Rule of thumb
Size minus pitch
Tap drill ≈ major diameter minus pitch. For inch that’s D − 1/TPI; for metric it’s D − P, so an M10 x 1.5 takes an 8.5 mm drill.
Percent of thread
About 75% is right
A 75% thread holds nearly as well as a full one and is far easier to tap. Go smaller in soft metals, bigger in tough stuff like stainless.
Drill names
Number & letter drills
Some sizes call for a number (#7) or letter (F) drill instead of a fraction. The decimal under each size gives you a metric or fraction substitute.
Big sizes
Over 1-1/2″
Large taps are usually cut on a lathe or boring bar instead. The drill sizes are still the right starting bore.
Inch threads
Source: Standard tap drill sizes for ASME B1.1 Unified threads, cross-checked by percent of thread (76.98 × TPI × (D − drill)). Most land between 72% and 79%; a few fine threads use the traditional drill at 65–67%. The fine column is UNF up to 1-1/2″ and 12UN above. See the thread pitch chart for which series a size comes in.
| Size | CoarseUNC | FineUNF / 12UN | 8 pitch8UN |
|---|---|---|---|
| 1/4″ | #7 drill1/4-20 UNC0.2010″ · 75% thread | #3 drill1/4-28 UNF0.2130″ · 80% thread | — |
| 5/16″ | F letter5/16-18 UNC0.2570″ · 77% thread | I letter5/16-24 UNF0.2720″ · 75% thread | — |
| 3/8″ | 5/16″3/8-16 UNC0.3125″ · 77% thread | Q letter3/8-24 UNF0.3320″ · 79% thread | — |
| 7/16″ | U letter7/16-14 UNC0.3680″ · 75% thread | 25/64″7/16-20 UNF0.3906″ · 72% thread | — |
| 1/2″ | 27/64″1/2-13 UNC0.4219″ · 78% thread | 29/64″1/2-20 UNF0.4531″ · 72% thread | — |
| 9/16″ | 31/64″9/16-12 UNC0.4844″ · 72% thread | 33/64″9/16-18 UNF0.5156″ · 65% thread | — |
| 5/8″ | 17/32″5/8-11 UNC0.5312″ · 79% thread | 37/64″5/8-18 UNF0.5781″ · 65% thread | — |
| 3/4″ | 21/32″3/4-10 UNC0.6562″ · 72% thread | 11/16″3/4-16 UNF0.6875″ · 77% thread | — |
| 7/8″ | 49/64″7/8-9 UNC0.7656″ · 76% thread | 13/16″7/8-14 UNF0.8125″ · 67% thread | — |
| 1″ | 7/8″1-8 UNC0.8750″ · 77% thread | 59/64″1-12 UNF0.9219″ · 72% thread | — |
| 1-1/8″ | 63/64″1-1/8-7 UNC0.9844″ · 76% thread | 1-3/64″1-1/8-12 UNF1.0469″ · 72% thread | 1″1-1/8-8 8UN1.0000″ · 77% thread |
| 1-1/4″ | 1-7/64″1-1/4-7 UNC1.1094″ · 76% thread | 1-11/64″1-1/4-12 UNF1.1719″ · 72% thread | 1-1/8″1-1/4-8 8UN1.1250″ · 77% thread |
| 1-3/8″ | 1-7/32″1-3/8-6 UNC1.2188″ · 72% thread | 1-19/64″1-3/8-12 UNF1.2969″ · 72% thread | 1-1/4″1-3/8-8 8UN1.2500″ · 77% thread |
| 1-1/2″ | 1-11/32″1-1/2-6 UNC1.3438″ · 72% thread | 1-27/64″1-1/2-12 UNF1.4219″ · 72% thread | 1-3/8″1-1/2-8 8UN1.3750″ · 77% thread |
| 1-3/4″ | 1-9/16″1-3/4-5 UNC1.5625″ · 72% thread | 1-43/64″1-3/4-12 12UN1.6719″ · 72% thread | 1-5/8″1-3/4-8 8UN1.6250″ · 77% thread |
| 2″ | 1-25/32″2-4.5 UNC1.7812″ · 76% thread | 1-59/64″2-12 12UN1.9219″ · 72% thread | 1-7/8″2-8 8UN1.8750″ · 77% thread |
Metric threads
Source: ISO 261 / ISO 262 threads, tap drill = major diameter − pitch (about 77% thread), rounded to the nearest stocked drill where that lands between sizes (M8 takes 6.8 mm, M12 takes 10.2 mm), per the standard ISO tap drill tables. Pitches follow our metric thread pitch chart.
