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Fractions, decimals, mm and drill sizes

An inch drawing, a #21 or an F drill — here it's all in decimals and millimeters. Enter a size in any form to see it in inches and mm with the nearest fraction and drills.

Quick answer

1″ = 25.4 mm, so multiply the fraction by 25.4: 1/64″ = 0.015625″ = 0.3969 mm, 3/8″ = 9.525 mm, 1/2″ = 12.7 mm. Number drill #7 = 0.201″ = 5.105 mm (for 1/4-20 UNC), letter drill F = 0.257″ = 6.528 mm.

Inches
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Millimeters
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Nearest fraction (1/64)
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Nearest number/letter drill
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Nearest metric drill (0.1 mm step)
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Fractions to decimal and mm

1″ = 25.4 mm exactly. Bold rows are the common 1/16 steps.

FractionDecimalmm
1/64″0.015630.397
1/32″0.031250.794
3/64″0.046881.191
1/16″0.062501.587
5/64″0.078131.984
3/32″0.093752.381
7/64″0.109382.778
1/8″0.125003.175
9/64″0.140633.572
5/32″0.156253.969
11/64″0.171884.366
3/16″0.187504.762
13/64″0.203135.159
7/32″0.218755.556
15/64″0.234385.953
1/4″0.250006.350
17/64″0.265636.747
9/32″0.281257.144
19/64″0.296887.541
5/16″0.312507.938
21/64″0.328138.334
11/32″0.343758.731
23/64″0.359389.128
3/8″0.375009.525
25/64″0.390639.922
13/32″0.4062510.319
27/64″0.4218810.716
7/16″0.4375011.112
29/64″0.4531311.509
15/32″0.4687511.906
31/64″0.4843812.303
1/2″0.5000012.700
FractionDecimalmm
33/64″0.5156313.097
17/32″0.5312513.494
35/64″0.5468813.891
9/16″0.5625014.287
37/64″0.5781314.684
19/32″0.5937515.081
39/64″0.6093815.478
5/8″0.6250015.875
41/64″0.6406316.272
21/32″0.6562516.669
43/64″0.6718817.066
11/16″0.6875017.462
45/64″0.7031317.859
23/32″0.7187518.256
47/64″0.7343818.653
3/4″0.7500019.050
49/64″0.7656319.447
25/32″0.7812519.844
51/64″0.7968820.241
13/16″0.8125020.637
53/64″0.8281321.034
27/32″0.8437521.431
55/64″0.8593821.828
7/8″0.8750022.225
57/64″0.8906322.622
29/32″0.9062523.019
59/64″0.9218823.416
15/16″0.9375023.813
61/64″0.9531324.209
31/32″0.9687524.606
63/64″0.9843825.003
1″1.0000025.400

Number drills #1–#80

DrillDecimalmm
#10.22805.79
#20.22105.61
#30.21305.41
#40.20905.31
#50.20555.22
#60.20405.18
#70.20105.11
#80.19905.05
#90.19604.98
#100.19354.91
#110.19104.85
#120.18904.80
#130.18504.70
#140.18204.62
#150.18004.57
#160.17704.50
#170.17304.39
#180.16954.31
#190.16604.22
#200.16104.09
DrillDecimalmm
#210.15904.04
#220.15703.99
#230.15403.91
#240.15203.86
#250.14953.80
#260.14703.73
#270.14403.66
#280.14053.57
#290.13603.45
#300.12853.26
#310.12003.05
#320.11602.95
#330.11302.87
#340.11102.82
#350.11002.79
#360.10652.71
#370.10402.64
#380.10152.58
#390.09952.53
#400.09802.49
DrillDecimalmm
#410.09602.44
#420.09352.37
#430.08902.26
#440.08602.18
#450.08202.08
#460.08102.06
#470.07851.99
#480.07601.93
#490.07301.85
#500.07001.78
#510.06701.70
#520.06351.61
#530.05951.51
#540.05501.40
#550.05201.32
#560.04651.18
#570.04301.09
#580.04201.07
#590.04101.04
#600.04001.02
DrillDecimalmm
#610.03900.99
#620.03800.97
#630.03700.94
#640.03600.91
#650.03500.89
#660.03300.84
#670.03200.81
#680.03100.79
#690.02920.74
#700.02800.71
#710.02600.66
#720.02500.64
#730.02400.61
#740.02250.57
#750.02100.53
#760.02000.51
#770.01800.46
#780.01600.41
#790.01450.37
#800.01350.34

Letter drills A–Z

DrillDecimalmm
A0.23405.94
B0.23806.05
C0.24206.15
D0.24606.25
E0.25006.35
F0.25706.53
G0.26106.63
H0.26606.76
I0.27206.91
J0.27707.04
K0.28107.14
L0.29007.37
M0.29507.49
DrillDecimalmm
N0.30207.67
O0.31608.03
P0.32308.20
Q0.33208.43
R0.33908.61
S0.34808.84
T0.35809.09
U0.36809.35
V0.37709.58
W0.38609.80
X0.397010.08
Y0.404010.26
Z0.413010.49

Number and letter drills follow the standard US series (Wikipedia "Drill bit sizes"). The higher the number, the smaller the drill; letter drills continue above #1 and grow from A to Z.

Which drill to use before tapping — see the tap drill chart.

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