Gauge Charts
Tap Drill Chart
Inch and metric tap drills follow two different conventions, and most charts publish them under one heading as though they did not. Inch tap drills target about 75 percent thread engagement. The metric rule is to subtract the pitch from the diameter, so M6 by 1.0 takes a 5.0 mm drill, and that lands at about 92 percent. Both are the stocked standard, and the table below shows what each listed drill actually gives.
Tap drill finder
35 threads. Clearance is close fit. Inch taps land near 75% thread, metric near 92%
| Thread | Series | Tap drill | Inches | % thread | At 75% | Clearance |
|---|---|---|---|---|---|---|
| #0-80 | UNF (fine) | 3/64 | 0.0469 | 81% | 0.0478 | #51 |
| #2-56 | UNC (coarse) | #50 | 0.0700 | 69% | 0.0686 | #43 |
| #4-40 | UNC (coarse) | #43 | 0.0890 | 71% | 0.0876 | #33 |
| #6-32 | UNC (coarse) | #36 | 0.1065 | 78% | 0.1076 | #28 |
| #8-32 | UNC (coarse) | #29 | 0.1360 | 69% | 0.1336 | #19 |
| #10-24 | UNC (coarse) | #25 | 0.1495 | 75% | 0.1494 | #11 |
| #10-32 | UNF (fine) | #21 | 0.1590 | 76% | 0.1596 | #11 |
| #12-24 | UNC (coarse) | #16 | 0.1770 | 72% | 0.1754 | #2 |
| 1/4-20 | UNC (coarse) | #7 | 0.2010 | 75% | 0.2013 | 17/64 |
| 1/4-28 | UNF (fine) | #3 | 0.2130 | 80% | 0.2152 | 17/64 |
| 5/16-18 | UNC (coarse) | F | 0.2570 | 77% | 0.2584 | 21/64 |
| 5/16-24 | UNF (fine) | I | 0.2720 | 75% | 0.2719 | 21/64 |
| 3/8-16 | UNC (coarse) | 5/16 | 0.3125 | 77% | 0.3141 | 25/64 |
| 3/8-24 | UNF (fine) | Q | 0.3320 | 79% | 0.3344 | 25/64 |
| 7/16-14 | UNC (coarse) | U | 0.3680 | 75% | 0.3679 | 29/64 |
| 7/16-20 | UNF (fine) | 25/64 | 0.3906 | 72% | 0.3888 | 29/64 |
| 1/2-13 | UNC (coarse) | 27/64 | 0.4219 | 78% | 0.4251 | 33/64 |
| 1/2-20 | UNF (fine) | 29/64 | 0.4531 | 72% | 0.4513 | 33/64 |
| 9/16-12 | UNC (coarse) | 31/64 | 0.4844 | 72% | 0.4813 | 37/64 |
| 5/8-11 | UNC (coarse) | 17/32 | 0.5312 | 79% | 0.5364 | 41/64 |
| 5/8-18 | UNF (fine) | 37/64 | 0.5781 | 65% | 0.5709 | 41/64 |
| 3/4-10 | UNC (coarse) | 21/32 | 0.6562 | 72% | 0.6526 | 49/64 |
| 3/4-16 | UNF (fine) | 11/16 | 0.6875 | 77% | 0.6891 | 49/64 |
| 7/8-9 | UNC (coarse) | 49/64 | 0.7656 | 76% | 0.7668 | 57/64 |
| 1-8 | UNC (coarse) | 7/8 | 0.8750 | 77% | 0.8782 | 1-1/32 |
| M2 × 0.4 | Metric coarse | 1.6 mm | 0.0630 | 92% | 0.0659 | 2.2 mm |
| M3 × 0.5 | Metric coarse | 2.5 mm | 0.0984 | 92% | 0.1021 | 3.2 mm |
| M4 × 0.7 | Metric coarse | 3.3 mm | 0.1299 | 93% | 0.1351 | 4.3 mm |
| M5 × 0.8 | Metric coarse | 4.2 mm | 0.1654 | 92% | 0.1713 | 5.3 mm |
| M6 × 1.0 | Metric coarse | 5.0 mm | 0.1969 | 92% | 0.2042 | 6.4 mm |
| M8 × 1.25 | Metric coarse | 6.8 mm | 0.2677 | 89% | 0.2750 | 8.4 mm |
| M10 × 1.5 | Metric coarse | 8.5 mm | 0.3346 | 92% | 0.3458 | 10.5 mm |
| M12 × 1.75 | Metric coarse | 10.2 mm | 0.4016 | 95% | 0.4165 | 13.0 mm |
| M14 × 2.0 | Metric coarse | 12.0 mm | 0.4724 | 92% | 0.4873 | 15.0 mm |
| M16 × 2.0 | Metric coarse | 14.0 mm | 0.5512 | 92% | 0.5660 | 17.0 mm |
Inch and metric use different conventions, and most charts hide it. Inch tap drills target about 75% thread, which holds roughly 95% of the strength of a full-depth thread while cutting tapping torque sharply. The metric rule is “subtract the pitch”, so M6 × 1.0 takes a 5.0 mm drill, and that lands at about 92%. Both are the stocked standard. The % thread column shows what the listed drill actually gives; amber marks anything above 85%, where tapping torque climbs and taps break in hard material. Use the slider to size a drill for a different engagement.
