DRILL SIZE REFERENCE
Drill Bit Decimal Equivalent Chart
Cross-reference fractional, number (#1–#80), letter (A–Z), and metric drill sizes with decimal inch and millimeter equivalents. Includes tap drill reference and common CNC hole-making applications.
✓
Fractional / Number / Letter / Metric
✓
Decimal Inch & Millimeter
✓
Tap Drill Reference Included
Drill Sizing Systems
Four ways to measure a drill bit
Fractional1/64″ to 1″+ increments
Number#80 (0.0135″) to #1 (0.2280″)
LetterA (0.2340″) to Z (0.4130″)
Metricmm sizes (0.1–50+ mm)
Conversion Reference
1″ = 25.4 mm
DRILL SIZING
The Four Drill Bit Sizing Systems
Drill bits come in fractional inch, number gauge, letter gauge, and metric — all converted to a common decimal inch or millimeter diameter.
A drill bit decimal equivalent chart unifies four independent sizing systems into one cross-reference table. Understanding how each system works makes the chart faster to read and harder to misread.
Fractional Inch
Sized in 1/64″ increments from 1/64″ (0.0156″) up to 1-3/4″ and beyond. Most common system in US manufacturing. Simple to read: 1/16″ = 0.0625″, 1/4″ = 0.2500″.
Number Drills (#80–#1)
Range from #80 (0.0135″) to #1 (0.2280″). Higher number = smaller bit. #7 (0.2010″) is the tap drill for 1/4-20 UNC. Essential for tap drilling.
Letter Drills (A–Z)
Span A (0.2340″) to Z (0.4130″), filling gaps between fractional sizes. Letter E = 0.2500″ (1/4″). Letter F = 0.2570″ (tap drill for 5/16-18 UNC).
DRILL SIZE DATA
Drill Bit Decimal Equivalent Chart
Fractional, number, letter, and metric drill sizes with decimal inch and millimeter equivalents — sorted by ascending decimal inch.
Note: Drill sizes per ANSI/ASME B94.11M. Number drills run backwards: #80 is the smallest (0.0135″), #1 is the largest (0.2280″). Higher gauge number = smaller bit. This trips up every beginner.
| Drill Size | System | Decimal (in) | Metric (mm) | Closest Fractional | Common Tap Use |
|---|---|---|---|---|---|
| #80 | Number | 0.0135 | 0.34 | — | — |
| #79 | Number | 0.0145 | 0.37 | — | — |
| 1/64″ | Fractional | 0.0156 | 0.40 | 1/64″ | — |
| #77 | Number | 0.0180 | 0.46 | — | — |
| #76 | Number | 0.0200 | 0.51 | — | — |
| #75 | Number | 0.0210 | 0.53 | — | — |
| #74 | Number | 0.0225 | 0.57 | — | — |
| #73 | Number | 0.0240 | 0.61 | — | — |
| #72 | Number | 0.0250 | 0.64 | — | — |
| #71 | Number | 0.0260 | 0.66 | — | — |
| #70 | Number | 0.0280 | 0.71 | — | — |
| #69 | Number | 0.0292 | 0.74 | — | — |
| #68 | Number | 0.0310 | 0.79 | 1/32″ | — |
| 1/32″ | Fractional | 0.0313 | 0.79 | 1/32″ | — |
| #67 | Number | 0.0320 | 0.81 | 1/32″ | — |
| #66 | Number | 0.0330 | 0.84 | — | — |
| #65 | Number | 0.0350 | 0.89 | — | — |
| #64 | Number | 0.0360 | 0.91 | — | — |
| #63 | Number | 0.0370 | 0.94 | — | — |
| #62 | Number | 0.0380 | 0.97 | — | — |
| #61 | Number | 0.0390 | 0.99 | — | — |
| #60 | Number | 0.0400 | 1.02 | 3/64″ | — |
| #59 | Number | 0.0410 | 1.04 | 3/64″ | — |
