---
title: "Drill Size Chart: Number, Letter, Fraction & Metric | OpenSpindle"
description: "Drill size chart with decimal inch and mm equivalents for number drills #1-#80, letter drills A-Z, fractional drills to 1 in, and metric drills."
canonical: https://openspindle.com/reference/drill-size-chart
---

# Drill Size Chart

Decimal inch and millimeter equivalents for every number, letter, fractional, and common metric drill, plus which tap or clearance hole each size is used for.

Drill bits in the US come in four size systems: **number** drills (#80, the smallest at 0.0135 in, up to #1 at 0.228 in), **letter** drills (A at 0.234 in up to Z at 0.413 in), **fractional** drills in 1/64 in steps, and **metric** drills in millimeters. Number and letter drills fill the gaps between fractional sizes, which is why most tap drills and clearance drills on inch drawings are called out by number or letter.

Note the direction: number drills get **smaller** as the number goes up, while letter drills get **larger** from A to Z. The largest number drill (#1, 0.228 in) is just under the smallest letter drill (A, 0.234 in).

The number and letter tables also list the tap or clearance hole each drill is standard for. For the drill to use with a specific thread, see the [tap drill size chart](/reference/tap-drill-thread-chart).

## Number drill sizes (#1 to #80)

Wire-gauge number drills, from #1 (largest) to #80 (smallest). These are standard in ASME B94.11M.

| Drill | Decimal (in) | Decimal (mm) | Standard use |
| --- | --- | --- | --- |
| #1 | 0.2280 | 5.79 | #12 free clearance |
| #2 | 0.2210 | 5.61 | #12 close clearance |
| #3 | 0.2130 | 5.41 | 1/4-28 tap |
| #4 | 0.2090 | 5.31 | - |
| #5 | 0.2055 | 5.22 | - |
| #6 | 0.2040 | 5.18 | - |
| #7 | 0.2010 | 5.11 | 1/4-20 tap; #10 free clearance |
| #8 | 0.1990 | 5.05 | - |
| #9 | 0.1960 | 4.98 | #10 close clearance |
| #10 | 0.1935 | 4.91 | - |
| #11 | 0.1910 | 4.85 | - |
| #12 | 0.1890 | 4.80 | - |
| #13 | 0.1850 | 4.70 | - |
| #14 | 0.1820 | 4.62 | #12-28 tap |
| #15 | 0.1800 | 4.57 | - |
| #16 | 0.1770 | 4.50 | #12-24 tap; #8 free clearance |
| #17 | 0.1730 | 4.39 | - |
| #18 | 0.1695 | 4.31 | #8 close clearance |
| #19 | 0.1660 | 4.22 | - |
| #20 | 0.1610 | 4.09 | - |
| #21 | 0.1590 | 4.04 | #10-32 tap |
| #22 | 0.1570 | 3.99 | - |
| #23 | 0.1540 | 3.91 | - |
| #24 | 0.1520 | 3.86 | - |
| #25 | 0.1495 | 3.80 | #10-24 tap; #6 free clearance |
| #26 | 0.1470 | 3.73 | - |
| #27 | 0.1440 | 3.66 | #6 close clearance |
| #28 | 0.1405 | 3.57 | - |
| #29 | 0.1360 | 3.45 | #8-32 tap; #8-36 tap |
| #30 | 0.1285 | 3.26 | #4 free clearance |
| #31 | 0.1200 | 3.05 | - |
| #32 | 0.1160 | 2.95 | #4 close clearance |
| #33 | 0.1130 | 2.87 | #6-40 tap |
| #34 | 0.1110 | 2.82 | - |
| #35 | 0.1100 | 2.79 | - |
| #36 | 0.1065 | 2.71 | #6-32 tap |
| #37 | 0.1040 | 2.64 | #5-44 tap |
| #38 | 0.1015 | 2.58 | #5-40 tap |
| #39 | 0.0995 | 2.53 | - |
| #40 | 0.0980 | 2.49 | - |
| #41 | 0.0960 | 2.44 | #2 free clearance |
| #42 | 0.0935 | 2.37 | #4-48 tap |
| #43 | 0.0890 | 2.26 | #4-40 tap; #2 close clearance |
| #44 | 0.0860 | 2.18 | - |
| #45 | 0.0820 | 2.08 | #3-56 tap |
| #46 | 0.0810 | 2.06 | - |
| #47 | 0.0785 | 1.99 | #3-48 tap |
| #48 | 0.0760 | 1.93 | - |
| #49 | 0.0730 | 1.85 | - |
| #50 | 0.0700 | 1.78 | #2-56 tap; #2-64 tap; #0 free clearance |
| #51 | 0.0670 | 1.70 | - |
| #52 | 0.0635 | 1.61 | #0 close clearance |
| #53 | 0.0595 | 1.51 | #1-64 tap; #1-72 tap |
| #54 | 0.0550 | 1.40 | - |
| #55 | 0.0520 | 1.32 | - |
| #56 | 0.0465 | 1.18 | - |
| #57 | 0.0430 | 1.09 | - |
| #58 | 0.0420 | 1.07 | - |
| #59 | 0.0410 | 1.04 | - |
| #60 | 0.0400 | 1.02 | - |
| #61 | 0.0390 | 0.99 | - |
| #62 | 0.0380 | 0.97 | - |
| #63 | 0.0370 | 0.94 | - |
| #64 | 0.0360 | 0.91 | - |
| #65 | 0.0350 | 0.89 | - |
| #66 | 0.0330 | 0.84 | - |
| #67 | 0.0320 | 0.81 | - |
| #68 | 0.0310 | 0.79 | - |
| #69 | 0.0292 | 0.74 | - |
| #70 | 0.0280 | 0.71 | - |
| #71 | 0.0260 | 0.66 | - |
| #72 | 0.0250 | 0.64 | - |
| #73 | 0.0240 | 0.61 | - |
| #74 | 0.0225 | 0.57 | - |
| #75 | 0.0210 | 0.53 | - |
| #76 | 0.0200 | 0.51 | - |
| #77 | 0.0180 | 0.46 | - |
| #78 | 0.0160 | 0.41 | - |
| #79 | 0.0145 | 0.37 | - |
| #80 | 0.0135 | 0.34 | - |

