Beads per Inch Bead Size Beads per Inch Beads per 16 Beads per 24 7 mm 3.6 57 86 8 mm 3.13 50 75 10 mm 2.5 40 60 12 mm 2.1 33 50 5 more rows …
If you stack two pennies together, you can estimate how big a 3 mm sized bead is or use a stack of four to estimate the size of a 6 mm bead. If you are out of pennies, nickels are another good reference. Nickels, at 1.95 mm thick, are just shy of 2 mm.
Remember there are two scales: the mm scale for standard beads, and the Aught size for seed beads. For standard beads, the number denoted in mm is the bead size (also diameter or the through hole length). For seed beads, a larger number is a smaller bead.