Holemaking and tapping
Tap drill chart
UNC and UNF #0 to 1 in, metric coarse M1.6 to M24 and common fine pitches, cut taps, 75% thread basis. 5/16-18 takes an F drill (0.257 in) for 77% thread.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Source: thread sizes and pitches from ASME B1.1 and ISO 261; drill sizes from ANSI/ASME B94.11M; percent of thread computed with the Machinery's Handbook formula.
Basis: each inch row is the standard drill nearest the 75% thread size, with the percent that drill gives. Cut taps only.
| Thread | Tap drill | Decimal (in) | Metric (mm) | Percent of thread (%) |
|---|---|---|---|---|
| #1-64 | #53 | 0.0595 | 1.51 | 67 |
| #2-56 | #50 | 0.0700 | 1.78 | 69 |
| #3-48 | #47 | 0.0785 | 1.99 | 76 |
| #4-40 | #43 | 0.0890 | 2.26 | 71 |
| #5-40 | #38 | 0.1015 | 2.58 | 72 |
| #6-32 | #36 | 0.1065 | 2.71 | 78 |
| #8-32 | #29 | 0.1360 | 3.45 | 69 |
| #10-24 | #25 | 0.1495 | 3.80 | 75 |
| #12-24 | #16 | 0.1770 | 4.50 | 72 |
| 1/4-20 | #7 | 0.2010 | 5.11 | 75 |
| 5/16-18 | F | 0.2570 | 6.53 | 77 |
| 3/8-16 | 5/16 | 0.3125 | 7.94 | 77 |
| 7/16-14 | U | 0.3680 | 9.35 | 75 |
| 1/2-13 | 27/64 | 0.4219 | 10.72 | 78 |
| 9/16-12 | 31/64 | 0.4844 | 12.30 | 72 |
| 5/8-11 | 17/32 | 0.5313 | 13.49 | 79 |
| 3/4-10 | 21/32 | 0.6563 | 16.67 | 72 |
| 7/8-9 | 49/64 | 0.7656 | 19.45 | 76 |
| 1-8 | 7/8 | 0.8750 | 22.23 | 77 |
Tap drill is the nearest number, letter or fractional (1/64 in) drill to the 75% size. Percent of thread is computed for that drill.
| Thread | Tap drill | Decimal (in) | Metric (mm) | Percent of thread (%) |
|---|---|---|---|---|
| #0-80 | 3/64 | 0.0469 | 1.19 | 81 |
| #1-72 | #53 | 0.0595 | 1.51 | 75 |
| #2-64 | #50 | 0.0700 | 1.78 | 79 |
| #3-56 | #45 | 0.0820 | 2.08 | 73 |
| #4-48 | #42 | 0.0935 | 2.37 | 68 |
| #5-44 | #37 | 0.1040 | 2.64 | 71 |
| #6-40 | #33 | 0.1130 | 2.87 | 77 |
| #8-36 | #29 | 0.1360 | 3.45 | 78 |
| #10-32 | #21 | 0.1590 | 4.04 | 76 |
| #12-28 | #14 | 0.1820 | 4.62 | 73 |
| 1/4-28 | #3 | 0.2130 | 5.41 | 80 |
| 5/16-24 | I | 0.2720 | 6.91 | 75 |
| 3/8-24 | Q | 0.3320 | 8.43 | 79 |
| 7/16-20 | 25/64 | 0.3906 | 9.92 | 72 |
| 1/2-20 | 29/64 | 0.4531 | 11.51 | 72 |
| 9/16-18 | 33/64 | 0.5156 | 13.10 | 65 |
| 5/8-18 | 37/64 | 0.5781 | 14.68 | 65 |
| 3/4-16 | 11/16 | 0.6875 | 17.46 | 77 |
| 7/8-14 | 13/16 | 0.8125 | 20.64 | 67 |
| 1-12 | 59/64 | 0.9219 | 23.42 | 72 |
Tap drill is the nearest number, letter or fractional (1/64 in) drill to the 75% size. Percent of thread is computed for that drill.
| Thread | Tap drill (mm) | Decimal (in) | Percent of thread (%) | Drill nearest 75% thread |
|---|---|---|---|---|
| M1.6 x 0.35 | 1.25 | 0.0492 | 77 | Same drill |
| M2 x 0.4 | 1.6 | 0.0630 | 77 | Same drill |
| M2.5 x 0.45 | 2.05 | 0.0807 | 77 | Same drill |
| M3 x 0.5 | 2.5 | 0.0984 | 77 | Same drill |
| M3.5 x 0.6 | 2.9 | 0.1142 | 77 | Same drill |
| M4 x 0.7 | 3.3 | 0.1299 | 77 | Same drill |
| M5 x 0.8 | 4.2 | 0.1654 | 77 | Same drill |
| M6 x 1 | 5.0 | 0.1969 | 77 | Same drill |
| M8 x 1.25 | 6.8 | 0.2677 | 74 | Same drill |
| M10 x 1.5 | 8.5 | 0.3346 | 77 | Same drill |
| M12 x 1.75 | 10.3 | 0.4055 | 75 | Same drill |
| M14 x 2 | 12.0 | 0.4724 | 77 | 12.1 mm (0.4764 in), 73% |
| M16 x 2 | 14.0 | 0.5512 | 77 | 14.1 mm (0.5551 in), 73% |
| M18 x 2.5 | 15.5 | 0.6102 | 77 | 15.6 mm (0.6142 in), 74% |
| M20 x 2.5 | 17.5 | 0.6890 | 77 | 17.6 mm (0.6929 in), 74% |
| M22 x 2.5 | 19.5 | 0.7677 | 77 | 19.6 mm (0.7717 in), 74% |
| M24 x 3 | 21.0 | 0.8268 | 77 | 21.1 mm (0.8307 in), 74% |
Tap drill is D - P (77% thread). Where D - P falls between 0.1 mm drill sizes (M8 x 1.25 and M12 x 1.75), the nearest 0.1 mm drill to 75% is listed instead. The last column gives the drill closest to 75% when it differs.
