Diamonds are sold by weight (carat), not size. A five-carat diamond weighs one gram. But it’s the cut and quality of the diamond that will determine the actual and apparent size, as well as its brightness. As a reference, though, a five-carat brilliant round cut diamond (the most popular diamond shape) will have a diameter of approximately 11mm, covering almost 70% of the average woman’s finger space. To put that in perspective, the average engagement ring size is 6.5mm for a one-carat brilliant round cut.