Drill Bit Size Chart

Fractional, number and letter drill bit sizes with decimal-inch and millimetre equivalents in one reference.

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Machine shops work with four overlapping drill size systems: fractional inch, number (wire) gauge, letter gauge and metric. This chart converts all of them to decimal inches and millimetres so you can pick the nearest available bit for a hole, a tapped thread or a reamer pilot without guessing.

Fractional drill sizes

Fraction	Decimal (in)	Millimetres
1/64	0.0156	0.397
1/32	0.0313	0.794
3/64	0.0469	1.191
1/16	0.0625	1.588
5/64	0.0781	1.984
3/32	0.0938	2.381
7/64	0.1094	2.778
1/8	0.1250	3.175
9/64	0.1406	3.572
5/32	0.1563	3.969
11/64	0.1719	4.366
3/16	0.1875	4.763
13/64	0.2031	5.159
7/32	0.2188	5.556
15/64	0.2344	5.953
1/4	0.2500	6.350
17/64	0.2656	6.747
9/32	0.2813	7.144
19/64	0.2969	7.541
5/16	0.3125	7.938
21/64	0.3281	8.334
11/32	0.3438	8.731
23/64	0.3594	9.128
3/8	0.3750	9.525
25/64	0.3906	9.922
13/32	0.4063	10.319
27/64	0.4219	10.716
7/16	0.4375	11.113
29/64	0.4531	11.509
15/32	0.4688	11.906
31/64	0.4844	12.303
1/2	0.5000	12.700
17/32	0.5313	13.494
9/16	0.5625	14.288
19/32	0.5938	15.081
5/8	0.6250	15.875
21/32	0.6563	16.669
11/16	0.6875	17.463
23/32	0.7188	18.256
3/4	0.7500	19.050
13/16	0.8125	20.638
7/8	0.8750	22.225
15/16	0.9375	23.813
1	1.0000	25.400

44 rows

Number (wire gauge) drill sizes

Number drills run from 80 (smallest, 0.0135 in) up to 1 (0.2280 in) and are the usual choice for small tapped holes.

Number	Decimal (in)	Millimetres
No. 80	0.0135	0.343
No. 79	0.0145	0.368
No. 78	0.0160	0.406
No. 77	0.0180	0.457
No. 76	0.0200	0.508
No. 75	0.0210	0.533
No. 74	0.0225	0.572
No. 73	0.0240	0.610
No. 72	0.0250	0.635
No. 71	0.0260	0.660
No. 70	0.0280	0.711
No. 69	0.0292	0.742
No. 68	0.0310	0.787
No. 67	0.0320	0.813
No. 66	0.0330	0.838
No. 65	0.0350	0.889
No. 64	0.0360	0.914
No. 63	0.0370	0.940
No. 62	0.0380	0.965
No. 61	0.0390	0.991
No. 60	0.0400	1.016
No. 59	0.0410	1.041
No. 58	0.0420	1.067
No. 57	0.0430	1.092
No. 56	0.0465	1.181
No. 55	0.0520	1.321
No. 54	0.0550	1.397
No. 53	0.0595	1.511
No. 52	0.0635	1.613
No. 51	0.0670	1.702
No. 50	0.0700	1.778
No. 49	0.0730	1.854
No. 48	0.0760	1.930
No. 47	0.0785	1.994
No. 46	0.0810	2.057
No. 45	0.0820	2.083
No. 44	0.0860	2.184
No. 43	0.0890	2.261
No. 42	0.0935	2.375
No. 41	0.0960	2.438
No. 40	0.0980	2.489
No. 39	0.0995	2.527
No. 38	0.1015	2.578
No. 37	0.1040	2.642
No. 36	0.1065	2.705
No. 35	0.1100	2.794
No. 34	0.1110	2.819
No. 33	0.1130	2.870
No. 32	0.1160	2.946
No. 31	0.1200	3.048
No. 30	0.1285	3.264
No. 29	0.1360	3.454
No. 28	0.1405	3.569
No. 27	0.1440	3.658
No. 26	0.1470	3.734
No. 25	0.1495	3.797
No. 24	0.1520	3.861
No. 23	0.1540	3.912
No. 22	0.1570	3.988
No. 21	0.1590	4.039
No. 20	0.1610	4.089
No. 19	0.1660	4.216
No. 18	0.1695	4.305
No. 17	0.1730	4.394
No. 16	0.1770	4.496
No. 15	0.1800	4.572
No. 14	0.1820	4.623
No. 13	0.1850	4.699
No. 12	0.1890	4.801
No. 11	0.1910	4.851
No. 10	0.1935	4.915
No. 9	0.1960	4.978
No. 8	0.1990	5.055
No. 7	0.2010	5.105
No. 6	0.2040	5.182
No. 5	0.2055	5.220
No. 4	0.2090	5.309
No. 3	0.2130	5.410
No. 2	0.2210	5.613
No. 1	0.2280	5.791

80 rows

Letter drill sizes

Letter drills bridge the gap between number drills and larger fractional sizes, from A (0.234 in) to Z (0.413 in).

