This drill bit size chart lists the decimal-inch and millimetre equivalents for every common twist-drill size — fractional, number (wire gauge), letter and metric — so you can quickly cross-reference sizes, find a substitute bit, or match a drill to a tap or screw. Bookmark it as your go-to twist-drill reference.

How to use this chart: Find your known size in one table, then read across to the decimal-inch and millimetre columns to find the exact equivalent (or the closest size in another system). Decimal values for fractional and metric sizes are exact conversions; number and letter drills use the standard industry decimal equivalents.

Fractional Drill Bit Sizes

Fractional bits are the most common sizes in the shop, running from tiny 1/64″ bits up to 1″ and beyond in 1/64″ steps.

Fractional inch drill sizes (1/64″ to 1″)
Fraction	Decimal (in)	Millimeters
1/64	0.0156	0.397
1/32	0.0312	0.794
3/64	0.0469	1.191
1/16	0.0625	1.587
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	0.1562	3.969
11/64	0.1719	4.366
3/16	0.1875	4.762
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	0.2812	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	0.4062	10.319
27/64	0.4219	10.716
7/16	0.4375	11.112
29/64	0.4531	11.509
15/32	0.4688	11.906
31/64	0.4844	12.303
1/2	0.5000	12.700
33/64	0.5156	13.097
17/32	0.5312	13.494
35/64	0.5469	13.891
9/16	0.5625	14.287
37/64	0.5781	14.684
19/32	0.5938	15.081
39/64	0.6094	15.478
5/8	0.6250	15.875
41/64	0.6406	16.272
21/32	0.6562	16.669
43/64	0.6719	17.066
11/16	0.6875	17.462
45/64	0.7031	17.859
23/32	0.7188	18.256
47/64	0.7344	18.653
3/4	0.7500	19.050
49/64	0.7656	19.447
25/32	0.7812	19.844
51/64	0.7969	20.241
13/16	0.8125	20.637
53/64	0.8281	21.034
27/32	0.8438	21.431
55/64	0.8594	21.828
7/8	0.8750	22.225
57/64	0.8906	22.622
29/32	0.9062	23.019
59/64	0.9219	23.416
15/16	0.9375	23.812
61/64	0.9531	24.209
31/32	0.9688	24.606
63/64	0.9844	25.003
1/1	1.0000	25.400

Number (Wire Gauge) Drill Sizes

Number or “wire gauge” drills (#1 through #80) fill in the small gaps between fractional sizes — essential for tapping and precision work. Note that a larger number means a smaller drill.

Number (wire gauge) drill sizes #1-#80
Number	Decimal (in)	Millimeters
#1	0.2280	5.791
#2	0.2210	5.613
#3	0.2130	5.410
#4	0.2090	5.309
#5	0.2055	5.220
#6	0.2040	5.182
#7	0.2010	5.105
#8	0.1990	5.055
#9	0.1960	4.978
#10	0.1935	4.915
#11	0.1910	4.851
#12	0.1890	4.801
#13	0.1850	4.699
#14	0.1820	4.623
#15	0.1800	4.572
#16	0.1770	4.496
#17	0.1730	4.394
#18	0.1695	4.305
#19	0.1660	4.216
#20	0.1610	4.089
#21	0.1590	4.039
#22	0.1570	3.988
#23	0.1540	3.912
#24	0.1520	3.861
#25	0.1495	3.797
#26	0.1470	3.734
#27	0.1440	3.658
#28	0.1405	3.569
#29	0.1360	3.454
#30	0.1285	3.264
#31	0.1200	3.048
#32	0.1160	2.946
#33	0.1130	2.870
#34	0.1110	2.819
#35	0.1100	2.794
#36	0.1065	2.705
#37	0.1040	2.642
#38	0.1015	2.578
#39	0.0995	2.527
#40	0.0980	2.489
#41	0.0960	2.438
#42	0.0935	2.375
#43	0.0890	2.261
#44	0.0860	2.184
#45	0.0820	2.083
#46	0.0810	2.057
#47	0.0785	1.994
#48	0.0760	1.930
#49	0.0730	1.854
#50	0.0700	1.778
#51	0.0670	1.702
#52	0.0635	1.613
#53	0.0595	1.511
#54	0.0550	1.397
#55	0.0520	1.321
#56	0.0465	1.181
#57	0.0430	1.092
#58	0.0420	1.067
#59	0.0410	1.041
#60	0.0400	1.016
#61	0.0390	0.991
#62	0.0380	0.965
#63	0.0370	0.940
#64	0.0360	0.914
#65	0.0350	0.889
#66	0.0330	0.838
#67	0.0320	0.813
#68	0.0310	0.787
#69	0.0292	0.742
#70	0.0280	0.711
#71	0.0260	0.660
#72	0.0250	0.635
#73	0.0240	0.610
#74	0.0225	0.571
#75	0.0210	0.533
#76	0.0200	0.508
#77	0.0180	0.457
#78	0.0160	0.406
#79	0.0145	0.368
#80	0.0135	0.343

Letter Drill Sizes

Letter drills (A through Z) cover the range just above 1/4″, from A (.234″) up to Z (.413″). They’re widely used for tap drills and clearance holes.

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

Metric Drill Sizes

Metric bits are sized directly in millimetres. Use this table to convert a metric size to its decimal-inch equivalent (or find the nearest inch size).

Common metric drill sizes (0.5 mm to 13 mm)
Metric	Decimal (in)	Inch (approx)
0.5 mm	0.0197	0.020″
1 mm	0.0394	0.039″
1.5 mm	0.0591	0.059″
2 mm	0.0787	0.079″
2.5 mm	0.0984	0.098″
3 mm	0.1181	0.118″
3.5 mm	0.1378	0.138″
4 mm	0.1575	0.157″
4.5 mm	0.1772	0.177″
5 mm	0.1969	0.197″
5.5 mm	0.2165	0.217″
6 mm	0.2362	0.236″
6.5 mm	0.2559	0.256″
7 mm	0.2756	0.276″
7.5 mm	0.2953	0.295″
8 mm	0.3150	0.315″
8.5 mm	0.3346	0.335″
9 mm	0.3543	0.354″
9.5 mm	0.3740	0.374″
10 mm	0.3937	0.394″
10.5 mm	0.4134	0.413″
11 mm	0.4331	0.433″
11.5 mm	0.4528	0.453″
12 mm	0.4724	0.472″
12.5 mm	0.4921	0.492″
13 mm	0.5118	0.512″

Twist Drill Cutting Speeds

Running a bit at the right speed protects the cutting edge and gives cleaner holes. The values below are typical surface speeds for standard HSS twist drills. To convert to spindle speed, use:

RPM = (3.82 × SFM) ÷ Drill diameter (in) — e.g. drilling mild steel (95 SFM) with a 1/4″ bit: RPM ≈ (3.82 × 95) ÷ 0.25 ≈ 1,450 RPM. Reduce speed for larger bits and harder materials; use cutting fluid on metal.

Typical cutting speeds for HSS twist drills (surface feet per minute)
Material	Speed (SFM)
Aluminium & alloys	200-300
Brass & bronze	120-200
Cast iron (soft)	80-120
Mild / low-carbon steel	80-110
Alloy & tool steel	50-70
Stainless steel	40-60
Hardwood	300-400
Softwood / plywood	300-500
Acrylic / plastics	100-200

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