Drill size chart: tap drill, clearance, number and letter — metric and ANSI.
Complete drill size reference for production engineers. Metric tap drill (M1–M48 coarse and fine), UNC and UNF tap drill, clearance drill per ISO 273 (close / normal / loose), number drills (#1–#80), letter drills (A–Z), NPT pipe tap drill, and Helicoil tap drill. All values for the standard 75% thread engagement unless noted.
The tap drill formula
For metric threads, the standard tap drill formula gives ~75% thread engagement:
Metric tap drill (75% engagement)
Tap drill = major diameter − pitch
For imperial threads, the equivalent formula is more involved due to the 60° thread geometry. The values in the imperial tables below come from ANSI B1.1 standard.
Metric tap drill chart — ISO coarse and fine thread
| Size | Coarse pitch | Tap drill (coarse) | Fine pitch | Tap drill (fine) |
|---|---|---|---|---|
| M1 | 0.25 | 0.75 | — | — |
| M1.6 | 0.35 | 1.25 | — | — |
| M2 | 0.40 | 1.60 | — | — |
| M2.5 | 0.45 | 2.05 | — | — |
| M3 | 0.50 | 2.50 | — | — |
| M3.5 | 0.60 | 2.90 | — | — |
| M4 | 0.70 | 3.30 | — | — |
| M5 | 0.80 | 4.20 | — | — |
| M6 | 1.00 | 5.00 | 0.75 | 5.25 |
| M8 | 1.25 | 6.80 | 1.00 | 7.00 |
| M10 | 1.50 | 8.50 | 1.25 | 8.75 |
| M12 | 1.75 | 10.20 | 1.25 | 10.75 |
| M14 | 2.00 | 12.00 | 1.50 | 12.50 |
| M16 | 2.00 | 14.00 | 1.50 | 14.50 |
| M18 | 2.50 | 15.50 | 1.50 | 16.50 |
| M20 | 2.50 | 17.50 | 1.50 | 18.50 |
| M22 | 2.50 | 19.50 | 1.50 | 20.50 |
| M24 | 3.00 | 21.00 | 2.00 | 22.00 |
| M27 | 3.00 | 24.00 | 2.00 | 25.00 |
| M30 | 3.50 | 26.50 | 2.00 | 28.00 |
| M33 | 3.50 | 29.50 | 2.00 | 31.00 |
| M36 | 4.00 | 32.00 | 3.00 | 33.00 |
| M39 | 4.00 | 35.00 | 3.00 | 36.00 |
| M42 | 4.50 | 37.50 | 3.00 | 39.00 |
| M45 | 4.50 | 40.50 | 3.00 | 42.00 |
| M48 | 5.00 | 43.00 | 3.00 | 45.00 |
Tap drill values in mm, for ~75% thread engagement. For 100% engagement, subtract approximately 50% of the pitch from the tap drill (e.g. M8 100% = 6.4 mm).
