Fastening Know-How

Drill Bit Size Chart: Fractional, Number, Letter and Metric

Decimal inch and millimeter equivalents for every standard drill size, plus the tap drills for common threads.

How to Read a Drill Bit Size Chart

US drills come in four series. Fractional drills step in 1/64 inch. Number drills run from #80 (0.0135 in, the smallest) to #1 (0.2280 in); the higher the number, the smaller the drill. Letter drills run from A (0.2340 in) to Z (0.4130 in) and fill the gaps above #1. Metric drills are sized in millimeters. Find the size you have in any table and read across to the decimal inch and millimeter equivalent. Values use 1 in = 25.4 mm exactly.

Common Tap Drill Sizes

The drill to use before tapping the most common threads.

Thread	Tap drill (about 75% thread)
#4-40 UNC	#43 (0.0890 in)
#6-32 UNC	#36 (0.1065 in)
#8-32 UNC	#29 (0.1360 in)
#10-24 UNC	#25 (0.1495 in)
#10-32 UNF	#21 (0.1590 in)
1/4-20 UNC	#7 (0.2010 in)
1/4-28 UNF	#3 (0.2130 in)
5/16-18 UNC	F (0.2570 in)
3/8-16 UNC	5/16 (0.3125 in)
1/2-13 UNC	27/64 (0.4219 in)
M5 x 0.8	4.2 mm
M6 x 1.0	5.0 mm
M8 x 1.25	6.8 mm
M10 x 1.5	8.5 mm
M12 x 1.75	10.2 mm

Tap drill sizes from standard machinists’ tap drill tables (about 75% thread); thread forms per ASME B1.1 and ISO 261. For every size see the tap drill chart.

Fractional Drill Sizes (1/64 to 1 inch)

Fractional drills in 1/64 inch steps with decimal and metric equivalents.

Fraction	Decimal inch	Millimeters
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
33/64	0.5156	13.097
17/32	0.5313	13.494
35/64	0.5469	13.891
9/16	0.5625	14.288
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	0.6563	16.669
43/64	0.6719	17.066
11/16	0.6875	17.463
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	0.7813	19.844
51/64	0.7969	20.241
13/16	0.8125	20.638
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	0.9063	23.019
59/64	0.9219	23.416
15/16	0.9375	23.813
61/64	0.9531	24.209
31/32	0.9688	24.606
63/64	0.9844	25.003
1	1.0000	25.400

Number Drill Sizes (#80 to #1)

Wire-gauge number drills, used mainly for tap drills and small holes.

Number	Decimal inch	Millimeters
#80	0.0135	0.343
#79	0.0145	0.368
#78	0.0160	0.406
#77	0.0180	0.457
#76	0.0200	0.508
#75	0.0210	0.533
#74	0.0225	0.572
#73	0.0240	0.610
#72	0.0250	0.635
#71	0.0260	0.660
#70	0.0280	0.711
#69	0.0292	0.742
#68	0.0310	0.787
#67	0.0320	0.813
#66	0.0330	0.838
#65	0.0350	0.889
#64	0.0360	0.914
#63	0.0370	0.940
#62	0.0380	0.965
#61	0.0390	0.991
#60	0.0400	1.016
#59	0.0410	1.041
#58	0.0420	1.067
#57	0.0430	1.092
#56	0.0465	1.181
#55	0.0520	1.321
#54	0.0550	1.397
#53	0.0595	1.511
#52	0.0635	1.613
#51	0.0670	1.702
#50	0.0700	1.778
#49	0.0730	1.854
#48	0.0760	1.930
#47	0.0785	1.994
#46	0.0810	2.057
#45	0.0820	2.083
#44	0.0860	2.184
#43	0.0890	2.261
#42	0.0935	2.375
#41	0.0960	2.438
#40	0.0980	2.489
#39	0.0995	2.527
#38	0.1015	2.578
#37	0.1040	2.642
#36	0.1065	2.705
#35	0.1100	2.794
#34	0.1110	2.819
#33	0.1130	2.870
#32	0.1160	2.946
#31	0.1200	3.048
#30	0.1285	3.264
#29	0.1360	3.454
#28	0.1405	3.569
#27	0.1440	3.658
#26	0.1470	3.734
#25	0.1495	3.797
#24	0.1520	3.861
#23	0.1540	3.912
#22	0.1570	3.988
#21	0.1590	4.039
#20	0.1610	4.089
#19	0.1660	4.216
#18	0.1695	4.305
#17	0.1730	4.394
#16	0.1770	4.496
#15	0.1800	4.572
#14	0.1820	4.623
#13	0.1850	4.699
#12	0.1890	4.801
#11	0.1910	4.851
#10	0.1935	4.915
#9	0.1960	4.978
#8	0.1990	5.055
#7	0.2010	5.105
#6	0.2040	5.182
#5	0.2055	5.220
#4	0.2090	5.309
#3	0.2130	5.410
#2	0.2210	5.613
#1	0.2280	5.791