M3 M3.5 M4 M5 M6 M7 M8 M10 M12 M14 M16 M18 M20 M22 M24 M27 M30 M33 M36 M39 M42 M45 M48 M52 M56 M60 M64
| Size | Coarsetap drill | Finetap drill | Extra finetap drill |
|---|---|---|---|
| M3 | 2.5mmM3 x 0.5≈ 0.0984″ | 2.65mmM3 x 0.35≈ 0.1043″ | — |
| M3.52nd choice | 2.9mmM3.5 x 0.6≈ 0.1142″ | 3.15mmM3.5 x 0.35≈ 0.1240″ | — |
| M4 | 3.3mmM4 x 0.7≈ 0.1299″ | 3.5mmM4 x 0.5≈ 0.1378″ | — |
| M5 | 4.2mmM5 x 0.8≈ 0.1654″ | 4.5mmM5 x 0.5≈ 0.1772″ | — |
| M6 | 5mmM6 x 1≈ 0.1969″ | 5.2mmM6 x 0.75≈ 0.2047″ | — |
| M72nd choice | 6mmM7 x 1≈ 0.2362″ | 6.2mmM7 x 0.75≈ 0.2441″ | — |
| M8 | 6.8mmM8 x 1.25≈ 0.2677″ | 7mmM8 x 1≈ 0.2756″ | 7.2mmM8 x 0.75≈ 0.2835″ |
| M10 | 8.5mmM10 x 1.5≈ 0.3346″ | 8.8mmM10 x 1.25≈ 0.3465″ | 9mmM10 x 1≈ 0.3543″ |
| M12 | 10.2mmM12 x 1.75≈ 0.4016″ | 10.5mmM12 x 1.5≈ 0.4134″ | 10.8mmM12 x 1.25≈ 0.4252″ |
| M142nd choice | 12mmM14 x 2≈ 0.4724″ | 12.5mmM14 x 1.5≈ 0.4921″ | — |
| M16 | 14mmM16 x 2≈ 0.5512″ | 14.5mmM16 x 1.5≈ 0.5709″ | — |
| M182nd choice | 15.5mmM18 x 2.5≈ 0.6102″ | 16.5mmM18 x 1.5≈ 0.6496″ | — |
| M20 | 17.5mmM20 x 2.5≈ 0.6890″ | 18.5mmM20 x 1.5≈ 0.7283″ | — |
| M222nd choice | 19.5mmM22 x 2.5≈ 0.7677″ | 20.5mmM22 x 1.5≈ 0.8071″ | — |
| M24 | 21mmM24 x 3≈ 0.8268″ | 22mmM24 x 2≈ 0.8661″ | — |
| M272nd choice | 24mmM27 x 3≈ 0.9449″ | 25mmM27 x 2≈ 0.9843″ | — |
| M30 | 26.5mmM30 x 3.5≈ 1.0433″ | 28mmM30 x 2≈ 1.1024″ | — |
| M332nd choice | 29.5mmM33 x 3.5≈ 1.1614″ | 31mmM33 x 2≈ 1.2205″ | — |
| M36 | 32mmM36 x 4≈ 1.2598″ | 33mmM36 x 3≈ 1.2992″ | — |
| M392nd choice | 35mmM39 x 4≈ 1.3780″ | 36mmM39 x 3≈ 1.4173″ | — |
| M42 | 37.5mmM42 x 4.5≈ 1.4764″ | 39mmM42 x 3≈ 1.5354″ | — |
| M452nd choice | 40.5mmM45 x 4.5≈ 1.5945″ | 42mmM45 x 3≈ 1.6535″ | — |
| M48 | 43mmM48 x 5≈ 1.6929″ | 45mmM48 x 3≈ 1.7717″ | — |
| M522nd choice | 47mmM52 x 5≈ 1.8504″ | 48mmM52 x 4≈ 1.8898″ | — |
| M56 | 50.5mmM56 x 5.5≈ 1.9882″ | 52mmM56 x 4≈ 2.0472″ | — |
| M602nd choice | 54.5mmM60 x 5.5≈ 2.1457″ | 56mmM60 x 4≈ 2.2047″ | — |
| M64 | 58mmM64 x 6≈ 2.2835″ | 60mmM64 x 4≈ 2.3622″ | — |
These are for cutting taps. Form (roll) taps need a larger drill, so check the tap maker’s chart. Reference information only — verify against the current published standard and your tooling before use. Need the wrench for the finished bolt? See the wrench size chart.
