Carat (weight): Of the Four C's, carat weight is the easiest to measure and understand. The carat is the standard unit of measurement used to indicate the weight of diamonds and precious gemstones. One carat equals one-fifth of one gram and is divided into 100 points. So when you discuss a 3/4 carat stone, you could say it weighs .75 carat, or 75 points.
Carat weight may seem the most obvious factor in determining the value of a diamond, but two diamonds of equal carat weights can have very different prices after taking into consideration the Cut, Colour and Clarity factors. Generally speaking, all other things being equal, bigger diamonds cost more. The key phrase is "all other things being equal." It is very common to see a ½ carat diamond cost more than a one carat diamond of lesser colour and/or clarity and/or cut.
The scale below depicts the carat weights and diameter sizes of Excellent Cut round diamonds. Other diamond shapes will vary.