Tapping Drill Size Chart: Metric and Imperial
Selected cutting-tap pre-drill sizes for metric coarse, UNC and UNF threads, with an approximate 75% theoretical-thread basis and a three-page printable A4 reference.
Find cutting-tap pre-drill sizes for selected metric coarse, UNC and UNF threads. This chart follows the approximate 75% theoretical-thread column in Guhring’s published cutting-tap reference, with drill designations kept intact and metric/inch conversions clearly labelled.
Download the workshop tapping chart (PDF, A4, 3 pages)
Print at actual size / 100% on A4. Each page is a separate workshop reference: metric coarse, UNC and UNF. Download size: 501 KiB.
Scope and how to use the chart
For cutting taps only. These are pre-drill diameters for an internal thread, not clearance holes for a bolt to pass through. They are not hole-depth instructions or finished-thread inspection limits. Forming, roll and fluteless taps displace material and require different pre-drill sizes; do not use these cutting-tap values for them.
Read the complete thread designation first. For example, M6 x 1.00 means a 6 mm nominal thread with 1 mm pitch; 1/4-20 UNC means a nominal 1/4-inch Unified coarse thread with 20 threads per inch. The drill diameter is a separate dimension. This chart covers selected metric coarse M2-M24 and UNC/UNF #2 through 1 inch. Metric fine, BSW, BSF, BA and pipe threads are excluded.
Use the specified drill label. A # prefix in the drill column identifies a number drill; Letter identifies a letter drill. These are different from the numbered Unified screw sizes in the thread column. Millimetre and inch equivalents are conversions of the specified drill, not alternative drill recommendations. Do not round a number, letter or fractional drill to the nearest metric size and assume the same result.
What the 75% target means
Guhring labels the source column as an approximate 75% theoretical percentage of thread. Available drill sizes approximate that target; the selected rows calculate to about 70.5-78.8% using its formulas and nominal drill diameters. This percentage describes thread depth under that calculation, not the length of engagement between a bolt and nut, and not a guaranteed strength rating.
Drill runout and hole-size variation change the actual result. The tap maker’s guidance for the workpiece material, cutting tap, lubrication, hole type and required fit takes precedence over a general chart. A different target percentage can require a different pre-drill. This reference makes no universal recommendation that 75% suits every material or job.
Metric coarse / M2 to M24
| Thread designation | Pitch (mm) | Specified cutting-tap drill | Drill diameter (mm) |
|---|---|---|---|
| M2 x 0.40 | 0.40 | 1.60 mm | 1.600 |
| M2.5 x 0.45 | 0.45 | #46 | 2.057 |
| M3 x 0.50 | 0.50 | 2.50 mm | 2.500 |
| M3.5 x 0.60 | 0.60 | 2.90 mm | 2.900 |
| M4 x 0.70 | 0.70 | 3.30 mm | 3.300 |
| M4.5 x 0.75 | 0.75 | #25 | 3.797 |
| M5 x 0.80 | 0.80 | 4.20 mm | 4.200 |
| M6 x 1.00 | 1.00 | 5.00 mm | 5.000 |
| M7 x 1.00 | 1.00 | 6.00 mm | 6.000 |
| M8 x 1.25 | 1.25 | Letter H | 6.756 |
| M10 x 1.50 | 1.50 | 8.50 mm | 8.500 |
| M12 x 1.75 | 1.75 | 10.30 mm | 10.300 |
| M14 x 2.00 | 2.00 | 12.00 mm | 12.000 |
| M16 x 2.00 | 2.00 | 14.00 mm | 14.000 |
| M18 x 2.50 | 2.50 | 15.50 mm | 15.500 |
| M20 x 2.50 | 2.50 | 17.50 mm | 17.500 |
| M22 x 2.50 | 2.50 | 19.50 mm | 19.500 |
| M24 x 3.00 | 3.00 | 53/64 in | 21.034 |
Unified National Coarse / #2 to 1 inch
| Thread designation | TPI | Specified cutting-tap drill | Drill diameter (in) | Drill diameter (mm) |
|---|---|---|---|---|
| #2-56 UNC | 56 | 1.73 mm | 0.0681 | 1.730 |
| #3-48 UNC | 48 | 2.00 mm | 0.0787 | 2.000 |
| #4-40 UNC | 40 | 2.20 mm | 0.0866 | 2.200 |
| #5-40 UNC | 40 | #39 | 0.0995 | 2.527 |