Letter	Decimal (in)	Millimetres
Letter A	0.2340	5.944
Letter B	0.2380	6.045
Letter C	0.2420	6.147
Letter D	0.2460	6.248
Letter E	0.2500	6.350
Letter F	0.2570	6.528
Letter G	0.2610	6.629
Letter H	0.2660	6.756
Letter I	0.2720	6.909
Letter J	0.2770	7.036
Letter K	0.2810	7.137
Letter L	0.2900	7.366
Letter M	0.2950	7.493
Letter N	0.3020	7.671
Letter O	0.3160	8.026
Letter P	0.3230	8.204
Letter Q	0.3320	8.433
Letter R	0.3390	8.611
Letter S	0.3480	8.839
Letter T	0.3580	9.093
Letter U	0.3680	9.347
Letter V	0.3770	9.576
Letter W	0.3860	9.804
Letter X	0.3970	10.084
Letter Y	0.4040	10.262
Letter Z	0.4130	10.490

26 rows

Recommended drilling speeds (HSS twist drill)

Material	Cutting speed (m/min)	Cutting speed (sfm)
Aluminium alloys	75–120	250–400
Brass and bronze	60–90	200–300
Cast iron (grey)	25–35	80–120
Mild steel	25–30	80–100
Alloy steel	15–20	50–65
Stainless steel (304)	10–15	35–50
Titanium alloys	6–12	20–40
Plastics (acrylic, nylon)	60–100	200–330

8 rows

About the Drill Bit Size Chart

Sources

Classification: Standard inch-series drill sizes

Sources

ANSI/ASME B94.11M — Twist drills (fractional, number, letter sizes)
ASME

Fractional, number and letter drill sizes are standardised inch-series designations per ASME B94.11M. The decimal-inch and millimetre equivalents are exact conversions of each designated size. Number drills (1–80) run from ~0.0138 in down to ~0.228 in; letter drills (A–Z) run from ~0.234 in up to ~0.413 in, overlapping the small fractional sizes.

How it was compiled

Inch drill bits are designated three ways: fractional (in 1/64-in increments and larger), number (1 smallest to 80 largest) and letter (A smallest to Z largest). The three series together cover a continuous range of diameters. Decimal-inch and millimetre columns are exact conversions of the designated size (1 in = 25.4 mm exactly).

How current it is

Classification: Stable (mature standard)

The fractional/number/letter drill size designations have been stable for decades. Confirm the current ASME B94.11M edition for procurement.

Limitations

The three series overlap; a given decimal size may correspond to a fractional, number or letter drill — choose by availability and the required precision.
Number and letter designations are not metric; use the metric drill chart for millimetre-designated bits.
Decimal equivalents are exact for the designated size; actual drill diameter has a manufacturing tolerance.

Using it

Use the decimal-inch or millimetre column to match a drill to a required hole size, then pick the nearest available designated drill.
For tap drills, pair this chart with the tap-drill chart and the relevant thread standard (UNC/UNF).
Prefer metric-designated drills for metric work to avoid rounding the hole size.

Units and notation

Designated sizes in fractional inches, number and letter codes; equivalents in decimal inches and millimetres (1 in = 25.4 mm exactly).

Frequently Asked Questions

How do I convert a drill size to millimetres?

Multiply the decimal inch size by 25.4. A 1/4 in drill is 0.2500 × 25.4 = 6.35 mm.

What is a number drill?

Number or wire gauge drills are a legacy small-diameter series running from No. 80 (0.0135 in) to No. 1 (0.2280 in). Sizes get larger as the number gets smaller.

Which drill is closest to 8 mm?

5/16 in measures 7.938 mm, about 0.06 mm undersize — usually acceptable for clearance holes but not for a press fit.

Why is my drilled hole oversize?

Common causes are an unequally ground point, excessive runout in the chuck, no centre drill or pilot hole, and feeding too aggressively in a flexible setup. Reaming is the reliable route to a precise diameter.

Do I need a pilot hole?

For diameters above roughly 10 mm a pilot hole or spot drill reduces thrust, improves position accuracy and stops the drill from walking.

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