*Tap uses are for about 75% thread with a cutting tap. Clearance uses follow common close and free fit values for inch screws.*

## Letter drill sizes (A to Z)

Letter drills continue above the number series, from A (0.234 in) to Z (0.413 in).

| Drill | Decimal (in) | Decimal (mm) | Standard use |
| --- | --- | --- | --- |
| A | 0.2340 | 5.94 | - |
| B | 0.2380 | 6.05 | - |
| C | 0.2420 | 6.15 | - |
| D | 0.2460 | 6.25 | - |
| E | 0.2500 | 6.35 | - |
| F | 0.2570 | 6.53 | 5/16-18 tap; 1/4 close clearance |
| G | 0.2610 | 6.63 | - |
| H | 0.2660 | 6.76 | 1/4 free clearance |
| I | 0.2720 | 6.91 | 5/16-24 tap |
| J | 0.2770 | 7.04 | - |
| K | 0.2810 | 7.14 | - |
| L | 0.2900 | 7.37 | - |
| M | 0.2950 | 7.49 | - |
| N | 0.3020 | 7.67 | - |
| O | 0.3160 | 8.03 | - |
| P | 0.3230 | 8.20 | 5/16 close clearance |
| Q | 0.3320 | 8.43 | 3/8-24 tap; 5/16 free clearance |
| R | 0.3390 | 8.61 | - |
| S | 0.3480 | 8.84 | - |
| T | 0.3580 | 9.09 | - |
| U | 0.3680 | 9.35 | 7/16-14 tap |
| V | 0.3770 | 9.58 | - |
| W | 0.3860 | 9.80 | 3/8 close clearance |
| X | 0.3970 | 10.08 | 3/8 free clearance |
| Y | 0.4040 | 10.26 | - |
| Z | 0.4130 | 10.49 | - |

## Fractional drill sizes (1/64 in to 1 in)

Fractional drills step in 1/64 in increments. Decimals are exact.