| #58 | Number | 0.0420 | 1.07 | — | — |
| #57 | Number | 0.0430 | 1.09 | — | — |
| #56 | Number | 0.0465 | 1.18 | 3/64″ | #2-56 UNC tap |
| 3/64″ | Fractional | 0.0469 | 1.19 | 3/64″ | — |
| #55 | Number | 0.0520 | 1.32 | — | — |
| #54 | Number | 0.0550 | 1.40 | — | — |
| #53 | Number | 0.0595 | 1.51 | 1/16″ | — |
| 1/16″ | Fractional | 0.0625 | 1.59 | 1/16″ | — |
| #52 | Number | 0.0635 | 1.61 | 1/16″ | — |
| #51 | Number | 0.0670 | 1.70 | — | — |
| #50 | Number | 0.0700 | 1.78 | — | #2-64 UNF tap |
| #49 | Number | 0.0730 | 1.85 | — | — |
| #48 | Number | 0.0760 | 1.93 | 5/64″ | — |
| 5/64″ | Fractional | 0.0781 | 1.98 | 5/64″ | — |
| #47 | Number | 0.0785 | 1.99 | 5/64″ | — |
| #46 | Number | 0.0810 | 2.06 | — | — |
| #45 | Number | 0.0820 | 2.08 | — | — |
| #44 | Number | 0.0860 | 2.18 | — | #4-40 UNC tap |
| #43 | Number | 0.0890 | 2.26 | — | #4-48 UNF tap |
| #42 | Number | 0.0935 | 2.37 | 3/32″ | — |
| 3/32″ | Fractional | 0.0938 | 2.38 | 3/32″ | — |
| #41 | Number | 0.0960 | 2.44 | — | — |
| #40 | Number | 0.0980 | 2.49 | — | — |
| #39 | Number | 0.0995 | 2.53 | — | — |
| #38 | Number | 0.1015 | 2.58 | — | — |
| #37 | Number | 0.1040 | 2.64 | — | — |
| #36 | Number | 0.1065 | 2.71 | — | #6-32 UNC tap |
| #35 | Number | 0.1100 | 2.79 | 7/64″ | — |
| #34 | Number | 0.1110 | 2.82 | 7/64″ | — |
| #33 | Number | 0.1130 | 2.87 | — | — |
| #32 | Number | 0.1160 | 2.95 | — | — |
| #31 | Number | 0.1200 | 3.05 | 1/8″ | — |
| 1/8″ | Fractional | 0.1250 | 3.18 | 1/8″ | — |
| #30 | Number | 0.1285 | 3.26 | 1/8″ | — |
| #29 | Number | 0.1360 | 3.45 | — | #8-32 UNC tap |
| #28 | Number | 0.1405 | 3.57 | 9/64″ | — |
| 9/64″ | Fractional | 0.1406 | 3.57 | 9/64″ | — |
| #27 | Number | 0.1440 | 3.66 | — | — |
| #26 | Number | 0.1470 | 3.73 | — | — |
| #25 | Number | 0.1495 | 3.80 | — | #10-24 UNC tap |
| #24 | Number | 0.1520 | 3.86 | 5/32″ | — |
| #23 | Number | 0.1540 | 3.91 | 5/32″ | — |
| 5/32″ | Fractional | 0.1563 | 3.97 | 5/32″ | — |
| #22 | Number | 0.1570 | 3.99 | 5/32″ | — |
| #21 | Number | 0.1590 | 4.04 | — | #10-32 UNF tap |
| #20 | Number | 0.1610 | 4.09 | — | — |
| #19 | Number | 0.1660 | 4.22 | — | — |
| #18 | Number | 0.1695 | 4.31 | 11/64″ | — |
| 11/64″ | Fractional | 0.1719 | 4.37 | 11/64″ | — |
| #17 | Number | 0.1730 | 4.39 | 11/64″ | — |
| #16 | Number | 0.1770 | 4.50 | — | — |
| #15 | Number | 0.1800 | 4.57 | — | — |
| #14 | Number | 0.1820 | 4.62 | — | — |
| #13 | Number | 0.1850 | 4.70 | 3/16″ | — |
| 3/16″ | Fractional | 0.1875 | 4.76 | 3/16″ | — |
| #12 | Number | 0.1890 | 4.80 | 3/16″ | — |
| #11 | Number | 0.1910 | 4.85 | — | — |
| #10 | Number | 0.1935 | 4.91 | — | — |
| #9 | Number | 0.1960 | 4.98 | — | — |
| #8 | Number | 0.1990 | 5.05 | — | — |
| #7 | Number | 0.2010 | 5.11 | 13/64″ | 1/4-20 UNC tap |
| 13/64″ | Fractional | 0.2031 | 5.16 | 13/64″ | — |
| #6 | Number | 0.2040 | 5.18 | 13/64″ | — |
| #5 | Number | 0.2055 | 5.22 | — | — |
