The standard measure for diamond weight is the "carat".

A carat is 200 milligrams and is divided into 100 points
(for example, a 25 point diamond is 1/4 carat and would weigh 50 milligrams).

Carat weight is the easiest of the four C's to determine, however the diamond MUST be loose.
Too many jewelers approximate carat weight, for instance, when you buy a 1.00 carat diamond,
which you assume to be 100 points, you may actually be getting less than that. To save yourself the headache of having to guess by only considering certified diamonds.

Many stores sell as low as 90 points, and present the stone as "approximately" a 1.00 carat.
Although the most obvious characteristic, carat weight is the least important in determining
the brilliance and overall beauty of a diamond.

Also important to note is that a heavier diamond with more weight may not in fact be larger than another diamond with a slightly lower carat weight. The size or dimensions of the diamond are more of a function of the proportions of the diamond.

Using Round Brilliant cut diamonds as our example, a diamond with a Depth of 62.0% will have less surface area than a diamond with the exact same carat weight but with a Depth of 60.0% given that all other proportions are identical.

Finally, Carat Weight is an important factor in determining value. The larger the stone, the more rare the stone and consequently the more valuable the diamond.

Additionally, there are price breaks on the Rap sheet (the wholesale diamond pricing sheet) at major carat weight marks. A 1.00 carat diamond will not cost the same per carat as a 1.50 carat diamond with the same characteristics. Excellent values may be found at carat weights just under these break points.