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Metric Bolt Thread Info

Metric size¹ (fine pitch)	Max. diameter	Wrench used²	Allen key used³	Tap drill used⁴
M1 x 0.25	1.0	???	???	0.75
M2 x 0.4	1.98	???	M1	1.60
M3 x 0.5	2.98	???	M2	2.50
M4 x 0.7	3.98	???	M3	3.30
M5 x 0.8	4.98	8	M4	4.20
M6 x 1.0	5.97	10	M5	5.00
M7 x 1.0	6.97	11	—	6.00
M8 x 1.25 (M8 x 1.0)	7.97	13	M6	6.80
M9 x 1.25	8.97	???	???	7.80
M10 x 1.5 (M10 x 1.25)* (M10 x 1.0)	9.97	17	M8	8.50
M11 x 1.5	10.97	???	???	9.50
M12 x 1.75 (M12 x 1.5) (M12 x 1.25)	11.97	19	M10	10.20
M14 x 2.0 (M14 x 1.5)	13.97	22	M12	12.00
M16 x 2.0 (M16 x 1.5)	15.97	24	M14	14.00

Notes:

(1) Metric bolts are referred to by their diameter (measured across the threads), pitch and length. Diameter is best measured using a vernier or dial caliper across the threaded portion of the bolt, only. Pitch, the density of the thread, is the number of threads per milimeter and is most easily determined by measuring the distance covered by 10 ridges or valleys (i.e., 10 threads in 15mm = 1.5 pitch; 10 threads in 12.5mm = 1.25 pitch). The most common pitch in metric bolts is called a "coarse" thread, and there is only one coarse thread per bolt size; accordingly, a metric bolt specified without a pitch is a coarse thread bolt. In parentheses are the typical "fine" pitch varieties; the most common of these are indicated by *. Metric bolt length is measured from under the bolt head to the tip of the bolt, except for countersunk bolts, where the head is included in the length.

(2) Bolts are not referred to by their head size (wrench size), as this can vary depending on the application, but the normal bolt head sizes are listed here, where known.

(3) Generally, one metric size (not necessarily number) smaller than the bolt size.

(4) Refers to coarse thread bolts, only.

(5) If you need more info than this, you really need to buy the Machinery's Handbook or spend a few hours at the Maryland Metrics Web site...