UNC tap drill chart — unified coarse thread
| Size | TPI | Tap drill (number/letter) | Tap drill (inch) | Tap drill (mm) |
|---|---|---|---|---|
| #0 | 80 | 3/64 | 0.0469 | 1.19 |
| #1 | 64 | #53 | 0.0595 | 1.51 |
| #2 | 56 | #50 | 0.0700 | 1.78 |
| #3 | 48 | #47 | 0.0785 | 1.99 |
| #4 | 40 | #43 | 0.0890 | 2.26 |
| #5 | 40 | #38 | 0.1015 | 2.58 |
| #6 | 32 | #36 | 0.1065 | 2.71 |
| #8 | 32 | #29 | 0.1360 | 3.45 |
| #10 | 24 | #25 | 0.1495 | 3.80 |
| #12 | 24 | #16 | 0.1770 | 4.50 |
| 1/4" | 20 | #7 | 0.2010 | 5.11 |
| 5/16" | 18 | F | 0.2570 | 6.53 |
| 3/8" | 16 | 5/16" | 0.3125 | 7.94 |
| 7/16" | 14 | U | 0.3680 | 9.35 |
| 1/2" | 13 | 27/64 | 0.4219 | 10.72 |
| 9/16" | 12 | 31/64 | 0.4844 | 12.30 |
| 5/8" | 11 | 17/32 | 0.5313 | 13.49 |
| 3/4" | 10 | 21/32 | 0.6563 | 16.67 |
| 7/8" | 9 | 49/64 | 0.7656 | 19.45 |
| 1" | 8 | 7/8" | 0.8750 | 22.23 |
| 1-1/4" | 7 | 1-7/64 | 1.1094 | 28.18 |
| 1-1/2" | 6 | 1-11/32 | 1.3438 | 34.13 |
UNF tap drill chart — unified fine thread
| Size | TPI | Tap drill (number/letter) | Tap drill (inch) | Tap drill (mm) |
|---|---|---|---|---|
| #0 | 80 | 3/64 | 0.0469 | 1.19 |
| #1 | 72 | #53 | 0.0595 | 1.51 |
| #2 | 64 | #50 | 0.0700 | 1.78 |
| #3 | 56 | #45 | 0.0820 | 2.08 |
| #4 | 48 | #42 | 0.0935 | 2.37 |
| #5 | 44 | #37 | 0.1040 | 2.64 |
| #6 | 40 | #33 | 0.1130 | 2.87 |
| #8 | 36 | #29 | 0.1360 | 3.45 |
| #10 | 32 | #21 | 0.1590 | 4.04 |
| #12 | 28 | #14 | 0.1820 | 4.62 |
| 1/4" | 28 | #3 | 0.2130 | 5.41 |
| 5/16" | 24 | I | 0.2720 | 6.91 |
| 3/8" | 24 | Q | 0.3320 | 8.43 |
| 7/16" | 20 | 25/64 | 0.3906 | 9.92 |
| 1/2" | 20 | 29/64 | 0.4531 | 11.51 |
| 9/16" | 18 | 33/64 | 0.5156 | 13.10 |
| 5/8" | 18 | 37/64 | 0.5781 | 14.68 |
| 3/4" | 16 | 11/16 | 0.6875 | 17.46 |
| 7/8" | 14 | 13/16 | 0.8125 | 20.64 |
| 1" | 12 | 59/64 | 0.9219 | 23.42 |
Metric clearance drill — ISO 273
Clearance hole sizes for through-bolt assembly. Close fit when the bolt locates the assembly. Normal fit is the standard default. Loose fit allows misalignment up to ~0.3× bolt diameter.
| Bolt | Close fit | Normal fit | Loose fit |
|---|---|---|---|
| M3 | 3.2 | 3.4 | 3.6 |
| M4 | 4.3 | 4.5 | 4.8 |
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7.0 |
| M8 | 8.4 | 9.0 | 10.0 |
| M10 | 10.5 | 11.0 | 12.0 |
| M12 | 13.0 | 13.5 | 14.5 |
| M14 | 15.0 | 15.5 | 16.5 |
| M16 | 17.0 | 17.5 | 18.5 |
| M18 | 19.0 | 20.0 | 21.0 |
| M20 | 21.0 | 22.0 | 24.0 |
| M22 | 23.0 | 24.0 | 26.0 |
| M24 | 25.0 | 26.0 | 28.0 |
| M27 | 28.0 | 30.0 | 32.0 |
| M30 | 31.0 | 33.0 | 35.0 |
| M36 | 37.0 | 39.0 | 42.0 |
Values in mm. Close = H12 hole, Normal = H13, Loose = H14 (per ISO 273-1979).
