Wrench and hex key sizes vary depending on the head type. This complete range guide provides the exact Thou and Metric dimensions for external hex, cap heads, and countersink fasteners.

Bolt Size Hex Bolt (Wrench) Cap Head (Hex Key) Countersink (Hex Key)
#4
250.0 thou
6.35 mm(1/4")
93.7 thou
2.38 mm(3/32")
62.5 thou
1.58 mm(1/16")
#6
312.5 thou
7.93 mm(5/16")
109.3 thou
2.77 mm(7/64")
78.1 thou
1.98 mm(5/64")
#8
312.5 thou
7.93 mm(5/16")
140.6 thou
3.57 mm(9/64")
93.7 thou
2.38 mm(3/32")
#10
375.0 thou
9.52 mm(3/8")
156.2 thou
3.96 mm(5/32")
125.0 thou
3.17 mm(1/8")
1/4"
437.5 thou
11.11 mm(7/16")
187.5 thou
4.76 mm(3/16")
156.2 thou
3.96 mm(5/32")
5/16"
500.0 thou
12.70 mm(1/2")
250.0 thou
6.35 mm(1/4")
187.5 thou
4.76 mm(3/16")
3/8"
562.5 thou
14.29 mm(9/16")
312.5 thou
7.94 mm(5/16")
218.7 thou
5.56 mm(7/32")
1/2"
750.0 thou
19.05 mm(3/4")
375.0 thou
9.52 mm(3/8")
312.5 thou
7.94 mm(5/16")
5/8"
937.5 thou
23.81 mm(15/16")
500.0 thou
12.70 mm(1/2")
375.0 thou
9.52 mm(3/8")
3/4"
1125.0 thou
28.57 mm(1-1/8")
625.0 thou
15.87 mm(5/8")
500.0 thou
12.70 mm(1/2")
M2
157.5 thou
4.00 mm
59.0 thou
1.50 mm
47.2 thou
1.20 mm
M3
216.5 thou
5.50 mm
98.4 thou
2.50 mm
78.7 thou
2.00 mm
M4
275.6 thou
7.00 mm
118.1 thou
3.00 mm
98.4 thou
2.50 mm
M5
315.0 thou
8.00 mm
157.5 thou
4.00 mm
118.1 thou
3.00 mm
M6
393.7 thou
10.00 mm
196.9 thou
5.00 mm
157.5 thou
4.00 mm
M8
511.8 thou
13.00 mm
236.2 thou
6.00 mm
196.9 thou
5.00 mm
M10
669.3 thou
17.00 mm
315.0 thou
8.00 mm
236.2 thou
6.00 mm
M12
748.0 thou
19.00 mm
393.7 thou
10.00 mm
315.0 thou
8.00 mm
M14
866.1 thou
22.00 mm
472.4 thou
12.00 mm
393.7 thou
10.00 mm
M16
944.9 thou
24.00 mm
551.2 thou
14.00 mm
393.7 thou
10.00 mm
M20
1181.1 thou
30.00 mm
669.3 thou
17.00 mm
472.4 thou
12.00 mm