XS 24-26, S 28-30, M 32-34, L 36-38, XL 40-42, XXL 44-46, XXXL 48-50, 4XL 52-56  
For reference, in most product photos you see here on my website I am wearing a Medium and my hips measure 34 inches. 
Note: It's really best to measure your hips where the top of the swimsuit will fit on you- instead of going for whichever pant size you normally buy.
If you are in between sizes and not sure which size to go with, I'd say go with the smaller size.
If there is a size you need that you don't see on my website please send me an email-