A man's shoe size is converted to a woman's shoe size by adding 1.5 to the men's size. To convert a man's pants size to the size a woman would wear, subtract 21 from the man's size.
For example, if a man wears size seven shoes but wants to know what the women's shoe size for his feet would be, he would add 1.5 to seven to learn that the women's size for his feet would be 8.5. Likewise, a size 10 for a man would be size 11.5 for a woman, and a size 10.5 for a man would be size 12 for a woman.
Though one who wants to convert the size of the pants worn by a man to the correct size in women's clothing simply needs to subtract 21 from his pants size, this doesn't take into account things like how the pants fit on the hips and how they accentuate a figure.
For example, a man who wears size 34 jeans would need to subtract 21 from that size to learn that the size in women's clothing would be size 13. Conversely, a woman who wears a size eight would add 21 to learn that she would wear a size 29 in men's jeans. However, men and women might not be happy with how tight or loose the pants are on different parts of the body.