Gemstone mm to Carat Calculator
Quick answer
Multiply the stone's size by its depth, its density (specific gravity) and a shape factor. For example, a 6.5 mm round diamond that is 4.0 mm deep is about 1.03 ct (likely 0.98 to 1.12 ct), and an 8 x 6 x 4 mm oval sapphire is about 1.54 ct.
Estimates from measurements, not weighed results. Data verified .
Estimate carat weight from millimetres
What this calculator does
Carat is a unit of weight: one carat is 200 milligrams. Millimetres measure size. You cannot convert one into the other with a fixed number, because the weight of a stone depends on three things: how big it is, how dense the material is, and how it is cut. This tool takes your measurements, the material and the shape, and estimates the weight with the same formulas gem dealers and appraisers use when a stone cannot be weighed.
The result is always a range, because a ruler cannot see everything that adds or removes weight. A thick girdle, a bulging pavilion or a high crown can all hide weight that the outline does not show.
How to use it
- Pick the material. Type a few letters to filter the list. If you have a measured specific gravity (SG) from a hydrostatic test or a lab report, enter it under More options.
- Pick the shape that matches the outline face up. Round stones take a diameter, and you can add a second diameter measured at 90 degrees for out-of-round stones.
- Enter length, width and depth. Depth runs from the flat table on top down to the culet or keel at the bottom. Switch to inches if your gauge reads in inches, or enter depth as a percent of width if that is what a listing gives you.
- Read the estimate and the range, then open "Assumptions used" to check the SG and factor. Tick a weight correction only if you can actually see the feature.
Not sure where to put the calipers? Our step by step measuring guide covers every shape.
Three worked examples
A round diamond
A round brilliant measures 6.5 mm across and 4.0 mm deep. Diamonds use a trade constant that already includes diamond's density: 6.5 squared x 4.0 x 0.0061. The result is about 1.03 ct, with a likely range of 0.98 to 1.12 ct. If the girdle looks thick under a loupe, ticking that correction moves the estimate to about 1.09 ct and the top of the range to 1.21 ct.
An oval sapphire
An oval sapphire measures 8 x 6 mm and is 4 mm deep. Colored stones use length x width x depth x SG x a shape factor. With sapphire's SG of 4.00 and the oval factor of 0.0020, the estimate is 1.54 ct, likely 1.43 to 1.73 ct. An alternative published oval factor (0.0021) is folded into that range.
An emerald cut emerald
A 9 x 7 mm emerald cut emerald, 4.9 mm deep, uses the rectangular step cut factor of 0.0025 and emerald's SG of 2.72. That gives about 2.10 ct (likely 1.94 to 2.27 ct). Step cuts often have a bulging pavilion. If yours does, the bulge correction can add up to 18%.
Why the answer is a range
Every estimate here has four sources of uncertainty, and the range adds them up honestly:
- Different published factors. Sources disagree slightly for some shapes, such as marquise and heart. We store each version and use the spread.
- Density varies. A garnet or zircon can vary widely in SG. Diamond barely varies at all. The range uses each material's typical SG span.
- Hidden proportions. We add a fixed allowance (4% for diamond formulas, 6% for colored stones, 10% for cabochons) for things a ruler cannot see.
- Corrections you tick. Each one shifts the estimate by its midpoint and widens the range to its full span.
The full method is on the How we calculate page.
Cabochons and other uncut-looking stones
Opal, turquoise, moonstone and many star stones are cut as cabochons: smooth domes with a flat or slightly curved base. Pick "Cabochon high" or "Cabochon low" depending on the dome. A 10 x 8 mm high dome opal that stands 5 mm tall works out to about 2.24 ct, likely 1.85 to 2.57 ct. The range is wider because cabochon bases vary a lot.
Round stone weights by size
This table is generated by the calculator itself for round stones at a typical 61% depth. Use it for a quick check, and use the tool above for your exact measurements.
