Drill Bit Size Chart: Fraction, Number, Letter and mm

Fractional drill bit sizes (1/64 to 1/2 in)

A standard 29-piece set covers 1/16 to 1/2 in in 1/64 in steps; a 21-piece set stops at 3/8 in. The table gives each bit's decimal and metric size, and the closest bit from a metric set when you need a substitute.

Fractional drill bit sizes with decimal and mm equivalents
Bit	Decimal (in)	Millimeters	Nearest metric bit
1/64 in	0.0156	0.40	0.5 mm
1/32 in	0.0313	0.79	1 mm
3/64 in	0.0469	1.19	1 mm
1/16 in	0.0625	1.59	1.5 mm
5/64 in	0.0781	1.98	2 mm
3/32 in	0.0938	2.38	2.5 mm
7/64 in	0.1094	2.78	3 mm
1/8 in	0.1250	3.18	3 mm
9/64 in	0.1406	3.57	3.5 mm
5/32 in	0.1563	3.97	4 mm
11/64 in	0.1719	4.37	4.5 mm
3/16 in	0.1875	4.76	5 mm
13/64 in	0.2031	5.16	5 mm
7/32 in	0.2188	5.56	5.5 mm
15/64 in	0.2344	5.95	6 mm
1/4 in	0.2500	6.35	6.5 mm
17/64 in	0.2656	6.75	6.5 mm
9/32 in	0.2813	7.14	7 mm
19/64 in	0.2969	7.54	7.5 mm
5/16 in	0.3125	7.94	8 mm
21/64 in	0.3281	8.33	8.5 mm
11/32 in	0.3438	8.73	8.5 mm
23/64 in	0.3594	9.13	9 mm
3/8 in	0.3750	9.52	9.5 mm
25/64 in	0.3906	9.92	10 mm
13/32 in	0.4063	10.32	10.5 mm
27/64 in	0.4219	10.72	10.5 mm
7/16 in	0.4375	11.11	11 mm
29/64 in	0.4531	11.51	11.5 mm
15/32 in	0.4688	11.91	12 mm
31/64 in	0.4844	12.30	12.5 mm
1/2 in	0.5000	12.70	12.5 mm

A metric substitute is fine for wood and plastic. For metal, taps and press fits, use the exact size: a 6 mm bit is 0.35 mm (0.014 in) smaller than 1/4 in.

Number drill sizes (#1 to #80)

Number (wire-gauge) drills are a machinist's series that fills the gaps between fractional sizes below 1/4 in. The number runs backward: #80 is the smallest at 0.0135 in, and #1 is the largest at 0.2280 in. A 60-piece number set covers #1 to #60.

Number drill sizes in decimal inches and mm
Drill	Inches	mm	Drill	Inches	mm
#1	0.2280	5.79	#41	0.0960	2.44
#2	0.2210	5.61	#42	0.0935	2.37
#3	0.2130	5.41	#43	0.0890	2.26
#4	0.2090	5.31	#44	0.0860	2.18
#5	0.2055	5.22	#45	0.0820	2.08
#6	0.2040	5.18	#46	0.0810	2.06
#7	0.2010	5.11	#47	0.0785	1.99
#8	0.1990	5.05	#48	0.0760	1.93
#9	0.1960	4.98	#49	0.0730	1.85
#10	0.1935	4.91	#50	0.0700	1.78
#11	0.1910	4.85	#51	0.0670	1.70
#12	0.1890	4.80	#52	0.0635	1.61
#13	0.1850	4.70	#53	0.0595	1.51
#14	0.1820	4.62	#54	0.0550	1.40
#15	0.1800	4.57	#55	0.0520	1.32
#16	0.1770	4.50	#56	0.0465	1.18
#17	0.1730	4.39	#57	0.0430	1.09
#18	0.1695	4.31	#58	0.0420	1.07
#19	0.1660	4.22	#59	0.0410	1.04
#20	0.1610	4.09	#60	0.0400	1.02
#21	0.1590	4.04	#61	0.0390	0.99
#22	0.1570	3.99	#62	0.0380	0.97
#23	0.1540	3.91	#63	0.0370	0.94
#24	0.1520	3.86	#64	0.0360	0.91
#25	0.1495	3.80	#65	0.0350	0.89
#26	0.1470	3.73	#66	0.0330	0.84
#27	0.1440	3.66	#67	0.0320	0.81
#28	0.1405	3.57	#68	0.0310	0.79
#29	0.1360	3.45	#69	0.0292	0.74
#30	0.1285	3.26	#70	0.0280	0.71
#31	0.1200	3.05	#71	0.0260	0.66
#32	0.1160	2.95	#72	0.0250	0.64
#33	0.1130	2.87	#73	0.0240	0.61
#34	0.1110	2.82	#74	0.0225	0.57
#35	0.1100	2.79	#75	0.0210	0.53
#36	0.1065	2.71	#76	0.0200	0.51
#37	0.1040	2.64	#77	0.0180	0.46
#38	0.1015	2.58	#78	0.0160	0.41
#39	0.0995	2.53	#79	0.0145	0.37
#40	0.0980	2.49	#80	0.0135	0.34

