The problem is that female bodies are proportioned differently than male bodies. And there is a lot of individual variation, so there is no way to tell what your hip size is from your waist size. Your best bet is to actually measure your hip size.
Sizing of women's clothing has a very large random component to it. The only sure way to see if a garment will fit is to try it on. I usually take a size 10, but sometimes I will fit an 8 or a 12, depending on the individual garment.