| #4 | Number | 0.2090 | 5.31 | — | — |
| #3 | Number | 0.2130 | 5.41 | — | — |
| 7/32″ | Fractional | 0.2188 | 5.56 | 7/32″ | — |
| #2 | Number | 0.2210 | 5.61 | 7/32″ | — |
| #1 | Number | 0.2280 | 5.79 | — | — |
| A | Letter | 0.2340 | 5.94 | 15/64″ | — |
| 15/64″ | Fractional | 0.2344 | 5.95 | 15/64″ | — |
| B | Letter | 0.2380 | 6.05 | — | — |
| C | Letter | 0.2420 | 6.15 | — | — |
| D | Letter | 0.2460 | 6.25 | — | — |
| E | Letter | 0.2500 | 6.35 | 1/4″ | — |
| 1/4″ | Fractional | 0.2500 | 6.35 | 1/4″ | — |
| F | Letter | 0.2570 | 6.53 | — | 5/16-18 UNC tap |
| G | Letter | 0.2610 | 6.63 | — | — |
| 17/64″ | Fractional | 0.2656 | 6.75 | 17/64″ | — |
| H | Letter | 0.2660 | 6.76 | 17/64″ | — |
| I | Letter | 0.2720 | 6.91 | — | — |
| J | Letter | 0.2770 | 7.04 | — | — |
| K | Letter | 0.2810 | 7.14 | 9/32″ | — |
| 9/32″ | Fractional | 0.2813 | 7.14 | 9/32″ | — |
| L | Letter | 0.2900 | 7.37 | — | — |
| M | Letter | 0.2950 | 7.49 | — | — |
| 19/64″ | Fractional | 0.2969 | 7.54 | 19/64″ | — |
| N | Letter | 0.3020 | 7.67 | — | — |
| 5/16″ | Fractional | 0.3125 | 7.94 | 5/16″ | — |
| O | Letter | 0.3160 | 8.03 | 5/16″ | — |
| P | Letter | 0.3230 | 8.20 | — | — |
| 21/64″ | Fractional | 0.3281 | 8.33 | 21/64″ | — |
| Q | Letter | 0.3320 | 8.43 | 21/64″ | — |
| R | Letter | 0.3390 | 8.61 | — | — |
| 11/32″ | Fractional | 0.3438 | 8.73 | 11/32″ | — |
| S | Letter | 0.3480 | 8.84 | — | — |
| T | Letter | 0.3580 | 9.09 | — | — |
| 23/64″ | Fractional | 0.3594 | 9.13 | 23/64″ | — |
| U | Letter | 0.3680 | 9.35 | — | 7/16-14 UNC tap |
| 3/8″ | Fractional | 0.3750 | 9.53 | 3/8″ | — |
| V | Letter | 0.3770 | 9.58 | 3/8″ | — |
| W | Letter | 0.3860 | 9.80 | — | — |
| 25/64″ | Fractional | 0.3906 | 9.92 | 25/64″ | — |
| X | Letter | 0.3970 | 10.08 | — | — |
| Y | Letter | 0.4040 | 10.26 | 13/32″ | — |
| 13/32″ | Fractional | 0.4063 | 10.32 | 13/32″ | — |
| Z | Letter | 0.4130 | 10.49 | — | — |
| 27/64″ | Fractional | 0.4219 | 10.72 | 27/64″ | 1/2-13 UNC tap |
| 7/16″ | Fractional | 0.4375 | 11.11 | 7/16″ | — |
| 29/64″ | Fractional | 0.4531 | 11.51 | 29/64″ | — |
| 15/32″ | Fractional | 0.4688 | 11.91 | 15/32″ | — |
| 31/64″ | Fractional | 0.4844 | 12.30 | 31/64″ | — |
| 1/2″ | Fractional | 0.5000 | 12.70 | 1/2″ | — |
| 33/64″ | Fractional | 0.5156 | 13.10 | 33/64″ | — |
| 17/32″ | Fractional | 0.5313 | 13.49 | 17/32″ | — |
| 35/64″ | Fractional | 0.5469 | 13.89 | 35/64″ | — |
| 9/16″ | Fractional | 0.5625 | 14.29 | 9/16″ | — |
| 37/64″ | Fractional | 0.5781 | 14.68 | 37/64″ | — |
| 19/32″ | Fractional | 0.5938 | 15.08 | 19/32″ | — |
| 39/64″ | Fractional | 0.6094 | 15.48 | 39/64″ | — |
| 5/8″ | Fractional | 0.6250 | 15.88 | 5/8″ | — |
| 3/4″ | Fractional | 0.7500 | 19.05 | 3/4″ | — |
| 1″ | Fractional | 1.0000 | 25.40 | 1″ | — |
ANSI/ASME B94.11M
Twist drill standard for drill sizes, decimal equivalents, and manufacturing tolerances. The authoritative reference for drill bit sizing.
