A 1.5 carat diamond (round cut) will, on average, have a diameter of 7.3mm. But as carat actually refers to weight, not size, this may be different from diamond to diamond. Each carat is equal to 0.2 grams, so a 1.50 carat diamond will weigh 0.3g. The table size - the area visible when you view the diamond from the top - can vary depending on each diamond’s cut and shape.