Drill Bit Size Chart — Metric, Imperial, Number and Letter Conversions

The three conversions you need:

Inches → mm: multiply by 25.4

mm → inches: divide by 25.4

Fraction → decimal: divide top by bottom (5 ÷ 16 = 0.3125″)

Number and letter drills follow no formula — each has a fixed diameter. Look them up below.

Four different sizing systems are still in daily use in the UK: metric, fractional inch, number gauge (#1–#80) and letter gauge (A–Z). Old drawings, imported machinery and inherited tooling mean you will meet all four. Every one of them is converted below.

The useful bit: we stock HSS drill bits in 0.1 mm increments from 0.4 mm to 20 mm. So if you're handed a number or letter drill size and you only work in metric, just round the millimetre figure to the nearest 0.1 mm — we'll have it, and it'll be within 0.05 mm of the original. That's close enough for anything short of a reamed hole.

Metric to imperial

Metric Decimal (in) Nearest fraction
1.0 mm 0.0394 3/64″
1.5 mm 0.0591 1/16″
2.0 mm 0.0787 5/64″
2.5 mm 0.0984 3/32″
3.0 mm 0.1181 1/8″
3.5 mm 0.1378 9/64″
4.0 mm 0.1575 5/32″
4.5 mm 0.1772 11/64″
5.0 mm 0.1969 13/64″
5.5 mm 0.2165 7/32″
6.0 mm 0.2362 15/64″
6.5 mm 0.2559 1/4″
7.0 mm 0.2756 9/32″
7.5 mm 0.2953 19/64″
8.0 mm 0.3150 5/16″
8.5 mm 0.3346 21/64″
9.0 mm 0.3543 23/64″
9.5 mm 0.3740 3/8″
10.0 mm 0.3937 25/64″
10.5 mm 0.4134 13/32″
11.0 mm 0.4331 7/16″
11.5 mm 0.4528 29/64″
12.0 mm 0.4724 15/32″
12.5 mm 0.4921 31/64″
13.0 mm 0.5118 33/64″

Nearest fraction is the closest standard 1/64″ drill — useful as a substitute, but not an exact match. For tapping, always use the exact size.

Fractional inch to metric

Fraction Decimal (in) Metric
1/64″ 0.0156 0.40 mm
1/32″ 0.0312 0.79 mm
3/64″ 0.0469 1.19 mm
1/16″ 0.0625 1.59 mm
5/64″ 0.0781 1.98 mm
3/32″ 0.0938 2.38 mm
7/64″ 0.1094 2.78 mm
1/8″ 0.1250 3.17 mm
9/64″ 0.1406 3.57 mm
5/32″ 0.1562 3.97 mm
11/64″ 0.1719 4.37 mm
3/16″ 0.1875 4.76 mm
13/64″ 0.2031 5.16 mm
7/32″ 0.2188 5.56 mm
15/64″ 0.2344 5.95 mm
1/4″ 0.2500 6.35 mm
17/64″ 0.2656 6.75 mm
9/32″ 0.2812 7.14 mm
19/64″ 0.2969 7.54 mm
5/16″ 0.3125 7.94 mm
21/64″ 0.3281 8.33 mm
11/32″ 0.3438 8.73 mm
23/64″ 0.3594 9.13 mm
3/8″ 0.3750 9.52 mm
25/64″ 0.3906 9.92 mm
13/32″ 0.4062 10.32 mm
27/64″ 0.4219 10.72 mm
7/16″ 0.4375 11.11 mm
29/64″ 0.4531 11.51 mm
15/32″ 0.4688 11.91 mm
31/64″ 0.4844 12.30 mm
1/2″ 0.5000 12.70 mm
33/64″ 0.5156 13.10 mm
17/32″ 0.5312 13.49 mm
35/64″ 0.5469 13.89 mm
9/16″ 0.5625 14.29 mm
37/64″ 0.5781 14.68 mm
19/32″ 0.5938 15.08 mm
39/64″ 0.6094 15.48 mm
5/8″ 0.6250 15.88 mm
41/64″ 0.6406 16.27 mm
21/32″ 0.6562 16.67 mm
43/64″ 0.6719 17.07 mm
11/16″ 0.6875 17.46 mm
45/64″ 0.7031 17.86 mm
23/32″ 0.7188 18.26 mm
47/64″ 0.7344 18.65 mm
3/4″ 0.7500 19.05 mm
49/64″ 0.7656 19.45 mm
25/32″ 0.7812 19.84 mm
51/64″ 0.7969 20.24 mm
13/16″ 0.8125 20.64 mm
53/64″ 0.8281 21.03 mm
27/32″ 0.8438 21.43 mm
55/64″ 0.8594 21.83 mm
7/8″ 0.8750 22.22 mm
57/64″ 0.8906 22.62 mm
29/32″ 0.9062 23.02 mm
59/64″ 0.9219 23.42 mm
15/16″ 0.9375 23.81 mm
61/64″ 0.9531 24.21 mm
31/32″ 0.9688 24.61 mm
63/64″ 0.9844 25.00 mm
1″ 1.0000 25.40 mm