Number Drills Run Backwards
#80 (0.0135″) is the smallest. #1 (0.2280″) is the largest. Higher gauge number = smaller bit. This trips up every beginner.
READING GUIDE
How to Use a Drill Bit Decimal Equivalent Chart
Find the diameter you need in the decimal column, then read left to see what drill size to use — in three common scenarios.
01
Decimal Hole Callout
The print calls for a 0.2010″ clearance hole. Find 0.2010 in the decimal column. Read left: #7 drill. If you don’t have #7, 13/64″ at 0.2031″ is 0.0021″ larger.
02
Metric Drill, Inch Drawing
The print calls for a #21 tap drill (0.1590″). Look up #21: 0.1590″ = 4.04 mm. The closest metric drill is 4.0 mm (0.1575″), which will work for a tap drill.
03
Drill for Reaming
The print calls for a 0.2500″ reamed hole. Find 0.2400″ in the decimal column. Letter C at 0.2420″ provides the right reaming allowance.
Common Tap Drill Sizes
| Tap Size | Thread Type | Tap Drill | Decimal (in) | Metric (mm) |
|---|---|---|---|---|
| #4-40 | UNC | #43 | 0.0890 | 2.26 |
| #6-32 | UNC | #36 | 0.1065 | 2.71 |
| #8-32 | UNC | #29 | 0.1360 | 3.45 |
| #10-24 | UNC | #25 | 0.1495 | 3.80 |
| #10-32 | UNF | #21 | 0.1590 | 4.04 |
| 1/4-20 | UNC | #7 | 0.2010 | 5.11 |
| 1/4-28 | UNF | #3 | 0.2130 | 5.41 |
| 5/16-18 | UNC | F | 0.2570 | 6.53 |
| 5/16-24 | UNF | I | 0.2720 | 6.91 |
| 3/8-16 | UNC | 5/16″ | 0.3125 | 7.94 |
| 3/8-24 | UNF | Q | 0.3320 | 8.43 |
| 7/16-14 | UNC | U | 0.3680 | 9.35 |
| 1/2-13 | UNC | 27/64″ | 0.4219 | 10.72 |
| 1/2-20 | UNF | 29/64″ | 0.4531 | 11.51 |
| M4×0.7 | Metric Coarse | 3.3 mm | 0.1299 | 3.30 |
| M5×0.8 | Metric Coarse | 4.2 mm | 0.1654 | 4.20 |
| M6×1.0 | Metric Coarse | 5.0 mm | 0.1969 | 5.00 |
| M8×1.25 | Metric Coarse | 6.8 mm | 0.2677 | 6.80 |
| M10×1.5 | Metric Coarse | 8.5 mm | 0.3346 | 8.50 |
| M12×1.75 | Metric Coarse | 10.2 mm | 0.4016 | 10.20 |
Example: 1/4-20 UNC Tap Drill
The 1/4-20 UNC thread is the most common fastener in North America. The tap drill formula: 0.250″ − (1/20 = 0.050″) = 0.200″. The closest standard drill is #7 at 0.2010″. This produces the standard 75% thread engagement for strong, durable threads.
1/4″Major Diameter
#7 (0.2010″)Tap Drill
75%Thread Engagement
Drill Size Systems Comparison
Each sizing system serves a different purpose. Fractional is the most common in US shops, number drills cover precision tap drilling, letter drills fill the gap between 1/4″ and 13/32″, and metric is the global standard.
| System | Range | Increment | Best For | Common Use |
|---|---|---|---|---|
| Fractional | 1/64″ to 1-3/4″+ | 1/64″ (small), 1/32″ (medium) | General purpose, clearance holes, standard drill indexes | Structural, woodworking, general fabrication |
| Number (#80–#1) | 0.0135″ to 0.2280″ | ≈0.001–0.005″ | Precision tap drilling, small holes | UNC/UNF tap drills (#7, #21, #29, #36) |
| Letter (A–Z) | 0.2340″ to 0.4130″ | ≈0.004–0.007″ | Intermediate sizes between fractional steps | Tap drills for 5/16″ and up (F, I, Q, U) |
| Metric (mm) | 0.1 to 50+ mm | 0.1, 0.5, 1.0 mm steps | Global standard, metric tap drills | M6 (5.0 mm), M8 (6.8 mm), M10 (8.5 mm) |
Key rule: Number drills run backwards. #80 is the smallest (0.0135″), #1 is the largest (0.2280″). Higher gauge number = smaller bit. This is the single most common mistake people make reading a drill decimal chart. Letter drills run forwards: A is smallest, Z is largest.
