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Comparative pearl sizes-- how much difference does 1mm larger really make?

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  • Comparative pearl sizes-- how much difference does 1mm larger really make?

    Have you ever wondered whether it's worth going up a size when buying pearls (or other round beads)?

    Wondered whether the increased cost will be worth it-- and if you'll really be able to tell the difference?

    The diameter of a pearl (its size in millimeters) doesn't tell the whole story and can be misleading when deciding what size pearls to buy.
    What really matters is not the diameter, but the volume.

    Why volume instead of diameter? Pearls are 3 dimensional (not flat circles on a page as they are pictured)-- volume is all about the 3 dimensional size. How much space the pearl takes up! How large it looks.
    Its visual impact.
    And its weight.

    Knowing the volume of a given pearl size lets you compare one size with another. It tells you how much larger (and heavier) any given pearl will be compared to another pearl.

    You can also use this to estimate out how much larger and heavier a strand may be compared to another strand.

    We do this by dividing the volume of the larger pearl by the volume of the smaller pearl (or in the case of strands, dividing volumes of the end pearls of the 2 strands and the center pearls of the 2 strands, and then averaging.)
    I will demonstrate with examples below.

    I've been having a little fun calculating volumes for different size round pearls.
    Take this as a little guide to help you estimate how large a given size pearl is compared to another size pearl.
    (Rounded off to the nearest whole number.)

    16mm pearl: 2144 cubic mm (21% larger than 15mm pearls, and over 3 times as large as 11mm pearls.)
    15mm pearl: 1766 cubic mm (23% larger than 14mm pearls)
    14mm pearl: 1436 cubic mm (25% larger than 13mm) (twice as large as 11mm, and over 5x as large as 8mm pearls)
    13mm pearl: 1150 cubic mm (27% larger than 12mm)
    12mm pearl: 904 cubic mm (30% larger than 11mm and 73% larger than 10mm; over 3 times as large as 8mm)
    11mm pearl: 697 cubic mm (33% larger than 10mm)
    10mm pearl: 523 cubic mm (37% larger than 9mm, 95% larger than 8mm)
    9mm pearl: 382 cubic mm (43% larger than 8mm; 2 times as large as 7mm pearls)
    8mm pearl: 268 cubic mm (49% larger than 7mm)
    7mm pearl: 180 cubic mm (59% larger than 6mm)
    6mm pearl: 113 cubic mm (74% larger than 5mm pearls)
    5mm pearl: 65 cubic mm

    Note: An increase of one millimeter makes more of a difference when the pearls are small than when they are large. (This is because 1mm is a greater per cent of the total size of a small pearl than of a large pearl.)

    You can do this calculation yourself for any pearl size
    (even half-sizes). All you need to know is the diameter.
    Here's how, using a 7mm pearl as an example:

    Volume (of a sphere) = 4/3 π (radius cubed)

    • Divide the diameter in half to get the radius. (The radius of a 7mm pearl is 3.5)
    • Multiply the radius by itself, and again by itself to get the radius cubed. (3.5 x 3.5 x 3.5 = 43, rounding it up)
    • Then multiply the radius cubed by 3.14 (π, or "pi") (43 x 3.14 = 135)
    • Now multiply that result by 4, and then divide by 3. (135 x 4 = 540, then divided by 3 = 180)
    • The final result is the volume of that pearl in cubic millimeters, rounded off. (7mm pearl is approximately 180 cubic mm)

    Alone, the volume is not very useful information, but it is very useful for comparing two pearl sizes so as to understand how much larger or smaller the pearls are.

    To compare two pearl sizes, divide the volume of the larger pearl by that of the smaller pearl.

    Example 1
    : How much larger is an 8mm pearl than a 7mm pearl?

    Divide 268 cubic mm (volume of 8mm pearl) by 180 cubic mm (volume of 7mm pearl) = 1.49

    Thus, 8mm pearls are 1.49 times as large as 7mm pearls, or stated differently, they are 49% larger by volume.
    Another way to state this is that 8mm pearls will look about one and a half times as large as 7mm pearls-- and will weigh one and a half times as much.

    Example 2: How much larger is a 10-12mm strand than a 9-11 mm strand?

    A 12mm pearl is 30% larger than an 11mm pearl, so the center pearls in a 10-12mm strand will be 30% larger.
    A 10mm pearl is 37% larger than a 9mm pearl, so the end pearls in a 10-12mm strand will be 37% larger.
    I averaged 37% and 30% to get 33.5%.

    Therefore a 10-12mm strand will be about a third larger overall (and a third heavier) than a 9-11mm strand.

    A 12-14mm strand will be two-thirds (66%) larger than a 10-12mm strand.
    And so on.

    You can do this to compare any 2 strands, but it is just an estimate when it comes to strands, as some strands have a single, oversized center pearl, or only a few small pearls on the ends. Still it will give you an idea of relative size.

    Going up by just a little can definitely impact appearance and weight.
    If you are buying a rope, it's all the more important to consider the weight.

    Last edited by Pearl Dreams; 04-25-2019, 05:12 AM. Reason: Added information to compare strands

  • #2
    Wow, Pearl Dreams, that is amazing! (both the information and all that math)


    • #3
      Thank goodness for calculators, is all I can say.