Tap drill and clearance sizes
| Thread | Series | Tap drill | Inches | % thread | Clearance (close) |
|---|---|---|---|---|---|
| #0-80 | UNF (fine) | 3/64 | 0.0469 | 81% | #51 |
| #2-56 | UNC (coarse) | #50 | 0.0700 | 69% | #43 |
| #4-40 | UNC (coarse) | #43 | 0.0890 | 71% | #33 |
| #6-32 | UNC (coarse) | #36 | 0.1065 | 78% | #28 |
| #8-32 | UNC (coarse) | #29 | 0.1360 | 69% | #19 |
| #10-24 | UNC (coarse) | #25 | 0.1495 | 75% | #11 |
| #10-32 | UNF (fine) | #21 | 0.1590 | 76% | #11 |
| #12-24 | UNC (coarse) | #16 | 0.1770 | 72% | #2 |
| 1/4-20 | UNC (coarse) | #7 | 0.2010 | 75% | 17/64 |
| 1/4-28 | UNF (fine) | #3 | 0.2130 | 80% | 17/64 |
| 5/16-18 | UNC (coarse) | F | 0.2570 | 77% | 21/64 |
| 5/16-24 | UNF (fine) | I | 0.2720 | 75% | 21/64 |
| 3/8-16 | UNC (coarse) | 5/16 | 0.3125 | 77% | 25/64 |
| 3/8-24 | UNF (fine) | Q | 0.3320 | 79% | 25/64 |
| 7/16-14 | UNC (coarse) | U | 0.3680 | 75% | 29/64 |
| 7/16-20 | UNF (fine) | 25/64 | 0.3906 | 72% | 29/64 |
| 1/2-13 | UNC (coarse) | 27/64 | 0.4219 | 78% | 33/64 |
| 1/2-20 | UNF (fine) | 29/64 | 0.4531 | 72% | 33/64 |
| 9/16-12 | UNC (coarse) | 31/64 | 0.4844 | 72% | 37/64 |
| 5/8-11 | UNC (coarse) | 17/32 | 0.5312 | 79% | 41/64 |
| 5/8-18 | UNF (fine) | 37/64 | 0.5781 | 65% | 41/64 |
| 3/4-10 | UNC (coarse) | 21/32 | 0.6562 | 72% | 49/64 |
| 3/4-16 | UNF (fine) | 11/16 | 0.6875 | 77% | 49/64 |
| 7/8-9 | UNC (coarse) | 49/64 | 0.7656 | 76% | 57/64 |
| 1-8 | UNC (coarse) | 7/8 | 0.8750 | 77% | 1-1/32 |
| M2 × 0.4 | Metric coarse | 1.6 mm | 0.0630 | 92% | 2.2 mm |
| M3 × 0.5 | Metric coarse | 2.5 mm | 0.0984 | 92% | 3.2 mm |
| M4 × 0.7 | Metric coarse | 3.3 mm | 0.1299 | 93% | 4.3 mm |
| M5 × 0.8 | Metric coarse | 4.2 mm | 0.1654 | 92% | 5.3 mm |
| M6 × 1.0 | Metric coarse | 5.0 mm | 0.1969 | 92% | 6.4 mm |
| M8 × 1.25 | Metric coarse | 6.8 mm | 0.2677 | 89% | 8.4 mm |
| M10 × 1.5 | Metric coarse | 8.5 mm | 0.3346 | 92% | 10.5 mm |
| M12 × 1.75 | Metric coarse | 10.2 mm | 0.4016 | 95% | 13.0 mm |
| M14 × 2.0 | Metric coarse | 12.0 mm | 0.4724 | 92% | 15.0 mm |
| M16 × 2.0 | Metric coarse | 14.0 mm | 0.5512 | 92% | 17.0 mm |
Working out a tap drill for any engagement
For unified threads the drill diameter is the major diameter minus the engagement fraction times 1.299 divided by threads per inch. For metric it is the major diameter minus the engagement fraction times the pitch times 1.0825.
A 1/4-20 thread has a major diameter of 0.250 inches and 20 threads per inch. At 75 percent engagement the theoretical drill is 0.250 minus 0.75 times 1.299 divided by 20, which is 0.2013 inches. The nearest standard drill is a number 7 at 0.201 inches, which is what every chart lists.
Softer materials such as aluminium and brass tolerate higher engagement because the tap cuts more easily. Harder materials and deep blind holes are usually tapped at 60 to 65 percent to keep torque down, which is exactly the case where a broken tap costs the most to remove.
Common questions
What size drill for a 1/4-20 tap?
- A number 7 drill, which is 0.201 inches. That gives about 75 percent thread engagement. If you do not have a number set, 13/64 at 0.2031 inches is slightly larger and gives about 72 percent, which is acceptable for most work.
What is the tap drill for M8 x 1.25?
- A 6.8 mm drill. The rule for ISO metric coarse threads is to subtract the pitch from the major diameter, so 8 minus 1.25 gives 6.75 mm and 6.8 mm is the nearest standard drill. Note that this lands at about 89 percent thread engagement rather than the 75 percent an inch tap drill targets, because metric and inch follow different conventions.
Why do metric tap drills give more thread engagement than inch ones?
- Because they follow different conventions. Inch tap drill charts are built around roughly 75 percent engagement. The metric convention is simply to subtract the pitch from the major diameter, which works out at about 92 percent. Neither is wrong, but a metric tap cuts noticeably harder than an inch tap of similar size, and in tough material many shops go up a tenth or two of a millimetre to bring it closer to 75 percent.
Why not tap to 100 percent thread?
- Because it adds almost nothing and costs a great deal. Going from 75 to 100 percent engagement increases thread strength by roughly 5 percent while more than doubling the tapping torque. In practice the fastener fails before a 75 percent thread strips.