Other than actually trying a belt on there is only one sure way to get a correct belt size:

MEASURE A BELT YOU ALREADY HAVE

Measure from the end of the buckle (where the pin hits the buckle) to the hole that you use

There are two rules-of-thumb regarding belt sizes:

1) A MAN'S BELT SIZE IS 2" LARGER* THAN HIS PANT SIZE
2) A WOMAN'S BELT SIZE IS 4-6" LARGER* THAN HER PANT SIZE
* pant sizing varies : these are general rules and are not set in stone...
ALDO ROSSI belt size is measured from the end of the buckle to the middle hole (of five or seven holes). There will always be 2 or 3 inches of holes on either side of the indicated size.