Drill sizes & decimal equivalents
Every drill in size order: number drills #40 to #1, letter drills A to Z, fractional 1/16″ to 1″ in 64ths, and metric drills to 25 mm in half-millimeter steps plus the in-between tap drill sizes. Rows with an amber edge are tap drills from the charts above, with the threads they’re for. Tap drills over 1″ or 25 mm are in the charts above only.
| Drill | Inchdecimal | Metricmm | Tap drill for |
|---|---|---|---|
| 1 mmMetric | 0.0394″ | 1.00 | — |
| 1.5 mmMetric | 0.0591″ | 1.50 | — |
| 1/16″Fraction | 0.0625″ | 1.59 | — |
| 5/64″Fraction | 0.0781″ | 1.98 | — |
| 2 mmMetric | 0.0787″ | 2.00 | — |
| 3/32″Fraction | 0.0938″ | 2.38 | — |
| #40Number | 0.0980″ | 2.49 | — |
| 2.5 mmMetric | 0.0984″ | 2.50 | M3 x 0.5 |
| #39Number | 0.0995″ | 2.53 | — |
| #38Number | 0.1015″ | 2.58 | — |
| #37Number | 0.1040″ | 2.64 | — |
| 2.65 mmMetric | 0.1043″ | 2.65 | M3 x 0.35 |
| #36Number | 0.1065″ | 2.71 | — |
| 7/64″Fraction | 0.1094″ | 2.78 | — |
| #35Number | 0.1100″ | 2.79 | — |
| #34Number | 0.1110″ | 2.82 | — |
| #33Number | 0.1130″ | 2.87 | — |
| 2.9 mmMetric | 0.1142″ | 2.90 | M3.5 x 0.6 |
| #32Number | 0.1160″ | 2.95 | — |
| 3 mmMetric | 0.1181″ | 3.00 | — |
| #31Number | 0.1200″ | 3.05 | — |
| 3.15 mmMetric | 0.1240″ | 3.15 | M3.5 x 0.35 |
| 1/8″Fraction | 0.1250″ | 3.17 | — |
| #30Number | 0.1285″ | 3.26 | — |
| 3.3 mmMetric | 0.1299″ | 3.30 | M4 x 0.7 |
| #29Number | 0.1360″ | 3.45 | — |
| 3.5 mmMetric | 0.1378″ | 3.50 | M4 x 0.5 |
| #28Number | 0.1405″ | 3.57 | — |
| 9/64″Fraction | 0.1406″ | 3.57 | — |
| #27Number | 0.1440″ | 3.66 | — |
| #26Number | 0.1470″ | 3.73 | — |
| #25Number | 0.1495″ | 3.80 | — |
| #24Number | 0.1520″ | 3.86 | — |
| #23Number | 0.1540″ | 3.91 | — |
| 5/32″Fraction | 0.1562″ | 3.97 | — |
| #22Number | 0.1570″ | 3.99 | — |
| 4 mmMetric | 0.1575″ | 4.00 | — |
| #21Number | 0.1590″ | 4.04 | — |
| #20Number | 0.1610″ | 4.09 | — |
| 4.2 mmMetric | 0.1654″ | 4.20 | M5 x 0.8 |
| #19Number | 0.1660″ | 4.22 | — |
| #18Number | 0.1695″ | 4.31 | — |
| 11/64″Fraction | 0.1719″ | 4.37 | — |
| #17Number | 0.1730″ | 4.39 | — |
| #16Number | 0.1770″ | 4.50 | — |
| 4.5 mmMetric | 0.1772″ | 4.50 | M5 x 0.5 |
| #15Number | 0.1800″ | 4.57 | — |
| #14Number | 0.1820″ | 4.62 | — |
| #13Number | 0.1850″ | 4.70 | — |
| 3/16″Fraction | 0.1875″ | 4.76 | — |
| #12Number | 0.1890″ | 4.80 | — |
| #11Number | 0.1910″ | 4.85 | — |
| #10Number | 0.1935″ | 4.91 | — |
| #9Number | 0.1960″ | 4.98 | — |
| 5 mmMetric | 0.1969″ | 5.00 | M6 x 1 |
| #8Number | 0.1990″ | 5.05 | — |
| #7Number | 0.2010″ | 5.11 | 1/4-20 UNC |
| 13/64″Fraction | 0.2031″ | 5.16 | — |
| #6Number | 0.2040″ | 5.18 | — |
