Seed bead sizing has always puzzled me. To me it’s counter intuitive to have an inverted system of numbering – the larger the number the smaller the bead. Logically a size 20 bead sounds larger than a size 5 bead – but of course, when it comes to seed beads it’s not. The tiniest seed bead is size 25 (25/0) and the largest size 5 (5/0). How did that come to be? I heard recently that the size number refers to number of beads that will fit side by side in an inch. That made sense and I thought, that's good a way to picture what size beads I need. However, as with many things in life it’s not that simple. It’s all to do with ‘aughts’.
What is an aught you might well ask? An aught is a unit of measurement for measuring seed bead sizes. It is denoted by the symbol a backslash with a zero (/0). Hence,
when you see a seed bead referred to as size 11/0 it means the bead is eleven aughts in size. The symbol makes some sense if you know that the word aught derived from the old English word naught which means zero. However, visualising the size of an ‘aught’ is somewhat harder and makes very little sense. An aught is a very old-fashioned measurement that refers to the number of beads lined up side by side that will fit in a given area. An 8 aught (8/0) seed bead means that you will get 8 of those beads in the space. An 11 (11/0) aught bead means that eleven beads will fit in the same amount of space. Hence, the inverted logic - the smaller the number the bigger the bead, as it takes fewer beads to fill the space. Still puzzled? Well there is a very handy chart on how aughts relate to more modern measurements such as inches and millimetres on several websites. I've put my own version in this post to help visualise an aught. The numbers per mm or inch may vary slightly according to manufacturers but it's a general guide. So, next time you hear that the number in seed bead sizes refers to the number of beads in an an inch - you aught to reply that its the number of beads in an aught!
If you want some other handy tools for sizing beady things Snazzy Cat has some great free downloads.
Sources • http://www.beadinglife.com/bead-sizes.html • http://www.unc.edu/~rowlett/units/dictA.html