Size reduction Solids and powder milling
One penny is millimeters thick. This gives you a point of reference for bead size. 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 mm thick, are just shy of 2 mm.