If you have ever purchased or shopped for a piece of jewelry online or in a catalog, perhaps you have noticed that most companies measure the size of their jewelry in millimeters, not inches. For many U.S. buyers who do not use the metric system on a daily basis, this causes difficulty in being able to accurately guage the size of a piece of jewelry in their mind, and can result in disappointment if the piece is much larger or smaller than they imagined.

However, the following chart will help you to accurately determine how large a piece of jewelry or a gemstone truly is, so you will better understand the dimensions of the piece you are looking to purchase:

1 mm = approximately 1/32" 21 mm = approximately 13/16"
2 mm = approximately 1/8" 22 mm = approximately 7/8"
3 mm = approximately 1/8" 23 mm = approximately 15/16"
4 mm = approximately 3/16" 24 mm = approximately 15/16"
5 mm = approximately 3/16" 25 mm = approximately 1"
6 mm = approximately 1/4" 26 mm = approximately 1"
7 mm = approximately 1/4" 27 mm = approximately 1 1/16"
8 mm = approximately 5/16" 28 mm = approximately 1 1/4"
9 mm = approximately 3/8" 29 mm = approximately 1 1/8"
10 mm = approximately 3/8" 30 mm = approximately 1 3/16"
11 mm = approximately 7/16" 31 mm = approximately 1 1/4"
12 mm = approximately 1/2" 32 mm = approximately 1 1/4"
13 mm = approximately 1/2" 33 mm = approximately 1 5/16"
14 mm = approximately 9/16" 34 mm = approximately 1 3/8"
15 mm = approximately 5/8" 35 mm = approximately 1 3/8"
16 mm = approximately 5/8" 36 mm = approximately 1 7/16"
17 mm = approximately 11/16" 37 mm = approximately 1 1/2"
18 mm = approximately 3/4" 38 mm = approximately 1 1/2"
19 mm = approximately 3/4" 39 mm = approximately 1 9/16"
20 mm = approximately 13/16" 40 mm = approximately 1 9/16"


For comparison purposes, here are the diameter measurements of some common items:

Dime = 17mm

Penny = 19mm

Nickel = 21mm

Quarter = 24mm

It is also important to make sure that you understand the definition of what the specific measurement is for. Here are some commonly misunderstood definitions:

Diameter: The distance of a straight line segment passing through the center of a figure, especially of a circle or sphere, and terminating at the edges

Circumference: The size of an item as given by the distance around it.