The unit of weight employed for diamonds and other gems is the metric carat. One carat equals 0.2grams or 200 milligrams. Diamond weight is given to two decimal places in carats or as a number of points. For example, 100 points equals 1.00ct and 1 point equals 0.01ct. Diamonds are measured by carat weight, and not size. Two diamonds having the same carat weight could have different millimeter sizes because a diamond is three dimensional meaning length, width, and depth.

 

As diamond carat size increases, both the diameter and the depth of the diamond increase. This is why a 1.00ct diamond (approximate 6.5mm diameter) does not look twice as wide as a 0.50ct diamond (approximate 5mm diameter).

The scale below is a comparison of the sizes based upon carat weight with cutting percentages being the same.