Letter Drill Sizes (A to Z)

Letter drills, larger than #1 and between the common fractions.

Letter	Decimal inch	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

Common metric jobber drill sizes with inch equivalents, including the standard metric tap drills (3.3, 4.2, 6.8, 8.5 and 10.2 mm).

Metric size	Decimal inch	Millimeters
1.0 mm	0.0394	1.0
1.5 mm	0.0591	1.5
2.0 mm	0.0787	2.0
2.5 mm	0.0984	2.5
3.0 mm	0.1181	3.0
3.3 mm	0.1299	3.3
3.5 mm	0.1378	3.5
4.0 mm	0.1575	4.0
4.2 mm	0.1654	4.2
4.5 mm	0.1772	4.5
5.0 mm	0.1969	5.0
5.5 mm	0.2165	5.5
6.0 mm	0.2362	6.0
6.5 mm	0.2559	6.5
6.8 mm	0.2677	6.8
7.0 mm	0.2756	7.0
7.5 mm	0.2953	7.5
8.0 mm	0.3150	8.0
8.5 mm	0.3346	8.5
9.0 mm	0.3543	9.0
9.5 mm	0.3740	9.5
10.0 mm	0.3937	10.0
10.2 mm	0.4016	10.2
10.5 mm	0.4134	10.5
11.0 mm	0.4331	11.0
11.5 mm	0.4528	11.5
12.0 mm	0.4724	12.0
12.5 mm	0.4921	12.5
13.0 mm	0.5118	13.0
14.0 mm	0.5512	14.0
15.0 mm	0.5906	15.0
16.0 mm	0.6299	16.0
17.0 mm	0.6693	17.0
18.0 mm	0.7087	18.0
19.0 mm	0.7480	19.0
20.0 mm	0.7874	20.0
21.0 mm	0.8268	21.0
22.0 mm	0.8661	22.0
23.0 mm	0.9055	23.0
24.0 mm	0.9449	24.0
25.0 mm	0.9843	25.0

Frequently Asked Questions

What size is a #7 drill?

0.2010 in (5.105 mm). It is the tap drill for 1/4-20 UNC.

What is a 1/4 inch drill in millimeters?

6.35 mm. The letter E drill is the same size, 0.2500 in.

What drill size is 5/16 in millimeters?

7.938 mm. It is the tap drill for 3/8-16 UNC.

Why do number drills get smaller as the number goes up?

The series comes from the old wire gauge system, where a higher gauge number meant a thinner wire. #1 is 0.2280 in and #80 is 0.0135 in.

What drill do I use for an M8 tap?

A 6.8 mm drill for M8 x 1.25, which gives about 75% thread.

Related Charts and Calculators

Work out any tap drill with the Tap Drill Size Calculator, see every thread in the tap drill size chart, check thread depth with the Minimum Thread Engagement Calculator, or browse the bolt size chart.

Specifying a Safety-Critical Joint?

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