Number drill sizes — #1 to #80
| # | Inch | mm | # | Inch | mm | # | Inch | mm |
|---|---|---|---|---|---|---|---|---|
| #1 | 0.2280 | 5.79 | #28 | 0.1405 | 3.57 | #55 | 0.0520 | 1.32 |
| #2 | 0.2210 | 5.61 | #29 | 0.1360 | 3.45 | #56 | 0.0465 | 1.18 |
| #3 | 0.2130 | 5.41 | #30 | 0.1285 | 3.26 | #57 | 0.0430 | 1.09 |
| #4 | 0.2090 | 5.31 | #31 | 0.1200 | 3.05 | #58 | 0.0420 | 1.07 |
| #5 | 0.2055 | 5.22 | #32 | 0.1160 | 2.95 | #59 | 0.0410 | 1.04 |
| #6 | 0.2040 | 5.18 | #33 | 0.1130 | 2.87 | #60 | 0.0400 | 1.02 |
| #7 | 0.2010 | 5.11 | #34 | 0.1110 | 2.82 | #61 | 0.0390 | 0.99 |
| #8 | 0.1990 | 5.05 | #35 | 0.1100 | 2.79 | #62 | 0.0380 | 0.97 |
| #9 | 0.1960 | 4.98 | #36 | 0.1065 | 2.71 | #63 | 0.0370 | 0.94 |
| #10 | 0.1935 | 4.91 | #37 | 0.1040 | 2.64 | #64 | 0.0360 | 0.91 |
| #11 | 0.1910 | 4.85 | #38 | 0.1015 | 2.58 | #65 | 0.0350 | 0.89 |
| #12 | 0.1890 | 4.80 | #39 | 0.0995 | 2.53 | #66 | 0.0330 | 0.84 |
| #13 | 0.1850 | 4.70 | #40 | 0.0980 | 2.49 | #67 | 0.0320 | 0.81 |
| #14 | 0.1820 | 4.62 | #41 | 0.0960 | 2.44 | #68 | 0.0310 | 0.79 |
| #15 | 0.1800 | 4.57 | #42 | 0.0935 | 2.37 | #69 | 0.0292 | 0.74 |
| #16 | 0.1770 | 4.50 | #43 | 0.0890 | 2.26 | #70 | 0.0280 | 0.71 |
| #17 | 0.1730 | 4.39 | #44 | 0.0860 | 2.18 | #71 | 0.0260 | 0.66 |
| #18 | 0.1695 | 4.31 | #45 | 0.0820 | 2.08 | #72 | 0.0250 | 0.64 |
| #19 | 0.1660 | 4.22 | #46 | 0.0810 | 2.06 | #73 | 0.0240 | 0.61 |
| #20 | 0.1610 | 4.09 | #47 | 0.0785 | 1.99 | #74 | 0.0225 | 0.57 |
| #21 | 0.1590 | 4.04 | #48 | 0.0760 | 1.93 | #75 | 0.0210 | 0.53 |
| #22 | 0.1570 | 3.99 | #49 | 0.0730 | 1.85 | #76 | 0.0200 | 0.51 |
| #23 | 0.1540 | 3.91 | #50 | 0.0700 | 1.78 | #77 | 0.0180 | 0.46 |
| #24 | 0.1520 | 3.86 | #51 | 0.0670 | 1.70 | #78 | 0.0160 | 0.41 |
| #25 | 0.1495 | 3.80 | #52 | 0.0635 | 1.61 | #79 | 0.0145 | 0.37 |
| #26 | 0.1470 | 3.73 | #53 | 0.0595 | 1.51 | #80 | 0.0135 | 0.34 |
| #27 | 0.1440 | 3.66 | #54 | 0.0550 | 1.40 |
Letter drill sizes — A to Z
| Letter | Inch | mm | Letter | Inch | mm |
|---|---|---|---|---|---|
| A | 0.2340 | 5.94 | N | 0.3020 | 7.67 |
| B | 0.2380 | 6.05 | O | 0.3160 | 8.03 |
| C | 0.2420 | 6.15 | P | 0.3230 | 8.20 |
| D | 0.2460 | 6.25 | Q | 0.3320 | 8.43 |
| E | 0.2500 | 6.35 | R | 0.3390 | 8.61 |
| F | 0.2570 | 6.53 | S | 0.3480 | 8.84 |
| G | 0.2610 | 6.63 | T | 0.3580 | 9.09 |
| H | 0.2660 | 6.76 | U | 0.3680 | 9.35 |
| I | 0.2720 | 6.91 | V | 0.3770 | 9.58 |
| J | 0.2770 | 7.04 | W | 0.3860 | 9.80 |
| K | 0.2810 | 7.14 | X | 0.3970 | 10.08 |
| L | 0.2900 | 7.37 | Y | 0.4040 | 10.26 |
| M | 0.2950 | 7.49 | Z | 0.4130 | 10.49 |
NPT pipe tap drill chart
National Pipe Taper threads (NPT) use tap drills sized for the small end of the tapered thread. Per ASME B1.20.1.