Number drill sizes (#1 – #80)

Number drills run backwards — #80 is the smallest at 0.34 mm, #1 is the largest at 5.79 mm. They're common on older engineering drawings and in electronics work.

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

Letter drill sizes (A – Z)

Letter drills pick up where the numbers stop. A is the smallest at 5.94 mm, Z the largest at 10.49 mm. Handy fact: letter E is exactly 1/4″.

Letter Decimal (in) Metric
A 0.2340 5.94 mm
B 0.2380 6.05 mm
C 0.2420 6.15 mm
D 0.2460 6.25 mm
E 0.2500 6.35 mm
F 0.2570 6.53 mm
G 0.2610 6.63 mm
H 0.2660 6.76 mm
I 0.2720 6.91 mm
J 0.2770 7.04 mm
K 0.2810 7.14 mm
L 0.2900 7.37 mm
M 0.2950 7.49 mm
N 0.3020 7.67 mm
O 0.3160 8.03 mm
P 0.3230 8.20 mm
Q 0.3320 8.43 mm
R 0.3390 8.61 mm
S 0.3480 8.84 mm
T 0.3580 9.09 mm
U 0.3680 9.35 mm
V 0.3770 9.58 mm
W 0.3860 9.80 mm
X 0.3970 10.08 mm
Y 0.4040 10.26 mm
Z 0.4130 10.49 mm

Whatever the system, we've got the size

Over a million drill bits in stock, HSS in 0.1 mm increments from 0.4 mm to 20 mm, plus imperial sizes. Same-day dispatch, free next-working-day UK delivery on orders over £50.

HSS drill bits  ·  M35 cobalt  ·  M42 cobalt  ·  Drill sets

Which bit, not just which size

Getting the diameter right is only half of it. Match the bit to the material:

  • Mild steel, aluminium, plastic, timber — standard HSS.
  • Stainless steel, steel, cast iron — M35 cobalt (5%). Stainless work-hardens under a blunt HSS bit and then stops cutting altogether.
  • Hardened and high-alloy steel — M42 cobalt (8%), for the jobs that eat drill bits.
  • Tapping a thread? — see our tap drill size chart for the exact hole size for every tap.

Frequently asked questions

What size is a #7 drill bit?

A #7 drill bit is 0.2010″ (5.11 mm). It's one of the most looked-up sizes because it's the tap drill for a 1/4″-20 UNC thread. If you work in metric, a 5.1 mm bit is within 0.01 mm of it.

What size is a letter F drill bit?

A letter F drill bit is 0.2570″ (6.53 mm) — the tap drill for 5/16″-18 UNC. The nearest metric size is 6.5 mm.

How do I convert inches to millimetres?

Multiply by 25.4. So 0.25″ × 25.4 = 6.35 mm. To go the other way, divide by 25.4.

What is 1/4 inch in mm?

6.35 mm. The nearest standard metric drill is 6.5 mm, which is 0.15 mm larger — fine for a clearance hole, but not for tapping.

Do number drill sizes go up or down?

Down. Number drills run backwards: #80 is the smallest (0.34 mm) and #1 the largest (5.79 mm). It catches people out constantly.

I have a number or letter drill size but only work in metric. What do I use?

Take the millimetre figure from the tables above and round it to the nearest 0.1 mm. We stock HSS in 0.1 mm increments from 0.4 mm to 20 mm, so the substitute will be within 0.05 mm of the original — close enough for everything except a reamed or precision-fit hole.

UK Drills has supplied drill bits to UK trade and industry since 1985, from our trade counter in Hindley, Wigan. Can't find the size you need? Email info@ukdrills.com — we almost certainly have it.