Letter drill sizes (A to Z)

Letter drills pick up where number drills end and run from A (0.234 in) to Z (0.413 in). A full 115-piece machinist index is 29 fractional, 26 letter and 60 number drills.

Letter drill sizes with decimal, mm and nearest fraction
Letter	Inches	mm	Nearest 1/64 in
A	0.234	5.94	15/64
B	0.238	6.05	15/64
C	0.242	6.15	15/64
D	0.246	6.25	1/4
E	0.250	6.35	1/4
F	0.257	6.53	1/4
G	0.261	6.63	17/64
H	0.266	6.76	17/64
I	0.272	6.91	17/64
J	0.277	7.04	9/32
K	0.281	7.14	9/32
L	0.290	7.37	19/64
M	0.295	7.49	19/64
N	0.302	7.67	19/64
O	0.316	8.03	5/16
P	0.323	8.20	21/64
Q	0.332	8.43	21/64
R	0.339	8.61	11/32
S	0.348	8.84	11/32
T	0.358	9.09	23/64
U	0.368	9.35	3/8
V	0.377	9.58	3/8
W	0.386	9.80	25/64
X	0.397	10.08	25/64
Y	0.404	10.26	13/32
Z	0.413	10.49	13/32

E is exactly 1/4 in. Letters matter mainly for tap drills: F for 5/16-18, I for 5/16-24, Q for 3/8-24 and U for 7/16-14.

Metric drill bit sizes and inch equivalents

Common metric drill bits converted to inches
Metric bit	Inches	Closest fractional bit	Typical use
2 mm	0.0787	5/64 in (0.0781)	Pilot for small screws
2.5 mm	0.0984	3/32 in (0.0938)	Tap drill for M3
3 mm	0.1181	1/8 in (0.1250)	Pilot for 4 mm wood screws
3.3 mm	0.1299	1/8 in (0.1250)	Tap drill for M4
4.2 mm	0.1654	11/64 in (0.1719)	Tap drill for M5
5 mm	0.1969	13/64 in (0.2031)	Tap drill for M6; 5 mm shelf pins; 5 mm wall plugs
6 mm	0.2362	15/64 in (0.2344)	6 mm wall plugs (the most common size)
6.8 mm	0.2677	17/64 in (0.2656)	Tap drill for M8
8 mm	0.3150	5/16 in (0.3125)	8 mm dowels and wall plugs
8.5 mm	0.3346	21/64 in (0.3281)	Tap drill for M10
10 mm	0.3937	25/64 in (0.3906)	10 mm wall plugs and anchors
10.2 mm	0.4016	13/32 in (0.4063)	Tap drill for M12
13 mm	0.5118	33/64 in (0.5156)	Largest bit in most 13 mm chucks

Tap drill chart

A tap drill leaves enough metal for about 75% thread engagement, the usual target for steel and aluminum. Metric tap drill size is simply diameter minus pitch: M8 × 1.25 needs 8 - 1.25 = 6.75, rounded to a 6.8 mm bit.

Tap drill sizes for common inch and metric threads
Thread	Tap drill	Decimal (in)	Thread	Tap drill
4-40	#43	0.0890	M3 × 0.5	2.5 mm
6-32	#36	0.1065	M4 × 0.7	3.3 mm
8-32	#29	0.1360	M5 × 0.8	4.2 mm
10-24	#25	0.1495	M6 × 1.0	5.0 mm
10-32	#21	0.1590	M8 × 1.25	6.8 mm
1/4-20	#7	0.2010	M10 × 1.5	8.5 mm
1/4-28	#3	0.2130	M12 × 1.75	10.2 mm
5/16-18	F	0.2570	M14 × 2.0	12.0 mm
3/8-16	5/16 in	0.3125	M16 × 2.0	14.0 mm
3/8-24	Q	0.3320	M8 × 1.0 (fine)	7.0 mm
1/2-13	27/64 in	0.4219	M10 × 1.25 (fine)	8.8 mm

Screw diameters, thread counts and wrench sizes are on the screw size chart.

