Carat is often confused with size even though it is actually a measure of weight. The cut of a diamond can make it appear much larger or smaller than its actual weight. One carat weighs 200 milligrams. As the relative weight of a diamond increases, so does its per carat cost. It is for this reason that a 1-carat (ct) diamond may cost 5 times more than a half-carat diamond. What carat diamond should you buy? It is usually a matter of personal taste. For engagement rings, many people seek diamonds that are approximately 1ct in weight. Obviously budget plays a large part in the decision. See also the discussion about budget.