Diamond weight is defined by a metric unit of measurement called the carat. The carat being equal to 0.2 grams is a fairly small unit of measurement and is measured most accurately by very sensitive scales where even slight air movements can interfere with measurement.
In instances where diamonds are mounted and an estimation of weight must be made, formulas have been developed which can mathematically estimate a diamond's weight based on general proportions of particular diamond cuts, relative to a diamonds specific gravity.
The most common and well defined estimator is for round brilliant cut diamonds that are generally cut to very specific proportions. In estimating a round brilliant diamonds weight the largest and smallest diameter of the diamond are averaged, squared and multiplied by the depth of the diamond, and then by .0061 (a defined variable for estimation of round brilliant diamonds) which will equal an estimated weight for the particular round brilliant diamond.
So for example if your round brilliant diamond has measurements of 7.54 x 7.56 x 4.58 millimeters, the diamonds diameter is averaged, in this case to 7.55, squared resulting in 57.003, multiplied by the depth, resulting in 261.07 and then by the estimation variable .0061 giving an estimated weight of 1.59 carats.
Of course there are additional variables and adjustment factors that can play into the diamonds weight being higher or lower than the estimate, but as a general rule, most estimates will be in a plus or minus 5% range of weight.
For more information or to learn how to estimate your diamonds weight, call our team of graduate gemologists at Meridian Diamond at 1-866-966-9454.