Diamond weight is measured in carats and points. When determining the value of a diamond, it is useful to know how to calculate diamond weight because, along with color, clarity and cut, a diamond's weight is a factor in determining its value. Diamond weight can be calculated precisely with a jeweler's scale. If you don't have a jeweler's scale, or if you don't want to remove your diamond from its mounting, you can still estimate its weight fairly accurately.
Learn your measurements and conversions. Diamond weight is measured in carats and points. One carat equals 200 milligrams. One point is one hundredth (1/00) of a carat. Smaller diamond weights are often expressed in terms of points. Diamonds larger than 25 points (1/4 carat) are generally expressed in terms of fractions of a carat.
Weigh the diamond. If you have a jeweler's scale, you may have the option of weighing the diamond in carats. If so, your task is easy. The scale will display the diamond's weight.
Weigh the diamond in milligrams if your scale does not measure in carats. Divide the result by 200. You now have your diamond's weight in carats.
Express the diamond's weight in fractions, if desired. A weight of .25 carats is equal to 1/4 carat, .33 is equal to 1/3 carat, .50 is equal to 1/2 carat, and .75 is equal to 3/4 carat. Most diamond weights will not equal an exact fraction, so note whether your diamond is slightly more or less than the stated fraction.
Calculate the weight of a diamond by measuring its diameter if you can't weigh your diamond. Use this method if you don't have an accurate scale or if you don't want to remove the diamond from its mount. While this method is not as precise as weighing a loose diamond, it still calculates diamond weight fairly accurately because the density of diamonds is known.
Measure your diamond's diameter (the distance from one point on the outside of the diamond to the opposite point on the outside of the diamond). If your diamond is oval or princess cut, measure the longer and shorter diameters, add them together and divide the answer by 2. The most accurate way to measure a diamond's dimensions is with a metric caliper. If you don't have a metric caliper, use a metric ruler.
Divide the diameter of the diamond by 6.5 (the approximate diameter of a 1-carat diamond) to obtain an estimated weight in carats. To make things easier for yourself, let a gemstone weight calculator like the one on csgnetwork.com do the math for you.
Express the diamond's weight in fractions, just as you would for a diamond that has been weighed.
Don't confuse carats, a measurement for gemstones, with karats, a measurement for gold. When using a jeweler's scale, make sure you are weighing diamonds in carats, not karats.
The estimation of diamond weight using diameter and density is not exact because of the differences in diamond cuts.
The densities of other gemstones differ from the density of diamonds. To use the estimation method for other gemstones, you will need to know their density.