Callicles, you asked a bigger question that you thought. Basically, caliber is measured only by diameter and you'll need a set of calipers to get an average measurement. As Pete points out, there are heavy and light bullets found in every caliber. I often use the weight to tell me if there might be air bubbles or other flaws in a bullet; or to illustrate how an especially deep cavity affects the weight; or as evidence toward a "solid" looking bullet being hollow (as in being possibly an incendiary or explosive round).
Joevann, sometimes weight does indeed have to do with caliber. Some bullets, most notably for the Tennessee rifle, were ordered "by the pound" ... such as 40, 50, or 60 bullets to the pound.
Callicles, measure the bullets to get the caliber.