| 5.2 mmMetric | 0.2047″ | 5.20 | M6 x 0.75 |
| #5Number | 0.2055″ | 5.22 | — |
| #4Number | 0.2090″ | 5.31 | — |
| #3Number | 0.2130″ | 5.41 | 1/4-28 UNF |
| 5.5 mmMetric | 0.2165″ | 5.50 | — |
| 7/32″Fraction | 0.2188″ | 5.56 | — |
| #2Number | 0.2210″ | 5.61 | — |
| #1Number | 0.2280″ | 5.79 | — |
| ALetter | 0.2340″ | 5.94 | — |
| 15/64″Fraction | 0.2344″ | 5.95 | — |
| 6 mmMetric | 0.2362″ | 6.00 | M7 x 1 |
| BLetter | 0.2380″ | 6.05 | — |
| CLetter | 0.2420″ | 6.15 | — |
| 6.2 mmMetric | 0.2441″ | 6.20 | M7 x 0.75 |
| DLetter | 0.2460″ | 6.25 | — |
| ELetter | 0.2500″ | 6.35 | — |
| 1/4″Fraction | 0.2500″ | 6.35 | — |
| 6.5 mmMetric | 0.2559″ | 6.50 | — |
| FLetter | 0.2570″ | 6.53 | 5/16-18 UNC |
| GLetter | 0.2610″ | 6.63 | — |
| 17/64″Fraction | 0.2656″ | 6.75 | — |
| HLetter | 0.2660″ | 6.76 | — |
| 6.8 mmMetric | 0.2677″ | 6.80 | M8 x 1.25 |
| ILetter | 0.2720″ | 6.91 | 5/16-24 UNF |
| 7 mmMetric | 0.2756″ | 7.00 | M8 x 1 |
| JLetter | 0.2770″ | 7.04 | — |
| KLetter | 0.2810″ | 7.14 | — |
| 9/32″Fraction | 0.2812″ | 7.14 | — |
| 7.2 mmMetric | 0.2835″ | 7.20 | M8 x 0.75 |
| LLetter | 0.2900″ | 7.37 | — |
| MLetter | 0.2950″ | 7.49 | — |
| 7.5 mmMetric | 0.2953″ | 7.50 | — |
| 19/64″Fraction | 0.2969″ | 7.54 | — |
| NLetter | 0.3020″ | 7.67 | — |
| 5/16″Fraction | 0.3125″ | 7.94 | 3/8-16 UNC |
| 8 mmMetric | 0.3150″ | 8.00 | — |
| OLetter | 0.3160″ | 8.03 | — |
| PLetter | 0.3230″ | 8.20 | — |
| 21/64″Fraction | 0.3281″ | 8.33 | — |
| QLetter | 0.3320″ | 8.43 | 3/8-24 UNF |
| 8.5 mmMetric | 0.3346″ | 8.50 | M10 x 1.5 |
| RLetter | 0.3390″ | 8.61 | — |
| 11/32″Fraction | 0.3438″ | 8.73 | — |
| 8.8 mmMetric | 0.3465″ | 8.80 | M10 x 1.25 |
| SLetter | 0.3480″ | 8.84 | — |
| 9 mmMetric | 0.3543″ | 9.00 | M10 x 1 |
| TLetter | 0.3580″ | 9.09 | — |
| 23/64″Fraction | 0.3594″ | 9.13 | — |
| ULetter | 0.3680″ | 9.35 | 7/16-14 UNC |
| 9.5 mmMetric | 0.3740″ | 9.50 | — |
| 3/8″Fraction | 0.3750″ | 9.52 | — |
| VLetter | 0.3770″ | 9.58 | — |
| WLetter | 0.3860″ | 9.80 | — |
| 25/64″Fraction | 0.3906″ | 9.92 | 7/16-20 UNF |
| 10 mmMetric | 0.3937″ | 10.00 | — |
| XLetter | 0.3970″ | 10.08 | — |
| 10.2 mmMetric | 0.4016″ | 10.20 | M12 x 1.75 |
| YLetter | 0.4040″ | 10.26 | — |
| 13/32″Fraction | 0.4062″ | 10.32 | — |
| ZLetter | 0.4130″ | 10.49 | — |
| 10.5 mmMetric | 0.4134″ | 10.50 | M12 x 1.5 |
| 27/64″Fraction | 0.4219″ | 10.72 | 1/2-13 UNC |
| 10.8 mmMetric | 0.4252″ | 10.80 | M12 x 1.25 |
| 11 mmMetric | 0.4331″ | 11.00 | — |
| 7/16″Fraction | 0.4375″ | 11.11 | — |
| 11.5 mmMetric | 0.4528″ | 11.50 | — |
| 29/64″Fraction | 0.4531″ | 11.51 | 1/2-20 UNF |
| 15/32″Fraction | 0.4688″ | 11.91 | — |