DRILLING PRACTICES
Drilling Best Practices
Selecting the right drill size from the chart is only the first step. How you drill determines whether the hole meets the print.
Drill size does not equal hole size. A drilled hole is typically 0.001–0.005″ oversize depending on drill point geometry, material, and setup rigidity. For precision holes, drill undersize and ream to final diameter.
Hole Size vs. Drill Size
Drill Oversize
Drilled holes are almost always larger than the drill diameter. Standard twist drills produce holes 0.001–0.005″ oversize. For tight tolerances, drill undersize and ream.
Peck Drilling
Deep Holes
Holes deeper than 3× diameter require peck drilling to clear chips and prevent tool breakage. For deep holes, use stub drills or carbide coolant-through drills.
Speed & Feed
Material Matters
Aluminum: high speed (300+ SFM). Steel: medium (80–120 SFM). Stainless: lower speed, higher feed. Material hardness determines cutting parameters.
Common Drilling Mistakes & How to Avoid Them
Using the wrong decimal equivalent: #7 is 0.2010″, not 0.2040″. Check the chart twice. Forgetting number drills run backwards: #21 is 0.1590″, not 0.0210″. Ignoring chip clearance: For holes deeper than 3× diameter, peck drill or use a coolant-through drill. Over-speeding in stainless: Stainless work-hardens when rubbed; maintain feed rate to keep the tool cutting.
#7 = 0.2010″1/4-20 UNC tap drill
#21 = 0.1590″#10-32 UNF tap drill
#29 = 0.1360″#8-32 UNC tap drill
Drill Size FAQs
Frequently Asked Questions
A #7 drill bit measures 0.2010″ in decimal inches and 5.11 mm in metric. It is the standard tap drill for 1/4-20 UNC threads and is one of the most commonly used number drills in any machine shop. If you memorize one number drill size, make it #7 = 0.2010″.
A letter F drill bit measures 0.2570″ in decimal inches and 6.53 mm in metric. It is the standard tap drill for 5/16-18 UNC threads. Letter F sits between 1/4″ (0.2500″) and 17/64″ (0.2656″) in the fractional series.
A 5/16″ drill bit has a decimal equivalent of 0.3125″. In metric, it equals 7.94 mm. Fractional sizes convert to decimal by dividing the numerator by the denominator: 5 ÷ 16 = 0.3125.
Number drills (#80–#1) cover 0.0135″ to 0.2280″. Letter drills (A–Z) cover 0.2340″ to 0.4130″. Number drills get larger as the gauge number decreases (#80 is smallest, #1 is largest). Letter drills get larger from A to Z.
Divide the numerator by the denominator. For 3/8″, divide 3 by 8 to get 0.3750″. For 11/64″, divide 11 by 64 to get 0.1719″. The decimal equivalents of common fractions (1/16″ = 0.0625″, 1/8″ = 0.125″, 1/4″ = 0.250″, 1/2″ = 0.500″) are worth memorizing.
Not exactly, but they are close enough for many non-critical operations. A 6.0 mm drill equals 0.2362″, which is between letter B (0.2380″) and letter C (0.2420″). For a tap drill, the difference matters more. A metric tap requires a metric tap drill for correct thread engagement; substituting the nearest fractional equivalent can produce weak threads.
The most commonly used drill bits in a general machine shop are: #7 (0.2010″, for 1/4-20 UNC), #21 (0.1590″, for #10-32 UNF), #29 (0.1360″, for #8-32 UNC), #36 (0.1065″, for #6-32 UNC), letter F (0.2570″, for 5/16-18 UNC), and 5/16″ (0.3125″, for 3/8-16 UNC). For metric shops, 5.0 mm (M6 tap), 6.8 mm (M8 tap), and 8.5 mm (M10 tap) are the most used.
Get an Instant Quote for Drilled & Tapped Parts
You have looked up the drill size. Now you need the part those holes go into, drilled and tapped to your specification. Upload your CAD file and get an instant quote with free DFM feedback. Parts ship in 3–7 days with hole diameters and thread quality verified by gauge inspection. ISO 9001 and AS9100 certified, no minimum order quantity.