Seed beads are sized in aughts, which are different than millimeters. The most popular school of thought of the sizing of seed beads states that the size (6/0) refers to the number of seed beads per inch (6 beads per inch). Another sizing theory is that the size is based on the rod used to make the beads. The larger the number, the smaller the bead (20 is small, 3 is large).
Seed Bead Sizes
|Aught Size||mm diameter||Beads per Inch|