Go size smaller!!  I wear 8 to 81/2 or small 9.  I need a 38 in women's Birkenstock.  Love these shoes!  They are helping my back and legs and hips and cute.  Toe post taking some getting used to as it is very thick.  This is my first try with Birkenstock.  I got carried away and ordered 3 pair(2 styles) which are all too big because I ordered the 39(which shows it SHOULD be right).  I am urging my daughter with bigger feet to wear them as I can't return them after wearing(please don't you do it either.. GROSS)  I am waiting on my correct size replacements to arrive from Amazon next week.