| NPT size | TPI | Tap drill (inch) | Tap drill (mm) |
|---|---|---|---|
| 1/16" | 27 | D (0.246) | 6.25 |
| 1/8" | 27 | R (0.339) | 8.61 |
| 1/4" | 18 | 7/16 (0.4375) | 11.11 |
| 3/8" | 18 | 37/64 (0.578) | 14.68 |
| 1/2" | 14 | 23/32 (0.7188) | 18.26 |
| 3/4" | 14 | 59/64 (0.9219) | 23.42 |
| 1" | 11.5 | 1-5/32 (1.156) | 29.36 |
| 1-1/4" | 11.5 | 1-1/2 (1.500) | 38.10 |
| 1-1/2" | 11.5 | 1-23/32 (1.7188) | 43.66 |
| 2" | 11.5 | 2-3/16 (2.1875) | 55.56 |
Helicoil tap drill chart — metric
Helicoil (wire thread insert) requires a larger tap drill than a standard thread because the insert sits inside an oversized tapped hole. Per Helicoil engineering manual.
| Insert | STI tap size | Tap drill (mm) |
|---|---|---|
| M3 × 0.5 | M3 STI | 3.20 |
| M4 × 0.7 | M4 STI | 4.30 |
| M5 × 0.8 | M5 STI | 5.40 |
| M6 × 1.0 | M6 STI | 6.40 |
| M8 × 1.25 | M8 STI | 8.50 |
| M10 × 1.5 | M10 STI | 10.50 |
| M12 × 1.75 | M12 STI | 12.60 |
| M14 × 2.0 | M14 STI | 14.75 |
| M16 × 2.0 | M16 STI | 16.75 |
| M20 × 2.5 | M20 STI | 20.90 |
5 common drilling mistakes
- Using the wrong tap drill for the desired engagement. Most charts default to 75% engagement. If the drawing specifies 50% engagement (common for soft materials), the tap drill is 0.5 × pitch larger. If 100% is specified, the tap drill is 0.5 × pitch smaller. Always check the drawing.
- Using a fine-thread tap drill for a coarse thread. M8 × 1.25 (coarse) takes a 6.8 mm tap drill; M8 × 1.0 (fine) takes 7.0 mm. The difference is small but enough to leave the wrong thread profile. Confirm pitch before drilling.
- Drilling at the wrong depth. For blind holes, tap drill depth must be at least 1.5–2× the desired thread depth to allow chips to fall clear. Drilling exactly to thread depth causes the tap to bottom out and break.
- Drilling without peck cycle for deep holes. Aspect ratios above 3:1 (depth > 3× diameter) need pecking to clear chips. Otherwise chip welding and tool breakage. Modern CNC controllers handle this automatically; manual machines need explicit peck-drill cycles.
- Choosing close-fit clearance when normal is enough. Close-fit clearance holes require tight position tolerance to avoid bolt binding. Normal fit accommodates ±0.1 mm position drift; close fit only ±0.05 mm. Use normal unless the bolt is designed to locate the assembly.
Need a thread or drill not in this chart?
The MetricMech Drill Size Calculator handles arbitrary thread inputs including non-standard pitches and custom engagement percentages. For thread pitch math (lead, helix angle, minor/pitch diameters), see the Thread Pitch Calculator.
References
- ISO 261:1998 — ISO general purpose metric screw threads — General plan
- ISO 262:1998 — ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts
- ISO 273:1979 — Fasteners — Clearance holes for bolts and screws
- ANSI B1.1-2003 — Unified Inch Screw Threads (UN and UNR Thread Form)
- ASME B1.20.1-2013 — Pipe Threads, General Purpose, Inch
- Helicoil Engineering Manual — Stanley Engineered Fastening
For interactive drill size lookup with custom thread specs, see the Drill Size Calculator. For tap selection and thread engineering math, see the Thread Pitch Calculator.