How do you calculate men's sizes?

In general, men's sizing (in the U.S.) is based on inch measurements of certain body parts. For instance, a man whose chest measures 45 inches would wear a size 45 in a suit jacket; a man whose waist measures 32 inches would wear a size 32 in jeans, etc. an inseam measurement on pants corresponds to the inches from a man's crotch to his ankle; etc. Different apparel manufacturers differ in their sizing, however, so there is no one standard that you can say always works.

(Men's shoes are not based on inches but on arbitrary numbers for incremental sizes; most men are size 9.)