What do the numerical sizes for seed beads mean? Basically, sizes used to refer to how many beads fit into a certain space. Now that different manufacturers in different countries produce beads, this measurement is no longer valid but the numerical bead sizing system remains. Essentially, the higher the number, the smaller the bead. (I always say “The older I get, the thinner I get” to remember. Its not true – but it does jog my memory!) As an example, size 13 seed beads are much smaller than size 10 seed beads and may require different thread and needle choices. The Japanese started creating seed beads (Toho and Miyuki are the brand names) that were more uniform and slightly larger than the corresponding Czech seed beads, but the number system is the same.
Other beads are sized by their millimeter measurement. Patterns will refer to 4mm, 6mm, etc. beads used and it means just what it sounds like. 6mm beads are larger than 4mm beads, measured around the widest part of the bead. Even Japanese beads other than seed beads (i.e. cubes, triangles, drops) are measured by millimeter.