How to Calculate Diamond CT Measurements

In the four "C's" of diamond measurements --- clarity, cut, color and carats --- it is the carat, or weight measurement, that determines how big that rock is or, more precisely, how much it weighs. The carat measurement, which is also sometimes referred to as the CT measurement, is the largest factor in determining the overall value --- and hence cost --- of that engagement ring you've had your eye on. The term comes from the long-ago practice of weighing diamonds against the number of carob seeds needed to balance it on a scale.

Instructions

1

Weigh the diamond on a scale. A one-carat diamond weighs 200 milligrams. However, diamonds smaller than one carat are measured in "points" with one carat equal to 100 points. So 0.5 carats could also be measured as 50 points.

2

Measure the diamond's diameter. A one-carat round diamond usually -- although not always -- measures 6.5 millimeters in diameter. A half-carat diamond usually measures about 5 millimeters and a quarter-carat diamond typically measures about 4 millimeters in diameter.

3

Take the diamond to a jeweler for a more precise measurement. The jeweler will also be able to give you a better idea of the diamond's value, because weight is only one factor in the value of a diamond. While larger diamonds tend to be more expensive, a 2-carat diamond with flaws or imperfect color will be worth less than a 1.5-carat diamond that is flawless and perfectly clear.