Gem Profile July 19: Jewelry Measurement Conversions

By on July 19, 2013
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by Narlene Allen,

Today's Gem Profile is...

Jewelry Measurement Conversions and Charts

Resource Center

This week were going to focus on a very rare and beautiful gemstone called Zultanite, however Layna, our Gem Profile contributor, was out ill. Since we don’t want to miss the wonderful information that she has to share, we are going to postpone that topic and come back to it in a couple of weeks! I can’t wait to hear what she’s discovered about it and we wish her a speedy recovery!

So, in the place of Zultanite, we are going to discuss the conversions of measurement for gemstones, beads, wire gauge and jump rings.

In the jewelry world, millimeters, as we all know, is a common form of measurement. However, if you’re like me, you have to think about the conversion. Today, I thought we’d take a quick look at how to figure it all out.

If you haven’t been to our Resource Center lately, there is a great article which covers a lot of this information as well. Today though,  I”m only going to touch on a small portion because I want you to all go on a little treasure hunt and see what you can find!


How to start:

Cabochons and beads are commonly measured in millimeters (mm), while many people in the United States are more familiar with inches (in). Here are some easy ways to figure out what size cabochon or bead to work with!.

  • To convert inches to mm, multiply inches by 25.4
  • To convert mm to inches, divide millimeters by 25.4


In most cabochon measurements, the height comes first, then the width (which may seem backwards).

Here are some common cabochon sizes, in millimeters and inches.

8 x 6mm Cab

8 x 6mm Cab

0.3″ x 0.25″ Cab

14 x 10mm Cab

14 x 10mm

0.6″ x 0.4″

18 x 13mm Cab

18 x 13mm

0.7″ x 0.5″

25 x 18mm Cab

25 x 18mm

1″ x 0.7″

30 x 22mm Cab

30 x 22mm

1.2″ x 0.7″

40 x 30mm Cab

40 x 30mm

1.6″ x 1.2″

  • Remember, there are 10 millimeters in 1 centimeter, so a 40 x 30mm cab can also be measured as 4 x 3 centimeters. Most rulers in the U.S. have one side for inches, and one side for centimeters.
  • For comparison, a U.S. Quarter is 24.26mm in diameter (across); a quarter is nearly the same size as a 25mm round cabochon.


U.S. Quarter


25mm Round Cab

25mm Round



A U.S. Penny is 19mm in diameter, or 3/4″ across. Here’s a penny compared to an 18 x 13mm cab:


U.S. Penny (19mm)

18 x 13mm Cab

18 x 13mm


Do you want to know the number of beads in a strand? If you know the length of the strand and the size of the beads, you can estimate the number of beads in any strand. Note: this method may not work on beads of different sizes on the same strand.

  1. Take the strand measurement and convert it to millimeters. On Wire-Sculpture, most of our strands are 16″ long, or 406.4mm
  2. Divide the strand measurement by the size of the bead.  For example, we have a 6mm round bead.6mm Bead406.4 ÷ 6 = 67.7

There are about 67 beads in a strand of 16″ 6mm beads. Each strand may vary slightly by a few beads in either direction.

What did you learn?

If you’d like to read more about Wire Gauges and Jump Rings – you can continue reading this article in our Resource Center! It’s great information for anyone from the beginner to the advanced wire artist.

Next week, we have some very exciting news – do you love new beads? I do! We will be hearing all about a NEW line of Large Hole Round Beads that are now available on Wire-Sculpture. You won’t want to miss next weeks profile!

Do you have any beautiful jewelry you’d like to share with us? Send us pictures at and they could be featured!


Gem Profile by Narlene Allen

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  1. avatar


    July 19, 2013 at 6:41 am

    This is very helpful. I can’t wait to see what the next few charts and conversions will be! Thank you.

    • avatar


      July 19, 2013 at 6:42 am

      Well, actually, I guess I will be checking out the resourse center!

  2. avatar

    Marcia Hooten

    July 19, 2013 at 8:42 am

    Great article. Very informative and a good keeper to put in my jewelry data file.

  3. avatar

    Mary Hallengren

    July 19, 2013 at 1:49 pm

    This is great info for buying beads at shows just a quick calculation cuts the guess work. I always buy too much.

  4. avatar


    July 25, 2013 at 7:49 am

    What do you do when you have a gemstone you want to use in your wire jewelry as an accent, but it’s too small to drill a hole in it? is there a glue you could use? if you wrap wire around it to secure it that would not work because the wire would simply cover it up given it’s size. Maybe the better question is what are the size limits of a gem to be able to drill holes without destroying it? What do you do with small spare chip sizes and sizes like say 6×4 or 7×5 pear shape gem?