| #6-32 UNC | 32 | #36 | 0.1065 | 2.705 |
| #8-32 UNC | 32 | 3.40 mm | 0.1339 | 3.400 |
| #10-24 UNC | 24 | #25 | 0.1495 | 3.797 |
| #12-24 UNC | 24 | 4.45 mm | 0.1752 | 4.450 |
| 1/4-20 UNC | 20 | #7 | 0.2010 | 5.105 |
| 5/16-18 UNC | 18 | Letter F | 0.2570 | 6.528 |
| 3/8-16 UNC | 16 | 5/16 in | 0.3125 | 7.938 |
| 7/16-14 UNC | 14 | Letter U | 0.3680 | 9.347 |
| 1/2-13 UNC | 13 | 27/64 in | 0.4219 | 10.716 |
| 9/16-12 UNC | 12 | 12.20 mm | 0.4803 | 12.200 |
| 5/8-11 UNC | 11 | 13.60 mm | 0.5354 | 13.600 |
| 3/4-10 UNC | 10 | 16.50 mm | 0.6496 | 16.500 |
| 7/8-9 UNC | 9 | 19.50 mm | 0.7677 | 19.500 |
| 1-8 UNC | 8 | 22.25 mm | 0.8760 | 22.250 |
Unified National Fine / #2 to 1 inch
| Thread designation | TPI | Specified cutting-tap drill | Drill diameter (in) | Drill diameter (mm) |
|---|---|---|---|---|
| #2-64 UNF | 64 | 1.80 mm | 0.0709 | 1.800 |
| #3-56 UNF | 56 | #46 | 0.0810 | 2.057 |
| #4-48 UNF | 48 | 2.32 mm | 0.0913 | 2.320 |
| #5-44 UNF | 44 | 2.60 mm | 0.1024 | 2.600 |
| #6-40 UNF | 40 | #33 | 0.1130 | 2.870 |
| #8-36 UNF | 36 | #29 | 0.1360 | 3.454 |
| #10-32 UNF | 32 | #21 | 0.1590 | 4.039 |
| #12-28 UNF | 28 | 4.60 mm | 0.1811 | 4.600 |
| 1/4-28 UNF | 28 | 5.45 mm | 0.2146 | 5.450 |
| 5/16-24 UNF | 24 | Letter I | 0.2720 | 6.909 |
| 3/8-24 UNF | 24 | 8.50 mm | 0.3346 | 8.500 |
| 7/16-20 UNF | 20 | 9.95 mm | 0.3917 | 9.950 |
| 1/2-20 UNF | 20 | 11.40 mm | 0.4488 | 11.400 |
| 9/16-18 UNF | 18 | 12.90 mm | 0.5079 | 12.900 |
| 5/8-18 UNF | 18 | 14.50 mm | 0.5709 | 14.500 |
| 3/4-16 UNF | 16 | 17.50 mm | 0.6890 | 17.500 |
| 7/8-14 UNF | 14 | 20.40 mm | 0.8031 | 20.400 |
| 1-12 UNF | 12 | 23.30 mm | 0.9173 | 23.300 |
Before using a listed size
- Confirm the exact thread series and pitch, and that the tool is a cutting tap.
- Check the tap manufacturer’s application advice for the material, hole type, thread depth and required fit.
- Measure the actual prepared hole where the specification requires it. A labelled drill size does not prove the hole finished at that diameter.
- For a blind hole, allow the depth required by the drill point, tap lead and chip clearance according to the tool and drawing; the diameter table supplies none of those depths.
- Inspect the finished thread to the job’s required specification. A drill choice alone does not establish tolerance or suitability.
For diameter, pitch and thread-form distinctions, see our imperial thread size chart. That chart is an identification reference, not a drill-selection table.
Sources and review
Data checked 14 October 2026. Revision 1.0. Every specified drill was checked against the rendered primary chart and a separate extraction of its cutting-tap column. Number/letter drill diameters were checked against the same manufacturer’s decimal-equivalents page. Independent arithmetic checked unit conversions and the source’s thread-percentage formulas. These checks establish source fidelity; they are not a second manufacturer’s endorsement or Workshop Standard machining tests.
Conversions use exactly 25.4 mm per inch. Calculated metric equivalents are displayed to 0.001 mm and calculated inch equivalents to 0.0001 inch, rounded half up. These display precisions are not drill or hole tolerances.
- Guhring Decimal Equivalents / Tap Drill Sizes pocket chart: PDF page 2, CUT TAPS approximate 75% column and percentage formulas; page 1, drill designations and decimal diameters.
- Wiseman Threading Tools: general technical information on taps: distinction between cutting and fluteless/forming taps, and application factors.
Other charts can specify different drill sizes or percentages. This guide keeps one named manufacturer’s stated target rather than combining unlike recommendations. It does not reproduce forming-tap, clearance-hole or pipe-thread charts. The HTML and printable reference use one source dataset.