| Fraction | Decimal (in) | Decimal (mm) |
| --- | --- | --- |
| 1/64" | 0.015625 | 0.40 |
| 1/32" | 0.03125 | 0.79 |
| 3/64" | 0.046875 | 1.19 |
| 1/16" | 0.0625 | 1.59 |
| 5/64" | 0.078125 | 1.98 |
| 3/32" | 0.09375 | 2.38 |
| 7/64" | 0.109375 | 2.78 |
| 1/8" | 0.125 | 3.18 |
| 9/64" | 0.140625 | 3.57 |
| 5/32" | 0.15625 | 3.97 |
| 11/64" | 0.171875 | 4.37 |
| 3/16" | 0.1875 | 4.76 |
| 13/64" | 0.203125 | 5.16 |
| 7/32" | 0.21875 | 5.56 |
| 15/64" | 0.234375 | 5.95 |
| 1/4" | 0.25 | 6.35 |
| 17/64" | 0.265625 | 6.75 |
| 9/32" | 0.28125 | 7.14 |
| 19/64" | 0.296875 | 7.54 |
| 5/16" | 0.3125 | 7.94 |
| 21/64" | 0.328125 | 8.33 |
| 11/32" | 0.34375 | 8.73 |
| 23/64" | 0.359375 | 9.13 |
| 3/8" | 0.375 | 9.53 |
| 25/64" | 0.390625 | 9.92 |
| 13/32" | 0.40625 | 10.32 |
| 27/64" | 0.421875 | 10.72 |
| 7/16" | 0.4375 | 11.11 |
| 29/64" | 0.453125 | 11.51 |
| 15/32" | 0.46875 | 11.91 |
| 31/64" | 0.484375 | 12.30 |
| 1/2" | 0.5 | 12.70 |
| 33/64" | 0.515625 | 13.10 |
| 17/32" | 0.53125 | 13.49 |
| 35/64" | 0.546875 | 13.89 |
| 9/16" | 0.5625 | 14.29 |
| 37/64" | 0.578125 | 14.68 |
| 19/32" | 0.59375 | 15.08 |
| 39/64" | 0.609375 | 15.48 |
| 5/8" | 0.625 | 15.88 |
| 41/64" | 0.640625 | 16.27 |
| 21/32" | 0.65625 | 16.67 |
| 43/64" | 0.671875 | 17.07 |
| 11/16" | 0.6875 | 17.46 |
| 45/64" | 0.703125 | 17.86 |
| 23/32" | 0.71875 | 18.26 |
| 47/64" | 0.734375 | 18.65 |
| 3/4" | 0.75 | 19.05 |
| 49/64" | 0.765625 | 19.45 |
| 25/32" | 0.78125 | 19.84 |
| 51/64" | 0.796875 | 20.24 |
| 13/16" | 0.8125 | 20.64 |
| 53/64" | 0.828125 | 21.03 |
| 27/32" | 0.84375 | 21.43 |
| 55/64" | 0.859375 | 21.83 |
| 7/8" | 0.875 | 22.23 |
| 57/64" | 0.890625 | 22.62 |
| 29/32" | 0.90625 | 23.02 |
| 59/64" | 0.921875 | 23.42 |
| 15/16" | 0.9375 | 23.81 |
| 61/64" | 0.953125 | 24.21 |
| 31/32" | 0.96875 | 24.61 |
| 63/64" | 0.984375 | 25.00 |
| 1" | 1 | 25.40 |

## Metric drill sizes to inches

Common metric drills converted to inches, with the nearest fractional drill and the nearest number or letter drill. Metric tap drill sizes (1.6, 2.05, 3.3, 4.2, 6.8, 8.5, and 10.2 mm) are included.

