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Momme (Japanese)
Equivalent round pearl size (mm)
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  • See the Glossary for an insight on each unit of measurement.
  • For a pearl each millimeter matters! Read an article below to understand why 🙂

Real Reason why each 1mm is so important for a Sea Pearl

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

We’re often asked for advice when customers are trying to determine what size of pearl is best suited for them. The question is typically whether or not it is worth it to go up a size or two in millimeters. Is the difference really that noticeable?

The image below is of a 6 mm akoya pearl next to a 12 mm South Sea pearl. The millimeter size is exactly double, but it’s easy to see that the true difference in size is much greater.

As a rule of thumb, I’ve always told customers that a 2 mm incremental increase in millimeter size would approximately double the overall size of the pearl. On the surface this tends to be confusing. How can an 8 mm pearl be twice the size of a 6 mm pearl?

The trick is that pearls are 3D odjects (not just flat circles on a page as they are pictured). Because of that their volume is all about the 3D size. It is about how much space pearls take up and how large their look. Their weight and visual impact.

Knowing the volume of a given pearl size lets you compare one size with another. It can tell how larger and heavier any pearl will be compared to another pearl. We could 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).

Lets look into some examples. Take them as a guide to help you estimate how large a given size pearl is compared to another.

  • 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 percent of the total size of a small pearl than of a large one 🙂

Anyone can do such calculations for any pearl size. All we need is to found out a diameter. Lets do such magic trick to a 7mm pearl:

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

  1. Divide the diameter in half to get the radius (the radius of a 7mm pearl is 3.5)
  2. 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)
  3. Then multiply the radius cubed by 3.14 (π, or “pi”) (43 x 3.14 = 135)
  4. Now multiply that result by 4, and then divide by 3 (135 x 4 = 540, then divided by 3 = 180)
  5. The final result is the volume of that pearl in cubic millimeters, rounded off (7mm pearl is approximately 180 cubic mm)

The volume alone is not much useful, but it is important for comparing two pearls by their sizes so as to understand how larger or smaller they are.

To compare two pearl sizes, divide the volume of the larger pearl by that of the smaller one. Lets deal with two tasks:


Task number One: lets found out how much 8mm pearl is larger than a 7mm pearl?

We should start by dividing 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 1/2 times as much.


Task number Two: How much a 10-12mm strand is larger than a 9-11 mm strand?

A 12mm single pearl is 30% larger than a 11mm pearl, so the central pearls in a 10-12mm strand will be 30% larger.

A 10mm pearl is 37% larger than a 9mm pearl, so the smallest pearls in a 10-12mm strand will be 37% larger.
In averaged we got 37% and 30%, and an averag of that is 33.5%.

Soo we see, that 10-12mm strand will be about a one third time larger (and a third heavier) than a 9-11mm strand overall.

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

You can do similar calculations to compare any 2 strands, but it is just an estimate when it comes to strands, as some strands have a single, oversized central pearl, or only a few small pearls close to a clasp. Still my method can give you an idea of a relative size for any product you want to compare with each other.

Going up by just a little in terms of millimeters can definitely impact appearance and weight your pearl jewelry. If you are buying a rope, it’s all the more important to consider the weight.