as Centcomm01 says, the general rule of thumb is to add 2 to the mens size to get the womens (assuming you are talking about normal US sizes). However, they do vary in actual size much like mens shoes.
For example, my foot is a bit under 11" long and 4" wide, this would place me as a about 12 in womens, and a about a 10 in mens, by most size charts (BTW the chart on the Payless site is pretty easy to follow), however, as online shoe buying has taught me not everything is sized even close to "correct" so sometime I can wear an 11 in womens and on at least one brand of mens sized converse rip offs, I wear an 11 too, which is crazy when you think on it.
The only true way to know the sizes is to try them on, however, I prefer shopping online, I find that brand tend to be more or less true to themselves sizewise, so I know if this brand runs big, I know that most of their shoes usually do.
