Carat weight is one of the four Cs – color, clarity, carat and cut — to look for when buying a diamond. Carat weight tells you how big the diamond size is by letting you know the weight of the gem.

Diamonds are measured in carats, with one carat equaling 200 milligrams of actual weight. Carats is often abbreviated with a ct. The abbreviation ct TW denotes carat total weight, which means the total weight of multiple diamonds in one piece of jewelry. So a necklace with a 1 ct diamond and two .25 ct diamonds has a ct TW of 1.5, for instance.

Size Guidelines

The carat size of the diamond or diamonds you buy will be determined by your budget, the cut of the diamond, personal preference and your size. If you are buying a ring for someone with small fingers, for instance, a diamond that is 1 ct or more may look quite large. The same ring on someone with thick fingers may appear average or even small, however. The cut of the diamond also determines how the size of the diamond looks, with oblong diamonds often appearing larger than square or circle cuts.

Here are some basic guidelines on carat weight:

For round diamonds, 0.10 carat is equal to about 3.0 millimeters in diameter. A 1 ct round diamond is about 6.55 mm in diameter, a 2 ct is about 8.2 mm in diameter, and 5 ct is about 11 mm in diameter.

For princess cut or square diamonds, .25 ct is about 3.4 x 3.4 mm, 1 ct is about 5.5 x 5.5 mm, 2 ct is 7.5 x 7.5 mm and 5 ct is 9.5 x 9.5 mm.

For oval diamonds, .25 ct is about 5.0 x 3.0 mm, 1 ct is 7.5 x 5.5 mm, 2 ct is 9.5 x 7.5 mm and 5 ct is 12.0 x 10.0 mm.