| Metric drill | Decimal (in) | Nearest fractional | Nearest number / letter |
| --- | --- | --- | --- |
| 1.0 mm | 0.0394 | 3/64" (0.0469) | #61 (0.0390) |
| 1.25 mm | 0.0492 | 3/64" (0.0469) | #56 (0.0465) |
| 1.5 mm | 0.0591 | 1/16" (0.0625) | #53 (0.0595) |
| 1.6 mm | 0.0630 | 1/16" (0.0625) | #52 (0.0635) |
| 2.0 mm | 0.0787 | 5/64" (0.0781) | #47 (0.0785) |
| 2.05 mm | 0.0807 | 5/64" (0.0781) | #46 (0.0810) |
| 2.5 mm | 0.0984 | 3/32" (0.0938) | #40 (0.0980) |
| 2.9 mm | 0.1142 | 7/64" (0.1094) | #33 (0.1130) |
| 3.0 mm | 0.1181 | 1/8" (0.1250) | #31 (0.1200) |
| 3.3 mm | 0.1299 | 1/8" (0.1250) | #30 (0.1285) |
| 3.5 mm | 0.1378 | 9/64" (0.1406) | #29 (0.1360) |
| 4.0 mm | 0.1575 | 5/32" (0.1563) | #22 (0.1570) |
| 4.2 mm | 0.1654 | 11/64" (0.1719) | #19 (0.1660) |
| 4.5 mm | 0.1772 | 11/64" (0.1719) | #16 (0.1770) |
| 5.0 mm | 0.1969 | 13/64" (0.2031) | #9 (0.1960) |
| 5.5 mm | 0.2165 | 7/32" (0.2188) | #3 (0.2130) |
| 6.0 mm | 0.2362 | 15/64" (0.2344) | B (0.2380) |
| 6.5 mm | 0.2559 | 1/4" (0.2500) | F (0.2570) |
| 6.8 mm | 0.2677 | 17/64" (0.2656) | H (0.2660) |
| 7.0 mm | 0.2756 | 9/32" (0.2813) | J (0.2770) |
| 7.5 mm | 0.2953 | 19/64" (0.2969) | M (0.2950) |
| 8.0 mm | 0.3150 | 5/16" (0.3125) | O (0.3160) |
| 8.5 mm | 0.3346 | 21/64" (0.3281) | Q (0.3320) |
| 9.0 mm | 0.3543 | 23/64" (0.3594) | T (0.3580) |
| 9.5 mm | 0.3740 | 3/8" (0.3750) | V (0.3770) |
| 10.0 mm | 0.3937 | 25/64" (0.3906) | X (0.3970) |
| 10.2 mm | 0.4016 | 13/32" (0.4063) | Y (0.4040) |
| 10.5 mm | 0.4134 | 13/32" (0.4063) | Z (0.4130) |
| 11.0 mm | 0.4331 | 7/16" (0.4375) | - |
| 11.5 mm | 0.4528 | 29/64" (0.4531) | - |
| 12.0 mm | 0.4724 | 15/32" (0.4688) | - |
| 12.5 mm | 0.4921 | 31/64" (0.4844) | - |
| 13.0 mm | 0.5118 | 33/64" (0.5156) | - |
| 14.0 mm | 0.5512 | 35/64" (0.5469) | - |
| 15.0 mm | 0.5906 | 19/32" (0.5938) | - |
| 16.0 mm | 0.6299 | 5/8" (0.6250) | - |
| 17.0 mm | 0.6693 | 43/64" (0.6719) | - |
| 18.0 mm | 0.7087 | 45/64" (0.7031) | - |
| 19.0 mm | 0.7480 | 3/4" (0.7500) | - |
| 20.0 mm | 0.7874 | 25/32" (0.7813) | - |
| 21.0 mm | 0.8268 | 53/64" (0.8281) | - |
| 22.0 mm | 0.8661 | 55/64" (0.8594) | - |
| 23.0 mm | 0.9055 | 29/32" (0.9063) | - |
| 24.0 mm | 0.9449 | 15/16" (0.9375) | - |
| 25.0 mm | 0.9843 | 63/64" (0.9844) | - |

*Nearest sizes are by diameter. A dash means the size is larger than the letter-drill range (Z, 0.413 in).*

Number and letter drill decimals follow ASME B94.11M. Millimeter values are exact conversions (1 in = 25.4 mm) rounded to 0.01 mm.

## On OpenSpindle

- [Tap drill size chart](https://openspindle.com/reference/tap-drill-thread-chart.md)
- [Thread callout guide](https://openspindle.com/reference/thread-callout-guide.md)
- [CNC machining tolerance chart](https://openspindle.com/reference/cnc-machining-tolerance-chart.md)
- [CNC machining capability](https://openspindle.com/capabilities/cnc-machining.md)

## Frequently asked questions

### What size is a #7 drill?

A #7 drill is 0.201 in (5.11 mm). It is the standard tap drill for a 1/4-20 UNC thread.

### What size is a #29 drill?

A #29 drill is 0.136 in (3.45 mm). It is the tap drill for 8-32 UNC and 8-36 UNF threads.

### What size is a #40 drill bit?

A #40 drill is 0.098 in (2.49 mm). It is close to a 2.5 mm drill (0.0984 in), which is the tap drill for M3 x 0.5.

### What size is a letter F drill?

A letter F drill is 0.257 in (6.53 mm). It is the tap drill for 5/16-18 UNC and the close-fit clearance drill for a 1/4 in screw.

### What size is an S drill bit?

A letter S drill is 0.348 in (8.84 mm).

### What is 7/32 as a decimal?

7/32 in is 0.21875 in (5.56 mm). The nearest number drill is #2 (0.221 in).

### What is a 6 mm drill in inches?

A 6 mm drill is 0.2362 in. The nearest fractional drill is 15/64 in (0.2344 in) and the nearest letter drill is B (0.238 in).

### Which is bigger, a #1 drill or an A drill?

An A drill (0.234 in) is slightly bigger than a #1 drill (0.228 in). Number drills get smaller as the number rises, and letter drills get bigger from A to Z, so the letter series picks up just above #1.

### Why do number and letter drills exist?

They fill the gaps between 1/64 in fractional steps. Many tap drills and clearance holes need a diameter between two fractions, such as the 0.201 in #7 drill for 1/4-20, so inch drawings often call out number and letter drills.