Spent this morning out shopping for a skirt for my Mistress. Only found one that was denim, which is what she wanted. Had to go to the dressing room and try it on till I found the size that fit me and then send her a picture before I went back to the rack and got her size. I hope I guessed right or my but will probably be very sore. Sad part is I really wish I was getting it for myself.
I don't know any conversion, I just had to try enough stuff on to know my sizes. Of coarse no two items are alike, so you really have to try almost everything. If it helps I have a 28 inch waist and 31 hips and wear a 5/6 in misses, and a 3/4 in womens.
Women's sizes aren't based off of real life measurements the way that men's sizes are... women's sizes are essentially completely made up. There is no industry standardization, there are no agreed upon size parameters and they have varied CONSIDERABLY over time (they are getting smaller and smaller). However, in the modern internet age, most women's clothing manufacturers POST their sizing charts with comparisons to inches or centimenters and a sizing guide.