216 | 24 | #16 | 0.177 | 4.496 | 28 | #14 | 0.182 | 4.623 |
| 1/4 | 0.25 | 20 | #7 | 0.201 | 5.105 | 28 | #3 | 0.213 | 5.41 |
| 5/16 | 0.3125 | 18 | F | 0.257 | 6.528 | 24 | I | 0.272 | 6.909 |
| 3/8 | 0.375 | 16 | 5/16 | 0.3125 | 7.938 | 24 | Q | 0.332 | 8.433 |
| 7/16 | 0.4375 | 14 | U | 0.368 | 9.347 | 20 | 25/64 | 0.390625 | 9.922 |
| 1/2 | 0.5 | 13 | 27/64 | 0.421875 | 10.716 | 20 | 29/64 | 0.453125 | 11.509 |

## Metric coarse tap drills, M1.6 to M12 (D − P rule)

| Thread | Major dia (mm) | Pitch (mm) | Tap drill (mm) | D − P (mm) | Tap drill (in) | Nearest inch drill |
| --- | --- | --- | --- | --- | --- | --- |
| M1.6x0.35 | 1.6 | 0.35 | 1.25 | 1.25 | 0.0492 | #55 (0.0520) |
| M2x0.4 | 2 | 0.4 | 1.6 | 1.6 | 0.063 | 1/16 (0.0625) |
| M2.5x0.45 | 2.5 | 0.45 | 2.05 | 2.05 | 0.0807 | #46 (0.0810) |
| M3x0.5 | 3 | 0.5 | 2.5 | 2.5 | 0.0984 | #40 (0.0980) |
| M3.5x0.6 | 3.5 | 0.6 | 2.9 | 2.9 | 0.1142 | #33 (0.1130) |
| M4x0.7 | 4 | 0.7 | 3.3 | 3.3 | 0.1299 | #30 (0.1285) |
| M5x0.8 | 5 | 0.8 | 4.2 | 4.2 | 0.1654 | #19 (0.1660) |
| M6x1 | 6 | 1 | 5 | 5 | 0.1969 | #9 (0.1960) |
| M7x1 | 7 | 1 | 6 | 6 | 0.2362 | 15/64 (0.2344) |
| M8x1.25 | 8 | 1.25 | 6.8 | 6.75 | 0.2677 | H (0.2660) |
| M10x1.5 | 10 | 1.5 | 8.5 | 8.5 | 0.3346 | Q (0.3320) |
| M12x1.75 | 12 | 1.75 | 10.2 | 10.25 | 0.4016 | Y (0.4040) |

## Notes

**The metric rule is exact enough to memorise.** Tap drill = major diameter
− pitch. Every listed metric drill is within 0.05 mm of D − P, and only M8
(6.80 against a computed 6.75) and M12 (10.20 against 10.25) are rounded to
the nearest stocked drill. Both of those are the standard catalog sizes.

**Published tap-drill decimals are usually rounded to three places.** Common
charts print #42 as .094, #38 as .102, #36 as .107 and #25 as .150. The true
diameters are 0.0935, 0.1015, 0.1065 and 0.1495. There is no conflict here,
only precision — but if you are computing thread engagement, use the true
value.

**#0-80 has two tap drills in circulation.** *Machinery's Handbook* gives
3/64 in (0.0469), while several shop charts give #56 (0.0465). The two
differ by 0.0004 in, about 3% of thread engagement, and either works. This
chart uses the *Machinery's Handbook* value.

Numbered-size major diameters were independently confirmed by formula:
major diameter = (screw number × 0.013) + 0.060 in.

## FAQ

### What is the tap drill for 1/4-20?

A #7 drill, 0.2010 in (5.105 mm), for approximately 75% thread engagement. For 1/4-28 UNF the tap drill is a #3, 0.2130 in.

### What is the tap drill for M6?

5.00 mm for M6 × 1 coarse. That follows directly from the metric rule: tap drill = major diameter − pitch, so 6 − 1 = 5 mm. The nearest inch drill is a #9 at 0.1960 in, which is 0.4% undersize.

### How do I work out a metric tap drill without a chart?

Subtract the pitch from the major diameter. M8 × 1.25 gives 6.75 mm, which rounds to the stocked 6.8 mm drill; M10 × 1.5 gives 8.5 mm exactly. The rule holds across the whole coarse series and produces about 76–77% thread engagement.

### Why is 75% thread engagement the standard target?

Because the last 25% is not worth paying for. Going from 75% to 100% engagement adds only about 5% to thread strength while roughly tripling tapping torque, which is the main cause of broken taps.

### What tap drill should I use in stainless steel?

Go one drill size larger than the 75% value to drop to roughly 60–65% engagement. The same applies to titanium and Inconel. The loss in thread strength is small; the reduction in tapping torque is not.

### What is the tap drill for 10-32?

A #21 drill, 0.1590 in (4.039 mm). For 10-24 UNC it is a #25, 0.1495 in.

## Sources

- [Wikipedia — Unified Thread Standard (cited to Machinery's Handbook 29th ed., p. 1816)](https://en.wikipedia.org/wiki/Unified_Thread_Standard)
- [Wikipedia — ISO metric screw thread (ISO 261/262 pitch table and the D − P rule)](https://en.wikipedia.org/wiki/ISO_metric_screw_thread)
- [Fastenal — Torque-Tension Reference Guide (diameter/TPI pairings, tensile stress areas)](https://www.fastenal.com/content/merch_rules/images/fcom/content-library/Torque-Tension%20Reference%20Guide.pdf)
- [Machining Doctor — Unified Threads (UNC/UNF/UNEF): Dimensions & Formulas](https://www.machiningdoctor.com/charts/unified-inch-threads-charts/)

## Related processes

- [CNC Milling](https://manufacturingprocesses.org/processes/cutting/cnc-milling.md)
- [Mechanical Fastening](https://manufacturingprocesses.org/processes/joining/mechanical-fastening.md)
- [CNC Machining](https://manufacturingprocesses.org/processes/cutting/cnc-machining.md)

## Related charts

- [Drill Size Chart](https://manufacturingprocesses.org/charts/drill-size-chart.md)
- [Thread Size Chart](https://manufacturingprocesses.org/charts/thread-size-chart.md)
- [Bolt Torque Chart](https://manufacturingprocesses.org/charts/bolt-torque-chart.md)

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/charts/tap-drill-chart)*

*Last updated: August 11, 2026*