Engineering Reference

Drill Size Chart

Every standard twist drill size — number (#1–#80), letter (A–Z) and fractional (1/64 in to 1 in) — with decimal inch and millimetre equivalents.

Page last changed 2026-10-19 · based on ASME B94.11M-1993 (R2017)

Quick Answer

Drills use three overlapping size systems: number drills (#1–#80, running large to small), letter drills (A–Z, running small to large) and fractional drills in 1/64 in steps. They interleave, so a #7 (0.2010 in) sits between 3/16 in and 13/64 in. Number drills cover the sizes fractional drills are too coarse for.

Standard Twist Drill Sizes

Size[3]Series[3]Decimal
inch[2]		Metric
mm[2]
#80 #Number0.01350.343
#79 #Number0.01450.368
1/64" #Fractional0.01560.397
#78 #Number0.01600.406
#77 #Number0.01800.457
#76 #Number0.02000.508
#75 #Number0.02100.533
#74 #Number0.02250.571
#73 #Number0.02400.610
#72 #Number0.02500.635
#71 #Number0.02600.660
#70 #Number0.02800.711
#69 #Number0.02920.742
#68 #Number0.03100.787
1/32" #Fractional0.03120.794
#67 #Number0.03200.813
#66 #Number0.03300.838
#65 #Number0.03500.889
#64 #Number0.03600.914
#63 #Number0.03700.940
#62 #Number0.03800.965
#61 #Number0.03900.991
#60 #Number0.04001.016
#59 #Number0.04101.041
#58 #Number0.04201.067
#57 #Number0.04301.092
#56 #Number0.04651.181
3/64" #Fractional0.04691.191
#55 #Number0.05201.321
#54 #Number0.05501.397
#53 #Number0.05951.511
1/16" #Fractional0.06251.587
#52 #Number0.06351.613
#51 #Number0.06701.702
#50 #Number0.07001.778
#49 #Number0.07301.854
#48 #Number0.07601.930
5/64" #Fractional0.07811.984
#47 #Number0.07851.994
#46 #Number0.08102.057
#45 #Number0.08202.083
#44 #Number0.08602.184
#43 #Number0.08902.261
#42 #Number0.09352.375
3/32" #Fractional0.09382.381
#41 #Number0.09602.438
#40 #Number0.09802.489
#39 #Number0.09952.527
#38 #Number0.10152.578
#37 #Number0.10402.642
#36 #Number0.10652.705
7/64" #Fractional0.10942.778
#35 #Number0.11002.794
#34 #Number0.11102.819
#33 #Number0.11302.870
#32 #Number0.11602.946
#31 #Number0.12003.048
1/8" #Fractional0.12503.175
#30 #Number0.12853.264
#29 #Number0.13603.454
#28 #Number0.14053.569
9/64" #Fractional0.14063.572
#27 #Number0.14403.658
#26 #Number0.14703.734
#25 #Number0.14953.797
#24 #Number0.15203.861
#23 #Number0.15403.912
5/32" #Fractional0.15623.969
#22 #Number0.15703.988
#21 #Number0.15904.039
#20 #Number0.16104.089
#19 #Number0.16604.216
#18 #Number0.16954.305
11/64" #Fractional0.17194.366
#17 #Number0.17304.394
#16 #Number0.17704.496
#15 #Number0.18004.572
#14 #Number0.18204.623
#13 #Number0.18504.699
3/16" #Fractional0.18754.762
#12 #Number0.18904.801
#11 #Number0.19104.851
#10 #Number0.19354.915
#9 #Number0.19604.978
#8 #Number0.19905.055
#7 #Number0.20105.105
13/64" #Fractional0.20315.159
#6 #Number0.20405.182
#5 #Number0.20555.220
#4 #Number0.20905.309
#3 #Number0.21305.410
7/32" #Fractional0.21885.556
#2 #Number0.22105.613
#1 #Number0.22805.791
A #Letter0.23405.944
15/64" #Fractional0.23445.953
B #Letter0.23806.045
C #Letter0.24206.147
D #Letter0.24606.248
E #Letter0.25006.350
1/4" #Fractional0.25006.350
F #Letter0.25706.528
G #Letter0.26106.629
17/64" #Fractional0.26566.747
H #Letter0.26606.756
I #Letter0.27206.909
J #Letter0.27707.036
K #Letter0.28107.137
9/32" #Fractional0.28127.144
L #Letter0.29007.366
M #Letter0.29507.493
19/64" #Fractional0.29697.541
N #Letter0.30207.671
5/16" #Fractional0.31257.938
O #Letter0.31608.026
P #Letter0.32308.204
21/64" #Fractional0.32818.334
Q #Letter0.33208.433
R #Letter0.33908.611
11/32" #Fractional0.34388.731
S #Letter0.34808.839
T #Letter0.35809.093
23/64" #Fractional0.35949.128
U #Letter0.36809.347
3/8" #Fractional0.37509.525
V #Letter0.37709.576
W #Letter0.38609.804
25/64" #Fractional0.39069.922
X #Letter0.397010.084
Y #Letter0.404010.262
13/32" #Fractional0.406210.319
Z #Letter0.413010.490
27/64" #Fractional0.421910.716
7/16" #Fractional0.437511.112
29/64" #Fractional0.453111.509
15/32" #Fractional0.468811.906
31/64" #Fractional0.484412.303
1/2" #Fractional0.500012.700
33/64" #Fractional0.515613.097
17/32" #Fractional0.531213.494
35/64" #Fractional0.546913.891
9/16" #Fractional0.562514.287
37/64" #Fractional0.578114.684
19/32" #Fractional0.593815.081
39/64" #Fractional0.609415.478
5/8" #Fractional0.625015.875
41/64" #Fractional0.640616.272
21/32" #Fractional0.656216.669
43/64" #Fractional0.671917.066
11/16" #Fractional0.687517.462
45/64" #Fractional0.703117.859
23/32" #Fractional0.718818.256
47/64" #Fractional0.734418.653
3/4" #Fractional0.750019.050
49/64" #Fractional0.765619.447
25/32" #Fractional0.781219.844
51/64" #Fractional0.796920.241
13/16" #Fractional0.812520.637
53/64" #Fractional0.828121.034
27/32" #Fractional0.843821.431
55/64" #Fractional0.859421.828
7/8" #Fractional0.875022.225
57/64" #Fractional0.890622.622
29/32" #Fractional0.906223.019
59/64" #Fractional0.921923.416
15/16" #Fractional0.937523.812
61/64" #Fractional0.953124.209
31/32" #Fractional0.968824.606
63/64" #Fractional0.984425.003
1/1" #Fractional1.000025.400