| Diameter | diamond | moissanite | sapphire | emerald | amethyst | CZ |
|---|---|---|---|---|---|---|
| 2 mm | 0.030 | 0.027 | 0.035 | 0.024 | 0.023 | 0.051 |
| 2.5 mm | 0.058 | 0.053 | 0.069 | 0.047 | 0.045 | 0.10 |
| 3 mm | 0.10 | 0.092 | 0.12 | 0.081 | 0.079 | 0.17 |
| 3.5 mm | 0.16 | 0.15 | 0.19 | 0.13 | 0.13 | 0.27 |
| 4 mm | 0.24 | 0.22 | 0.28 | 0.19 | 0.19 | 0.41 |
| 4.5 mm | 0.34 | 0.31 | 0.40 | 0.27 | 0.27 | 0.58 |
| 5 mm | 0.47 | 0.43 | 0.55 | 0.37 | 0.36 | 0.80 |
| 5.5 mm | 0.62 | 0.57 | 0.73 | 0.50 | 0.48 | 1.06 |
| 6 mm | 0.80 | 0.74 | 0.95 | 0.65 | 0.63 | 1.38 |
| 6.5 mm | 1.02 | 0.94 | 1.21 | 0.82 | 0.80 | 1.75 |
| 7 mm | 1.28 | 1.17 | 1.51 | 1.02 | 1.00 | 2.18 |
| 7.5 mm | 1.57 | 1.44 | 1.85 | 1.26 | 1.23 | 2.69 |
| 8 mm | 1.91 | 1.74 | 2.25 | 1.53 | 1.49 | 3.26 |
| 8.5 mm | 2.29 | 2.09 | 2.70 | 1.83 | 1.79 | 3.91 |
| 9 mm | 2.71 | 2.48 | 3.20 | 2.18 | 2.12 | 4.64 |
| 10 mm | 3.72 | 3.40 | 4.39 | 2.99 | 2.91 | 6.37 |
| 11 mm | 4.95 | 4.53 | 5.85 | 3.98 | 3.87 | 8.48 |
| 12 mm | 6.43 | 5.88 | 7.59 | 5.16 | 5.03 | 11.01 |
Notice how quickly weight climbs. Doubling the diameter multiplies the weight by about eight, because the stone gets wider, longer and deeper at the same time.
Common mistakes
- Measuring the setting, not the stone. Prongs and bezels add width. Measure girdle to girdle, between the prongs.
- Using total carat weight. A ring listed as "1 ct TW" may have a 0.5 ct center and many small stones. Work out the center stone alone, or use the total carat weight calculator.
- Treating moissanite "DEW" as real weight. A moissanite sold as 1 ct DEW really weighs less. See the DEW calculator.
- Guessing depth. If you cannot measure it, use the typical depth from the carat to mm finder and accept a wider range.
Related questions
How many carats is a 6 mm stone?
It depends on the material. At a typical 61% depth, a 6 mm round diamond is about 0.80 ct, a 6 mm moissanite about 0.74 ct, a 6 mm sapphire about 0.95 ct and a 6 mm emerald about 0.65 ct. Denser materials weigh more at the same size.
How many carats is a 5 mm round diamond?
About 0.47 ct at typical proportions, with a likely range of 0.44 to 0.50 ct. A 5 mm stone with a thick girdle or deep pavilion can weigh a little more.
How many carats is an 8 mm round diamond?
About 1.91 ct at 61% depth, likely 1.82 to 2.06 ct. Because weight grows with the cube of size, 8 mm is only 23% wider than 6.5 mm but roughly 1.87 times the weight.
Can I find the exact carat weight from measurements?
No. Measurements give an estimate that is usually within a few percent for well-cut stones, and further off for stones with thick girdles, bulging pavilions or unusual shapes. Only a calibrated scale gives the exact weight.
Do I need the depth, or is length and width enough?
You need the depth. Two stones with the same face-up size can differ in weight by 20% or more because one is cut deeper. If the stone is set and you cannot reach the culet, see our guide to estimating weight in a setting.
Why does a CZ weigh so much more than a diamond of the same size?
Cubic zirconia has a specific gravity of about 5.80, against 3.52 for diamond, so the same volume holds about 65% more mass. A 6.5 mm round CZ comes out near 1.75 ct.
Which shape should I pick for a cushion or radiant cut?
Pick the outline that matches the stone face up: cushion for rounded corners, cut-corner rectangle for radiant style stones, and emerald cut for step cut rectangles with long parallel facets. If unsure, try both and read the range.
Does the calculator work for lab-grown stones?
Yes. Lab-grown diamond, ruby, sapphire and emerald have the same or nearly the same density as natural stones, so they are listed separately only for clarity. Moissanite and CZ are different materials and have their own entries.
Sources and method
Weight estimates use the colored-stone shape factors published by the International Gem Society and the Dendritics standard formula, and the diamond trade constants and length to width ratio tables published in jewelry trade references (Esslinger, Omni, Octonus). Specific gravity values are cross-checked against the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect.
Every number on this page is generated by the same formula engine as the calculators. Read the full method, constants and limits and the editorial policy. Data last verified 2026-11-04.
Estimates only. This is not an appraisal, grading report or valuation. A calibrated scale or a lab report is the only real weight.