Carat Weight Calculator
Enter your gemstone measurements in millimetres, choose the cut and species, and get the estimated carat weight instantly. The calculator uses the industry-standard formula accepted by jewellers and gemologists: length x width x depth x specific gravity x shape factor. Results are given in carats, grams and troy ounces, and a full worked example shows every step.
How gemstone carat weight is calculated from dimensions
Jewellers and gemologists estimate carat weight from millimetre measurements using a simple volumetric formula. The core idea is that the weight of a stone equals its approximate volume multiplied by its density. Volume is estimated as the product of length, width and depth scaled by a shape factor (SF) that corrects for the material removed during cutting. The formula for coloured gemstones is: weight (ct) = L x W x D x SG x SF, where L, W and D are in millimetres, SG is the specific gravity of the mineral species, and SF is a cut-specific coefficient. For diamonds, the SG is already incorporated into the published SF values, so the formula simplifies to: weight (ct) = L x W x D x SF. Shape factors range from about 0.0016 for a marquise to 0.0085 for a princess cut, reflecting how much of the bounding-box volume each style of cut actually uses.
Specific gravity and why it matters
Specific gravity (SG) is the ratio of a mineral's density to the density of water. It determines how heavy a given volume of stone feels. Diamond has an SG of 3.52, meaning a cubic centimetre of diamond weighs 3.52 grams. Ruby and sapphire (both corundum) have an SG of about 4.00, so they are denser than diamond and a ruby the same size as a diamond will weigh more carats. Conversely, opal has an SG of only 2.25, so an opal of the same diameter weighs noticeably less than a diamond of identical dimensions. This is why you cannot size-match gemstones of different species by diameter alone and expect equal carat weights. The table below lists specific gravity values for 17 commonly traded species used in this calculator.
Accuracy and limitations of dimensional estimation
Dimensional estimation is a trade tool for mounted or inaccessible stones where placing the gem on a scale is impractical. Typical accuracy is within about 5 to 10 percent for well-cut stones of known species. Error increases when the stone deviates from standard proportions, for example an unusually deep or shallow pavilion, an abnormally thick girdle, or a modified cut. For certified diamonds, the GIA grading report lists the exact weight measured on a calibrated scale and should always be used in preference to a calculated estimate. For unmounted stones, a direct weighing on a 0.001-carat precision scale is more reliable than any formula.
Carat as a unit of weight
The metric carat was standardised internationally in 1907 and equals exactly 0.200 grams or 200 milligrams. The word derives from the carob seed (Greek keration), which was historically used as a counterweight in scales because of its relatively consistent size. One troy ounce equals approximately 141.75 carats. Carat should not be confused with karat (sometimes spelled carat in British English), which measures the fineness of gold alloys on a scale of 0 to 24. In gemology, "carat" always refers to weight.
Round brilliant diamond: diameter vs carat weight reference
| Diameter (mm) | Approx. weight (ct) | Common size label |
|---|---|---|
| 3.00 | 0.10 | Melee |
| 3.40 | 0.15 | Melee |
| 3.70 | 0.20 | Melee |
| 4.00 | 0.25 | 1/4 ct |
| 4.10 | 0.28 | |
| 4.40 | 0.33 | |
| 4.50 | 0.37 | |
| 4.70 | 0.40 | |
| 4.80 | 0.44 | |
| 5.00 | 0.50 | 1/2 ct |
| 5.20 | 0.55 | |
| 5.50 | 0.66 | |
| 5.70 | 0.75 | 3/4 ct |
| 5.80 | 0.80 | |
| 6.00 | 0.90 | |
| 6.20 | 1.00 | 1 ct |
| 6.50 | 1.10 | |
| 6.80 | 1.25 | |
| 7.00 | 1.40 | |
| 7.40 | 1.50 | 1.5 ct |
| 7.80 | 1.75 | |
| 8.00 | 2.00 | 2 ct |
| 8.20 | 2.15 | |
| 8.50 | 2.40 | |
| 8.80 | 2.65 | |
| 9.00 | 2.85 | |
| 9.10 | 3.00 | 3 ct |
| 9.40 | 3.25 | |
| 9.75 | 3.50 | |
| 10.00 | 3.87 |
Approximate carat weights for ideal-cut round brilliant diamonds at standard depth ratios. Actual weight varies with exact proportions.
Frequently asked questions
How accurate is the carat weight formula?
For a well-cut stone with standard proportions, the formula is typically accurate to within 5 to 10 percent. Accuracy decreases for unusual proportions such as a very thick girdle, an extra-deep or extra-shallow pavilion, or a non-standard modified cut. For certified diamonds, always use the weight listed on the GIA or AGS grading report. For unmounted coloured stones, a direct weighing on a precision gem scale is more reliable.
Why does depth percentage matter for a round brilliant diamond?
Depth percentage is the depth divided by the average diameter, expressed as a percentage. A well-cut round brilliant typically has a depth of 59 to 63 percent of the diameter. A stone that is too shallow (under about 57 percent) loses light through the pavilion and can look glassy or "fisheye." A stone that is too deep (over about 65 percent) traps light in the base, making the stone appear smaller face-up than its carat weight would suggest. Depth percentage does not affect the calculated weight, but it is a key indicator of cut quality.
Can I use this calculator for rubies, sapphires and emeralds?
Yes. Select the appropriate species from the gemstone list and the relevant cut. The calculator uses the correct specific gravity for each mineral - 4.00 for ruby and sapphire, 2.72 for emerald. The same formula applies across all coloured gemstones; only the SG and shape factor change.
What is the difference between carat and karat?
Carat (ct) is a unit of weight for gemstones: 1 ct = 0.200 g. Karat (kt or k) measures the purity of gold alloys: 24 kt is pure gold, 18 kt is 75 percent gold. The words share the same etymological root but are completely separate units used for different purposes. In gemology and this calculator, "carat" always means weight.
How do I measure a mounted stone?
For a stone set in a ring or pendant, measure the face dimensions (length and width) with a millimetre gauge or digital caliper across the top of the stone. Estimating depth is harder because the setting obscures the pavilion. As a rough guide, assume depth is about 60 to 63 percent of the diameter for a round brilliant, or use the girdle-to-table visible portion and add an estimate for the hidden pavilion. Because depth is often an estimate for mounted stones, treat the resulting carat weight as approximate.