| 12 mmMetric | 0.4724″ | 12.00 | M14 x 2 |
| 31/64″Fraction | 0.4844″ | 12.30 | 9/16-12 UNC |
| 12.5 mmMetric | 0.4921″ | 12.50 | M14 x 1.5 |
| 1/2″Fraction | 0.5000″ | 12.70 | — |
| 13 mmMetric | 0.5118″ | 13.00 | — |
| 33/64″Fraction | 0.5156″ | 13.10 | 9/16-18 UNF |
| 17/32″Fraction | 0.5312″ | 13.49 | 5/8-11 UNC |
| 13.5 mmMetric | 0.5315″ | 13.50 | — |
| 35/64″Fraction | 0.5469″ | 13.89 | — |
| 14 mmMetric | 0.5512″ | 14.00 | M16 x 2 |
| 9/16″Fraction | 0.5625″ | 14.29 | — |
| 14.5 mmMetric | 0.5709″ | 14.50 | M16 x 1.5 |
| 37/64″Fraction | 0.5781″ | 14.68 | 5/8-18 UNF |
| 15 mmMetric | 0.5906″ | 15.00 | — |
| 19/32″Fraction | 0.5938″ | 15.08 | — |
| 39/64″Fraction | 0.6094″ | 15.48 | — |
| 15.5 mmMetric | 0.6102″ | 15.50 | M18 x 2.5 |
| 5/8″Fraction | 0.6250″ | 15.88 | — |
| 16 mmMetric | 0.6299″ | 16.00 | — |
| 41/64″Fraction | 0.6406″ | 16.27 | — |
| 16.5 mmMetric | 0.6496″ | 16.50 | M18 x 1.5 |
| 21/32″Fraction | 0.6562″ | 16.67 | 3/4-10 UNC |
| 17 mmMetric | 0.6693″ | 17.00 | — |
| 43/64″Fraction | 0.6719″ | 17.07 | — |
| 11/16″Fraction | 0.6875″ | 17.46 | 3/4-16 UNF |
| 17.5 mmMetric | 0.6890″ | 17.50 | M20 x 2.5 |
| 45/64″Fraction | 0.7031″ | 17.86 | — |
| 18 mmMetric | 0.7087″ | 18.00 | — |
| 23/32″Fraction | 0.7188″ | 18.26 | — |
| 18.5 mmMetric | 0.7283″ | 18.50 | M20 x 1.5 |
| 47/64″Fraction | 0.7344″ | 18.65 | — |
| 19 mmMetric | 0.7480″ | 19.00 | — |
| 3/4″Fraction | 0.7500″ | 19.05 | — |
| 49/64″Fraction | 0.7656″ | 19.45 | 7/8-9 UNC |
| 19.5 mmMetric | 0.7677″ | 19.50 | M22 x 2.5 |
| 25/32″Fraction | 0.7812″ | 19.84 | — |
| 20 mmMetric | 0.7874″ | 20.00 | — |
| 51/64″Fraction | 0.7969″ | 20.24 | — |
| 20.5 mmMetric | 0.8071″ | 20.50 | M22 x 1.5 |
| 13/16″Fraction | 0.8125″ | 20.64 | 7/8-14 UNF |
| 21 mmMetric | 0.8268″ | 21.00 | M24 x 3 |
| 53/64″Fraction | 0.8281″ | 21.03 | — |
| 27/32″Fraction | 0.8438″ | 21.43 | — |
| 21.5 mmMetric | 0.8465″ | 21.50 | — |
| 55/64″Fraction | 0.8594″ | 21.83 | — |
| 22 mmMetric | 0.8661″ | 22.00 | M24 x 2 |
| 7/8″Fraction | 0.8750″ | 22.22 | 1-8 UNC |
| 22.5 mmMetric | 0.8858″ | 22.50 | — |
| 57/64″Fraction | 0.8906″ | 22.62 | — |
| 23 mmMetric | 0.9055″ | 23.00 | — |
| 29/32″Fraction | 0.9062″ | 23.02 | — |
| 59/64″Fraction | 0.9219″ | 23.42 | 1-12 UNF |
| 23.5 mmMetric | 0.9252″ | 23.50 | — |
| 15/16″Fraction | 0.9375″ | 23.81 | — |
| 24 mmMetric | 0.9449″ | 24.00 | M27 x 3 |
| 61/64″Fraction | 0.9531″ | 24.21 | — |
| 24.5 mmMetric | 0.9646″ | 24.50 | — |
| 31/32″Fraction | 0.9688″ | 24.61 | — |
| 25 mmMetric | 0.9843″ | 25.00 | M27 x 2 |
| 63/64″Fraction | 0.9844″ | 25.00 | 1-1/8-7 UNC |
| 1″Fraction | 1.0000″ | 25.40 | 1-1/8-8 8UN |