Number and letter drills exist to fill the gaps between fractional sizes — the three series interleave rather than being alternatives. Decimal values are the defining dimensions; metric values are converted at 25.4 mm per inch.

Why Three Overlapping Series Exist

Fractional drills in 1/64 in steps leave gaps of roughly 0.016 in, which is too coarse for fitting work — a clearance hole for a given screw has to be within a few thousandths. The number and letter series fill those gaps.

Because they interleave, "the nearest drill" is a real question — this is the table that answers it, which is why tap drill size selection depends on it.

Drill Size and Tap Drill Selection

Tap drills are cut from these series. For a thread, the pilot diameter is major diameter minus pitch, and the tap drill is the nearest standard size to that — which is almost always a number or letter drill rather than a fraction.

For example the pilot for a 1/4-20 thread is 0.2500 − 0.0500 = 0.2000 in. There is no fractional drill within 0.012 in of that, but #7 at 0.2010 in is 0.001 in away. That is why the number series exists.

Frequently Asked Questions

What size is a #7 drill bit?
#7 is 0.2010 in, or 5.105 mm. It is the standard tap drill for a 1/4-20 thread — the pilot diameter is 0.2000 in and #7 is the nearest standard drill.
What is the difference between number, letter and fractional drill bits?
They are three interleaved size series covering the same overall range at different granularity. Number drills run #1 (0.2280 in) down to #80 (0.0135 in); letter drills run A (0.2340 in) to Z (0.4130 in); fractional drills step in 1/64 in. A given size exists in only one series.
Why do number drills count backwards?
The numbering is historical, inherited from the Stubs steel wire gauge where higher gauge numbers meant thinner wire. It has no functional meaning today — #1 is simply the largest number drill.
What is an F drill used for?
F is 0.2570 in and is the standard tap drill for 5/16-18. The pilot diameter is 0.3125 − 0.0556 = 0.2569 in, and F at 0.2570 in is within 0.0001 in of it.
Is a #7 drill the same as 13/64?
No. #7 is 0.2010 in and 13/64 is 0.2031 in — a difference of 0.002 in. Close, but not interchangeable for tapping: the wrong one changes thread engagement.
What is the largest standard drill size?
Fractional drills continue past 1 in in larger steps, and letter drills stop at Z (0.4130 in). Above roughly 1/2 in, core drills and boring are usually more practical than twist drills.

Related

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#SourceTypeRevision / method
[1]ASME B94.11M — Twist DrillsreferenceASME B94.11M-1993 (R2017) — source
[2]Values computed from the defining relationshipderivedDecimal inch values are the defining dimensions of each drill series; millimetre values are inch × 25.4. Fraction-to-decimal values are exact arithmetic (numerator ÷ denominator).
[3]Standard twist drill size series (number, letter and fractional)referencen/a — long-established industry series, standardised in ASME B94.11M — source

Data Sources

StandardRevisionWhat it covers on this page
ASME B94.11M — Twist DrillsASME B94.11M-1993 (R2017)the number, letter and fractional drill size series and their defining dimensions

Verification scope

No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

Derived values — the following values on this page are calculated, not taken directly from the standard:

ValueHow it is derived
Decimal inch valuesThese are the defining dimensions of each drill series.
Metric valuesInch value × 25.4.

Decimal values are nominal. Drill manufacturing tolerances apply and vary by size and quality grade.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

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