Depending on the quality of the cut, a diamond that measures 6.2 mm at the girdle may weigh about .85 carats.
A two carat round brilliant cut stone will measure about 8.2 mm. A different cut will have a different measurement.
A 1.00 carat round brilliant, when properly cut, should measure approximately 6.5 mm in diameter.
In terms of a round brilliant diamond, a properly cut stone should measure approximately 5.2 mm in diameter.
A carat diamonds size is 3.5mm.
The diamond weighs one carat. Its other dimensions depend on its cut: round, princess, emerald and so forth. A carat is 0.2 grams; a pure diamond has a density of 3.5 gm cm-3 which translates to a volume of 0.057 cm3 per carat. As stated in the first answer the linear dimensions depend on the shape that is cut.
A round cut diamond weighing 0.07 carats measures 2.7 mm at the girdle. If the diamond is of a different cut, the measurement given will roughly equate to a different carat weight.
If the stone is a round diamond, the stone measures about 10.6mm at the girdle. Remember carat is an indicator of weight, and millimeters is an indicator of length: the two are only loosely related in this case.
If a diamond is a round cut, the mm size at the girdle is about seven if the diamond weighs about 1.25 carats. Other gemstones have different weights relative to their mm measurements.
The designation .30 carats indicates that the diamond weighs 30/100ths of a carat. The mm size will depend on its cut.
A common misconception. Carat is nothing to do with size. Carat is related to weight and therefore a 1 carat stone depends on the gemstone in question and its cut. 1 carat is 0.2g and a 1 carat round-cut diamond is approximately 6.5mm in diameter. Similarly, since CZ is a diamond simulant, a 1 carat round-cut CZ is also deemed to be one that is 6.5mm in diameter but such a stone actually weighs more than 0.2g because CZ is more dense than diamond.
Carats refers to weight, not size. You should ask for measurements. Two round diamonds of equal carat weight are not necessarily the same size. Also the shape is a factor. A 1.80 ct pear shape diamond can look much bigger than a 1.80 ct round cut diamond.
The carat size of a diamond should be determined by a certified jeweler. There are two different sizes the is the millimeter size which as stated was 7mm and then a jeweler can give a carat size.