| Thread | Tap drill (mm) | Decimal (in) | Percent of thread (%) | Drill nearest 75% thread |
|---|---|---|---|---|
| M8 x 1 | 7.0 | 0.2756 | 77 | Same drill |
| M10 x 1.25 | 8.8 | 0.3465 | 74 | Same drill |
| M10 x 1 | 9.0 | 0.3543 | 77 | Same drill |
| M12 x 1.5 | 10.5 | 0.4134 | 77 | Same drill |
| M12 x 1.25 | 10.8 | 0.4252 | 74 | Same drill |
| M14 x 1.5 | 12.5 | 0.4921 | 77 | Same drill |
| M16 x 1.5 | 14.5 | 0.5709 | 77 | Same drill |
| M18 x 1.5 | 16.5 | 0.6496 | 77 | Same drill |
| M20 x 1.5 | 18.5 | 0.7283 | 77 | Same drill |
| M22 x 1.5 | 20.5 | 0.8071 | 77 | Same drill |
| M24 x 2 | 22.0 | 0.8661 | 77 | 22.1 mm (0.8701 in), 73% |
Tap drill is D - P (77% thread). Where D - P falls between 0.1 mm drill sizes (M10 x 1.25 and M12 x 1.25), the nearest 0.1 mm drill to 75% is listed instead.
How to read the table
Find the thread, drill the tap drill, and read the percent of thread it leaves. 75% is the target the chart is built on; the percent column shows how close the nearest real drill lands. A number above 75 means a tighter hole and more tapping torque; below 75 means an easier tap and a shallower thread.
How the chart is computed
Inch
drill = D - (0.01299 x %thread / TPI)
| Symbol | Meaning | Unit |
|---|---|---|
| drill | Drill diameter for the target percent of thread | in |
| D | Basic major diameter of the thread | in |
| %thread | Target percent of thread as a whole number (75, not 0.75) | % |
| TPI | Threads per inch | 1/in |
| 0.01299 | Constant: 1 / 76.98 | in |
Metric
drill = D - (%thread x P / 76.98)
| Symbol | Meaning | Unit |
|---|---|---|
| drill | Drill diameter for the target percent of thread | mm |
| D | Basic major diameter of the thread | mm |
| %thread | Target percent of thread as a whole number | % |
| P | Pitch, the number after the x in the callout | mm |
Check any drill with the percent-of-thread formula: %thread = 76.98 x TPI x (D - drill) in inch, or %thread = 76.98 x (D - drill) / P in metric. The shop rule of thumb drill = D - P computes to 77% thread.
Worked example: 5/16-18
5/16-18 UNC: D = 0.3125 in, 18 TPI, 75% target.
- Target drill: 0.3125 - (0.01299 x 75 / 18) = 0.3125 - 0.0541 = 0.2584 in.
- Nearest standard drill: F, 0.257 in.
- Percent of thread for F: 76.98 x 18 x (0.3125 - 0.257) = 76.98 x 18 x 0.0555 = 76.9%.
Result: F drill (0.257 in, 6.53 mm), about 77% thread.
Worked example: M6 x 1
M6 x 1: D = 6 mm, P = 1 mm, 75% target.
- Target drill: 6 - (75 x 1 / 76.98) = 6 - 0.974 = 5.03 mm.
- Nearest drill: 5.0 mm, which is also D - P.
- Percent of thread: 76.98 x (6 - 5.0) / 1 = 77%.
Result: 5.0 mm drill (0.1969 in), 77% thread.
Shop notes
- Percent of thread by material, forum rule of thumb (not a standard): about 60 to 65% in hard steel, about 70% in steel, 75 to 80% in aluminum.
- Form (roll) taps need a larger drill than this chart lists. Use the tap maker's chart.
- No chart drill on hand? Pick the nearest drill you have and check its percent of thread with the formula above.
- Holes drill a little oversize, so the real percent of thread runs slightly lower than the chart value.
- Percent of thread is a shop check on the drill. The finished thread is inspected against the class 2B (inch) or 6H (metric) internal thread limits.
FAQ
What size drill for 5/16-18?
An F drill (0.257 in, 6.53 mm) gives about 77% thread. A G drill (0.261 in) gives about 71% and taps easier.
What drill for 1/4-20?
A #7 drill (0.201 in, 5.11 mm), 75% thread.
What drill is close to 2.5 mm for M3 if you have no 2.5 mm drill?
A #40 drill (0.098 in, 2.49 mm) gives 79% thread in M3 x 0.5. A #39 (0.0995 in, 2.53 mm) gives 73%.
What drill for M8 x 1.25?
A 6.8 mm drill, 74% thread. The D - P rule computes 6.75 mm (77%), which falls between 0.1 mm drill sizes.
What size drill for a form (roll) tap?
Larger than the cut-tap drill in this chart. Form taps push metal into the thread instead of cutting it; use the tap maker's chart.
What is the difference between a tap drill and a clearance drill?
The tap drill leaves metal for the tap to cut the thread. A clearance drill makes a hole the bolt passes through, and close fit and free fit clearance sizes differ.
Last updated