Which drill bit size to use for common jobs

Bit sizes for everyday drilling
Job	Bit	Notes
Pilot for #8 wood screw	3/32 in (softwood), 7/64 in (hardwood)	Full pilot chart on the screws page
Plastic wall plug	Same as the plug: 5, 6 or 8 mm	Use a masonry bit in brick or concrete
3/16 in concrete screw (Tapcon type)	5/32 in carbide masonry bit	Drill deeper than the screw will go
1/4 in concrete screw	3/16 in carbide masonry bit	Check the maker's sheet for stainless versions
Door lock (cylinder)	2 1/8 in hole saw	Standard bore through the door face
Door latch	1 in spade or Forstner bit	Bored into the door edge; see door sizes
European cup hinge	35 mm Forstner bit	About 1/2 in (11-13 mm) deep in cabinet doors
Shelf pins	5 mm or 1/4 in brad-point	Match the pins you bought
Cable through a stud	3/4-1 in spade bit	Keep the hole 1 1/4 in from the stud face
1/2 in PEX through framing	3/4 in or 7/8 in bit	PEX OD is 5/8 in; see pipe sizes

Holes in framing

Codes limit bored holes in studs. Under IRC Section R602.6, a hole in a single bearing stud may be up to 40% of its width (1.4 in in a 2 × 4); 60% (2.1 in) is allowed in nonbearing walls or doubled studs. Keep every hole at least 5/8 in from the stud edge, and protect any pipe or cable closer than 1 1/4 in to the face with a steel plate.

Bit lengths, shanks and chuck sizes

Jobber length is the standard twist drill. Under ASME B94.11M, a 1/8 in jobber is 2 3/4 in long overall, a 1/4 in is 4 in and a 1/2 in is 6 in.
Stubby (screw machine) bits are about half as long and stiffer, good for metal and tight spaces. Aircraft extension bits are 6 or 12 in long.
Chuck capacity: compact drills take up to 3/8 in (10 mm) shanks; most cordless drills and hammer drills take 1/2 in (13 mm). Larger bits come with reduced shanks.
Hex shank: 1/4 in hex bits fit impact drivers and quick-change chucks.
SDS-plus bits have a 10 mm shank for rotary hammers up to about 1 in holes in concrete; SDS-max has an 18 mm shank for heavier work.

A drill and a basic bit index fit in the end pocket of most tool bags. When choosing fasteners for the hole, compare nail sizes and screws first: a nail needs no hole at all in softwood.

Frequently asked questions

What fraction drill size is 0.125 inch?

0.125 in is exactly 1/8 in (3.18 mm). The nearest metric bit is 3 mm (0.118 in), which drills a slightly smaller hole, and the nearest number drill is #30 (0.1285 in).

What is another name for a 1/4 inch drill bit?

A 1/4 in bit is the same as 16/64 in, 0.250 in, 6.35 mm and a letter E drill. All of these drill an identical hole.

What is the difference between a 1/16 and a 5/64 drill bit?

1/64 in, or 0.0156 in (0.40 mm). A 1/16 in bit is 0.0625 in (1.59 mm) and a 5/64 in bit is 0.0781 in (1.98 mm).

What drill bit sizes are between 1/2 and 9/16 inch?

In fractions: 33/64 (0.5156 in), 17/32 (0.5313 in) and 35/64 (0.5469 in). In metric: 13 mm (0.5118 in), 13.5 mm (0.5315 in) and 14 mm (0.5512 in). Bits above 1/2 in usually have a reduced 3/8 or 1/2 in shank to fit common chucks.

What is a letter Q drill bit equivalent to?

Q is 0.332 in (8.43 mm). The nearest fraction is 21/64 in (0.3281 in) and the nearest metric bit is 8.4 mm. There is no number-drill equivalent, because number drills stop at #1 (0.228 in). Q is the tap drill for a 3/8-24 thread.

Which Forstner bit sizes are most useful?

Sets usually run from 1/4 in to 2 1/8 in, or 15 to 50 mm. The sizes used most are 35 mm for European cup hinges, 1 in and 1 1/2 in for cable and pipe holes, and 2 1/8 in, which matches a standard door lock bore.