"Carat" of a diamond is often confused with "Karat" as a measure of gold purity. "Carat" does not measure size, instead, it is a measure of the physical weight of a diamond. The formula for calculation therefore is Total Price = Weight x Price per Carat
As you might expect, larger diamonds often cost more per carat due to the rarity of available large size raw diamonds. What determines diamond carat?
In Diamonds, there are 100 points to each carat, so a 50 point diamond is 1/2 a carat. When selecting a diamond it is important to get the actual point size of the diamond rather than a fractional weight. "Discount" or Internet Jewelers have been known to sell a .90 diamond as a 1 carat diamond for example. Always insist on point values.
The personal style, size of your finger, as well as the size and style of the setting is more important than just overall diamond size alone.
It is important to note that the smaller the finger, the larger the diamond will appear. A 1.5 carat diamond looks much larger when the recipient is a size 4 than a size 8 for example.
If you have already chosen a setting it is extremely important that you choose a diamond that is appropriate for your ring.