Source: Wikimedia Commons

Bolts and studs come in an incredible array of sizes, and those numbers are typically indicated by a series of numbers and letters found somewhere on the hardware.

In most cases, the first number usually represents the bolt diameter expressed in either metric or imperial measurements. The second denotes the distance between the thread pitch, while the third number, if present, represents the length of the bolt.

Different standards have differently arranged numbers, but in most cases, all the important measurements are arranged in columns, with the leftmost column usually showing the bolt diameter or thread size. It's usually followed by thread pitch, then specific bolt length (if present), and lastly, the tensile strength grade.

All of this information is crucial when selecting a bolt, as it allows you to match the appropriate size and strength of your bolt to your application.

Standard Bolt Size Chart

Metric Size	Imperial Size	Diameter (mm)	Diameter (inches)	Common Thread Pitches (mm)	Common Thread Pitches (TPI)	Common Lengths (mm)	Common Lengths (inches)
M2	#2	2	5/64	0.4	56	16, 20	5/8, 3/4
M3	#4	3	7/64	0.5	48	20, 30	3/4, 1-1/4
M4	#6	4	1/8	0.7	40	25, 30	1, 1-1/4
M5	#8	5	3/16	0.8	32	30, 40	1-1/4, 1-1/2
M6	#10	6	7/32	1.0	24	40, 50	1-1/2, 2
M8	5/16	8	5/16	1.25	18	50, 60	2, 2-1/2
M10	3/8	10	3/8	1.5	16	60, 70	2-1/2, 3
M12	1/2	12	1/2	1.75	13	70, 80	3, 3-1/2

Difference Between Metric and Imperial

The ISO metric system is the most commonly used system of measures used by most of the world, with the exception of the United States, Liberia, and Myanmar (also known as Burma). Metric bolt sizes are measured in millimeters; they're identified by the letter "M" followed by a number that denotes bolt diameter. A common example of a metric bolt size would look like this: M8-1.25×30.

Imperial bolt sizes are common in the US. They rely on inches and inch fractions and are typically denoted with a "#" for smaller sizes or a fraction for larger bolts, followed by a count of bolt threads per inch. A common example would look like this: "#8-32 or 1/2" -13.

Difference Between Coarse and Fine Threads

Common thread pitches denote nominal-size threads but it is measured differently between imperial or metric threads. For metric threads, the common thread pitch refers to the distance between two high points of individual threads. For the M2 bolt, that's 0.4 millimeters. However, for an imperial bolt, the number refers to the number of threads per inch of thread length. For example, the #2 bolt has 56 TPI.

The aforementioned numbers apply to a coarse thread (UNC). Fine threads for both imperial measurements (UNF) would be #2-64. Metric threads differentiate fine and coarse only in bolt sizes whose body diameters exceed size M8.

Tensile Strength

Tensile strength isn't typically marked along with other measurements. In most cases, the designation is imprinted on the head of the screw; you will most often see a number on the head of a heavy hex bolt head.

The most common grades for metric bolts and fasteners are 8.8, 10.9, and 12.9, with a higher number denoting a stronger bolt. The same applies to the imperial system, though the most common denotations are 2, 5, and 8. There isn't any correlation between imperial and metric measurements when it comes to tensile strength.

Choosing the Right Bolt Size and Strength

It's important to remember that while some measurements may seem close, like a metric M10 and an imperial 3/8", these two bolts aren't interchangeable due to different standards of measurement. The diameters, thread pitch, and length of bolts in these two measurement systems vary, which means that they're not interchangeable.

If you want to learn more about bolts of different types and sizes for your specific application, don’t hesitate to contact our team to find out which bolts we can provide at the lowest lead time for your schedule.