Millimeter size charts can be found on BeadsGaloreInternational.com, SomethingGlassy.com and BergerBeads.com. Each site includes inches to millimeters conversion charts with reference images. All charts are online only.
Beads Galore and Something Glassy show one set of bead-sized images along with a chart. Beads Galore shows images of a credit card and a quarter for comparison. Berger Beads offers charts with size images for sequins, beads, stones and pearls.
Beads Galore also includes a formula to calculate the estimated number of beads per strand needed. Something Glassy and Berger Beads include beads per inch charts, with separate charts for seed beads.