Roughly, how long is 8mm. measured as an inch?

It's the thickness of a piece of paper more than 5/16"

Bead necklaces from all over the world are listed by diameter in mm and length in inches, so you need to learn the system perfectly. Get a ruler in your hands. Measure things until you start to understand how a ruler works. Measure some stuff and figure out where the center is. Say you measure a book and it's 7/8" thick. You look at your ruler and see that every eighth is divided into two sixteenths, so obviously half of 7/8" is going to be 7/16". If you write that out you have 1/2 x 7/8 = 7/16. And you notice that 1/2 is divided into 2/4 and then into 4/8 and so on, so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom.

Other rulers are divided into 10 and 100 parts. But an inch is still and inch, so anything on one ruler can be translated to the other ruler. A half inch on one ruler is 5/10 or 50/100 on the other. An eighth inch is just 12.5 marks when you have 100 marks per inch. A metric ruler divides an inch into 25.4 parts, so a half inch would be 12.7 of those parts. Pretty simple, isn't it? Practice this a bit and people will think you went to wizard school.

An inch is 25.4 mm so you are talking about 8/25.4 = 0.315 inch which is close to 5/16". (5/16 = 0.3125)

Leaving off the long lecture but being more supportive than a simple answer - an inch is exactly 25.4mmm by definition. so for practical purposes you can use 8/25 to get the fraction.

How Long Is 8mm

How Big Is 8mm

Closer to a third of an inch.

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Yea you got a big head, are you implying your smarter?.

About 0.32"