If you're eyeing an Italian size 38, you'll typically need a U.S. size 4, which corresponds to an "extra small." For Italian size 40, you'll look for a U.S. size 6, commonly considered "small."

If you're wondering what size 28 corresponds to in American measurements for women's jeans, subtracting 15-14 sizes from the Italian size (which is 42) will give you the answer. Note that this trick applies specifically to jeans, as they follow a different scale than pants.