So I attempted to measure the beads and I am pretty sure I'm doing something wrong because the measurements seem to not fit quite right. So I pulled out my bead books and tried to do a comparison of the bead and pictures and the best I was able to gather is that it's too big to be 15mm and a tad bit to small to be a 20mm bead so it's somewhere in between. I'm sorry for not knowing the correct way to measure this type of bead but I hope I was able to give you a better idea as to the size.