      • #4
        Great work! I know often I'll try to explain how big a difference just 1mm makes, seeing the actual volume really explains it perfectly.
        Kevin Canning
        Pearls Of Joy
        Read My Blog
        FaceBook Fan Page


        • #5
          Pearl Dreams, that really says it all! Thank you!!





          • #6
            Wow. You just came up with another sticky, gal! and a 5 star rating and a few reputation points for reporting this one! Here is good. It is kind of crafty info - but sellers could put it on their sites, huh?.

            How to hand-knot pearls without a tool

            My avatar is a Sea of Cortez mabe pearl. One of a pair of Mexican handmade earrings.


            • #7
              My pleasure! It's a powerful tool and has made me look differently at pearl sizes already.

              It also helps one make sense of the price of larger pearls. Just knowing that a 10mm pearl weighs about double what an 8mm pearl weighs explains a lot-- aside from the increased rarity of the larger pearl.

              For oval pearls we'd need to use the formula for an ellipsoid -- but there are so many size and shape variations that it would not be easy to make a standard chart. But if you feel like doing the math, here are the formulas for 2 different kinds of ellipsoid shapes: http://www.web-formulas.com/Math_For...Ellipsoid.aspx

              In simple terms, the flatter (oblate) ellipsoid is the shape of a "plain" M&M candy (Smarties for you British folks)-- rather like a plump coin pearl-- and the longer (prolate) ellipsoid is the shape of a "peanut" M&M-- rather like an oval pearl.
              So for oval pearls use the formula for the prolate ellipsoid: 4/3 a(b cubed) with a being the length and b being the width of the pearl. (Notice we are not using the radius this time, but the full length and width.)

              So here's an example: Volume of an oval pearl that is 10mm long and 8mm wide.
              a is 10 and b is 8.
              b cubed is 512.
              10 x 512 = 5120
              4/3 (5120) = 6827 cubic mm. That is the volume of the pearl.

              If you want to compare that with any other size oval, do the math for the other pearl, then divide the cubic mm of the larger by that of the smaller.
              The result will tell you how many times the larger pearl is larger than the smaller pearl. (e.g. A result of 2 means the larger pearl is twice as large; 1.5 is one and a half times as large. etc.)
              Last edited by Pearl Dreams; 12-19-2013, 08:14 PM. Reason: further explanation of formula of ovals. Now I feel like buying M&Ms!


              • #8
                Girlfriend -- you are a total pearl geek!


                • #9
                  Thanks, GemGeek!

                  If only the pearl size photos on vendor websites could be 3-D-- a flat image of a spherical object doesn't show the real size.


                  • #10
                    Well this is great stuff ! Thanks so much for posting the information...really interesting. And it's a great way of better explaining the difference in prices. Well done !!


                    • #11
                      Makes us look at pearl sizes in a whole new way and explains a lot. You are great at maths Pearl Dreams. Thank you for all that work.

                      Dawn - Bodecia
                      eBay Seller ID dawncee333 Natural pearl collector & seller. And all round pearl lover.


                      • #12
                        Thank you so much for this chart Pearl Dreams! Thanks to you I ended up ordering my freshadama strand in the next size larger (9-9.5mm). I wouldn't have thought the size would have been that much larger, but according to your calculations IMO there is enough of a difference to justify the extra expense. And I just like big pearls

                        I hope you don't mind my adding a few visuals???

                        Click image for larger version

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                        8.5-9mm freshadama left, 7-8mm metallic freshwater middle, 5.5-6mm AAA akoya rope right

                        Click image for larger version

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                        Earrings from left: 11.8 mm wSS studs, 13mm wSS baroques, and 8.6mm natural white handama akoya studs
                        Necklaces from top to bottom: 8.5-9mm freshadama, 7-8mm metallic freshwater, 5.5-6mm AAA akoya rope

                        The pearls are all from Pearl Paradise


                        • #13
                          So you went up half a size from 8.5-9mm to 9-9.5mm?

                          Then the strand as a whole will be between 17.5% (at the center) and 19% (at the ends) larger in volume and weight. I'd say that's a significant difference!

                          The math: I calculated the volume of the end pearls of the two strands, and the volume of the center pearls of the two strands, and then divided the volume of the larger strand's pearls by those of the smaller strand's pearls.
                          center pearl 9.5mm/9mm, volumes are 449/382 = 1.175 or 17.5% larger at the center (18% rounded up).
                          end pearls 9mm/8.5mm, volumes are 382/321 = 1.19 or 19% larger at the ends.

                          Obviously the difference will be greater if increasing by a whole size up, but 17.5-19% is still quite a nice increase.
                          Last edited by Pearl Dreams; 12-19-2013, 03:29 PM.


                          • #14
                            Originally posted by Pearl Dreams View Post
                            So you went up half a size from 8.5-9mm to 9-9.5mm?

                            Then the strand as a whole will be between 18% (at the center) and 19% (at the ends) larger in volume and weight. I'd say that's a significant difference!
                            Yes, 18/19% is definitely significant Any excuse to buy bigger pearls right? Lol!


                            • #15
                              I added my math for what it's worth. Thanks for the photos!

                              You know, this volume thing cuts both ways. There are petite people who worry that a particular pearl size will be too large. In the smaller sizes, a 1mm difference in diameter makes for a huge difference in volume. This may help them not buy larger than they want.