• The most comprehensive guide to drill sizes anywhere on the web, with complete charts covering every size in metric, imperial and fractional measurements.
• As well as detailed sizing charts, this article contains a brief outline of drill bit sizing systems, how gauge relates to diameter, and some of the different lengths used in various applications.
• Instantly find drill bit conversions, just refer to one of our tables for standard gauges, fractional inch sizes and centre drill bit sizes.

Every drill size chart you’ll ever need is in this article! No more scratching your head wondering which gauge relates to which size or how to convert inches to mm or vice versa. This is your one-stop-shop for all drill bit size conversions and tables.

You will also get some background on how the various drill sizes came about, how they are calculated and some of their applications. If you’ve ever wondered why there are so many different gauge designations and drill bit sizes, then read on.

## How to Measure Drill Bits

To measure the diameter of a drill bit, it’s best to measure the cutting edge rather than the shank (the shank is the part that is held by the drill chuck). To get an accurate measurement, use Vernier callipers, but be aware that depending on the quality of the drill bit, there could be a tolerance error of between 5 and 15 thousandths of a unit.

## Metric Drill Bit Sizes

Metric drill sizes are used primarily in Europe and use metric units to measure the drill bit diameter. There are a variety of standards used to define drill sizes, the most common one in the UK is the British Standard BS 328 set which includes twist drill bit sizes between 0.2 mm and 25 mm.

BS 328 drill sizes are defined as shown in the drill size chart below:

 ø0.2 to 0.98 mm: N. 0.1 mm N. 0.1 + 0.02 mm N. 0.1 + 0.05 mm N. 0.1 + 0.08 mm N is an integer between 2 and 9 ø1.0 to 2.95 mm: N. 0.1 mm N. 0.1 + 0.05 mm N is an integer between 10 and 29 ø3.0 to 13.9 mm: N. 0.1 mm N is an integer between 30 and 139 ø14.0 to 25.0 mm: M. 1mm M. 1 + 0.25 mm M. 1 + 0.5 mm M. 1 + 0.75 mm M is an integer between 14 to 25

Please note that BS 328 sizing only applies to twist drill bits. Other bit types, such as Forstner bits, will contain different size cutting surfaces.

Another example of a metric set of drill sizes is the Renard series, which uses a simple factor to determine the diameter:

• The R5 series uses a factor of 1.58 and contains the following sizes: M2.5, M4, M6, M10, M16 and M24
• The R10 series uses a factor of 1.26 and contains the following sizes: M3, M5, M8, M12, M20 and M30

## Drill Size Chart for Number and Letter Gauges

Number and letter gauges do not follow a formula or standard increments. They are loosely based on the Stubs Steel Wire Gauge, with gauge sizes that are similar (but not entirely identical) to that system.

Although they originated in the 19thcentury, these gauges are still commonly used in the U.S. They are also seen in the UK to a lesser degree, having been largely replaced by metric sizes. Most other countries, especially in Europe, have abandoned them entirely in favour of metric systems.

The drill size chart below contains imperial and metric drill bit diameter measurements for all number and letter gauges.

 Gauge inch mm Gauge inch mm Gauge inch mm 104 0.0031 0.079 52 0.0635 1.613 A 0.234 5.944 103 0.0035 0.089 51 0.067 1.702 B 0.238 6.045 102 0.0039 0.099 50 0.07 1.778 C 0.242 6.147 101 0.0043 0.109 49 0.073 1.854 D 0.246 6.248 100 0.0047 0.119 48 0.076 1.93 E 0.25 6.35 99 0.0051 0.13 47 0.0785 1.994 F 0.257 6.528 98 0.0055 0.14 46 0.081 2.057 G 0.261 6.629 97 0.0059 0.15 45 0.082 2.083 H 0.266 6.756 96 0.0063 0.16 44 0.086 2.184 I 0.272 6.909 95 0.0067 0.17 43 0.089 2.261 J 0.277 7.036 94 0.0071 0.18 42 0.0935 2.375 K 0.281 7.137 93 0.0075 0.191 41 0.096 2.438 L 0.29 7.366 92 0.0079 0.201 40 0.098 2.489 M 0.295 7.493 91 0.0083 0.211 39 0.0995 2.527 N 0.302 7.671 90 0.0087 0.221 38 0.1015 2.578 O 0.316 8.026 89 0.0091 0.231 37 0.104 2.642 P 0.323 8.204 88 0.0095 0.241 36 0.1065 2.705 Q 0.332 8.433 87 0.01 0.254 35 0.11 2.794 R 0.339 8.611 86 0.0105 0.267 34 0.111 2.819 S 0.348 8.839 85 0.011 0.279 33 0.113 2.87 T 0.358 9.093 84 0.0115 0.292 32 0.116 2.946 U 0.368 9.347 83 0.012 0.305 31 0.12 3.048 V 0.377 9.576 82 0.0125 0.318 30 0.1285 3.264 W 0.386 9.804 81 0.013 0.33 29 0.136 3.454 X 0.397 10.08 80 0.0135 0.343 28 0.1405 3.569 Y 0.404 10.26 79 0.0145 0.368 27 0.144 3.658 Z 0.413 10.49 78 0.016 0.406 26 0.147 3.734 R 0.339 8.611 77 0.018 0.457 25 0.1495 3.797 S 0.348 8.839 76 0.02 0.508 24 0.152 3.861 T 0.358 9.093 75 0.021 0.533 23 0.154 3.912 U 0.368 9.347 74 0.0225 0.572 22 0.157 3.988 V 0.377 9.576 73 0.024 0.61 21 0.159 4.039 72 0.025 0.635 20 0.161 4.089 71 0.026 0.66 19 0.166 4.216 70 0.028 0.711 18 0.1695 4.305 69 0.0292 0.742 17 0.173 4.394 68 0.031 0.787 16 0.177 4.496 67 0.032 0.813 15 0.18 4.572 66 0.033 0.838 14 0.182 4.623 65 0.035 0.889 13 0.185 4.699 64 0.036 0.914 12 0.189 4.801 63 0.037 0.94 11 0.191 4.851 62 0.038 0.965 10 0.1935 4.915 61 0.039 0.991 9 0.196 4.978 60 0.04 1.016 8 0.199 5.055 59 0.041 1.041 7 0.201 5.105 58 0.042 1.067 6 0.204 5.182 57 0.043 1.092 5 0.2055 5.22 56 0.0465 1.181 4 0.209 5.309 55 0.052 1.321 3 0.213 5.41 54 0.055 1.397 2 0.221 5.613 53 0.0595 1.511 1 0.228 5.791

