A carat is a measure of weight used when describing a diamond, though it is not true measure of size, due to the various shapes and volumes provided by the diamond's cut. One carat however, is equal to 200 milligrams or 0.2 grams. One carat equals 100 points. A 3/4 carat diamond is 75 points.
Carat weight effects the value of a diamond exponentially. A 2 carat diamond is not twice as valuable as a 1 carat diamond, but more valuable. The reason is that a 2 carat diamond is much more rare than a 1 carat diamond, so it carries a price premium above and beyond its weight alone.
The value of a diamond is not based solely on carat weight. Cut, color, and clarity have just as much influence on the overall value of a diamond as its carat weight. The bottom line is that bigger is not always better.
When choosing your diamond, determine your preference for each one of the 4C's. Doing so will ensure you get the right diamond for you at the right price for you. For fancy shape diamonds (not round) such as ovals, cushions, radiants and emerald cut, the ratio of length to width also affects the look and value. There are desirable length to width ratios, but also personal preferences.