## Fractional-inch Drill Size Chart

Fractional drill bits are definedby the ANSI B94.11M-1979 standard.

Most commonly used in the US, but can still be found occasionally in the UK, these imperial measurement drill bits come in 1/64 inch increments. The fractions used are always in their simplest form, i.e. reduced as far as possible, so 2/64 becomes 1/32 for instance.

The drill size chart below gives fractional sizes and their metric conversions up to 1 inch.

 Fraction of an inch inch mm Fraction of an inch inch mm Fraction of an inch inch mm 1/64 0.0156 0.3969 23/64 0.3594 9.1281 45/64 0.7031 17.8594 1/32 0.0313 0.7938 3/8 0.3750 9.5250 23/32 0.7188 18.2563 3/64 0.0469 1.1906 25/64 0.3906 9.9219 47/64 0.7344 18.6531 1/16 0.0625 1.5875 13/32 0.4063 10.3188 3/4 0.7500 19.0500 5/64 0.0781 1.9844 27/64 0.4219 10.7156 49/64 0.7656 19.4469 3/32 0.0938 2.3813 7/16 0.4375 11.1125 25/32 0.7813 19.8438 7/64 0.1094 2.7781 29/64 0.4531 11.5094 51/64 0.7969 20.2406 1/8 0.1250 3.1750 15/32 0.4688 11.9063 13/16 0.8125 20.6375 9/64 0.1406 3.5719 31/64 0.4844 12.3031 53/64 0.8281 21.0344 5/32 0.1563 3.9688 1/2 0.5000 12.7000 27/32 0.8438 21.4313 11/64 0.1719 4.3656 33/64 0.5156 13.0969 55/64 0.8594 21.8281 3/16 0.1875 4.7625 17/32 0.5313 13.4938 7/8 0.8750 22.2250 13/64 0.2031 5.1594 35/64 0.5469 13.8906 57/64 0.8906 22.6219 7/32 0.2188 5.5563 9/16 0.5625 14.2875 29/32 0.9063 23.0188 15/64 0.2344 5.9531 37/64 0.5781 14.6844 59/64 0.9219 23.4156 1/4 0.2500 6.3500 19/32 0.5938 15.0813 15/16 0.9375 23.8125 17/64 0.2656 6.7469 39/64 0.6094 15.4781 61/64 0.9531 24.2094 9/32 0.2813 7.1438 5/8 0.6250 15.8750 31/32 0.9688 24.6063 19/64 0.2969 7.5406 41/64 0.6406 16.2719 63/64 0.9844 25.0031 5/16 0.3125 7.9375 21/32 0.6563 16.6688 1 1.0000 25.4000 21/64 0.3281 8.3344 43/64 0.6719 17.0656 11/32 0.3438 8.7313 11/16 0.6875 17.4625

## Centre Drill Bit Sizes

These specialist drill bits are used for drilling centre holes at two different angles, 60 degrees and 90 degrees. They are commonly used to drill lathe centres and spotting drills, i.e.marking holes to guide twist drilling.

The drill size chart below contains all commonly used centre drill bit sizes.

 Size designation Drill diameter [inches (mm)] 5/0 0.010 in (0.254 mm) 4/0 0.015 in (0.381 mm) 3/0 0.020 in (0.508 mm) 2/0 0.025 in (0.635 mm) 0 1/32 in (0.794 mm) 1 3/64 in (1.191 mm) 2 5/64 in (1.984 mm) 3 7/64 in (2.778 mm) 4 1/8 in (3.175 mm) 4½ 9/64 in (3.572 mm) 5 3/16 in (4.763 mm) 6 7/32 in (5.556 mm) 7 1/4 in (6.350 mm) 8 5/16 in (7.938 mm) Gauge Body diameter [inches (mm)] BS1 1/8 in (3.175 mm) BS2 3/16 in (4.763 mm) BS3 1/4 in (6.350 mm) BS4 5/16 in (7.938 mm) BS5 7/16 in (11.113 mm) BS5A 1/2 in (12.700 mm) BS6 5/8 in (15.875 mm) BS7 3/4 in (19.050 mm)

## Drill Bit Lengths

The length of drill bits vary depending on their application.

• Jobber-length drill bits are the most common, with a length somewhere between 9 to 14 times the diameter.
• Screw-machine-length drills arethe shortest available standard sized bits. They are used primarily in machinist workshops for screw machinesand are not usually available in hardware stores.
• Aircraft-length drills are extended length drill bits that are manufactured in fixed length such as 4, 6, 8, 12, 18, 24-inch bits, rather than proportional lengths.