Diamond MM to Carat Calculator
Convert a diamond's size in millimetres to its approximate carat weight, and back, for round and fancy shapes. Carats to grams and cents too.
- Free
- No signup
- Works on your phone
Estimates from the standard trade formulas; cut proportions change the real weight. Charts and factors checked 9 November 2026.
Your stone
Shape
Measured across the girdle. On a certificate, average the smallest and largest diameter.
Table to culet, in mm or as a % of the diameter. Leave it blank to assume 61%, typical for a well-cut round.
Estimated weight
Round, 6.5 mm
1.02 ct
About 0.204 g · 102 cents
- Formula
- 6.5² × 3.96 × 0.0061
- Total depth
- 3.96 mm (61.0%)
- At 59–63% depth
- 0.99–1.06 ct
- In grams
- 0.204 g
- In cents (points)
- 102
Typical sizes for well-cut rounds
| Weight | Trade charts | Formula, 61% |
|---|---|---|
| 0.75 ct | 5.7–5.9 mm | 5.86 mm |
| 1.00 ct | 6.4–6.5 mm | 6.45 mm |
| 1.25 ct | 7 mm | 6.95 mm |
Diameters from Stuller's size chart (2025) and Walt's Jewelers' chart. The closest is highlighted.
An estimate from the stone's measurements. A thick girdle or a bulging pavilion adds weight; weigh a loose stone to be sure.
How to use it
Pick the shape, then choose mm to carat, or carat to mm to find the size for a weight.
Type the diameter (or length and width) in millimetres, and the total depth if you know it.
Read the carat weight with grams, cents and typical sizes for comparison, and copy it with one tap.
How the mm to carat formula works
A diamond's weight follows its volume, and its volume follows its measurements. Jewellers estimate the weight of a stone they can't put on a scale, such as one already set in a ring, by multiplying its measurements by a constant for its shape:
- Round brilliant: carats = average diameter² × total depth × 0.0061
- Fancy shapes: carats = length × width × total depth × shape factor
Measurements are in millimetres, and total depth runs from the table to the culet. If you leave the depth blank, the calculator assumes 61% of the diameter for a round. One carat is 0.2 g and 100 cents.
Shape factors used
| Shape | Factor (× length × width × depth) |
|---|---|
| Round | 0.0061 (× diameter² × depth) |
| Oval | 0.0062 |
| Pear | 0.00615 at 1.25 : 1 down to 0.00575 at 2 : 1 |
| Marquise | 0.00565 at 1.5 : 1 up to 0.00595 at 3 : 1 |
| Emerald | 0.0080 at 1 : 1 up to 0.0106 at 2.5 : 1 |
| Radiant | 0.0081 at 1 : 1, 0.0084 at 1.5 : 1 |
| Princess | 0.0083 |
| Cushion | 0.00815 |
| Asscher | 0.0080 |
| Heart | 0.0059 |
For pear, marquise, emerald and radiant the factor depends on the length-to-width ratio, so the calculator reads it between the published points.
Worked example
A round measures 6.48–6.52 mm across and 3.96 mm deep. Its average diameter is 6.50 mm, so 6.50 × 6.50 × 3.96 × 0.0061 = 1.02 ct, which is 0.204 g or 102 cents. An 8 × 6 mm oval that is 3.6 mm deep comes to 8 × 6 × 3.6 × 0.0062 = 1.07 ct.
Round diamond mm to carat chart
| Diameter | Formula, 61% depth | Trade charts | Cents |
|---|---|---|---|
| 1 mm | 0.0037 ct | 0.005 ct | 0.37 |
| 1.5 mm | 0.013 ct | 0.015 ct | 1.3 |
| 2 mm | 0.03 ct | 0.03 ct | 3 |
| 2.5 mm | 0.058 ct | 0.05–0.06 ct | 5.8 |
| 3 mm | 0.10 ct | 0.10 ct | 10 |
| 3.5 mm | 0.16 ct | 0.16 ct | 16 |
| 4 mm | 0.24 ct | 0.23–0.24 ct | 24 |
| 4.5 mm | 0.34 ct | 0.35 ct | 34 |
| 5 mm | 0.47 ct | 0.45–0.47 ct | 47 |
| 5.5 mm | 0.62 ct | 0.61–0.63 ct | 62 |
| 6 mm | 0.80 ct | 0.78–0.80 ct | 80 |
| 6.5 mm | 1.02 ct | 1.00–1.05 ct | 102 |
| 7 mm | 1.28 ct | 1.25 ct | 128 |
| 7.5 mm | 1.57 ct | 1.56–1.60 ct | 157 |
| 8 mm | 1.91 ct | 1.88–2.00 ct | 191 |
| 8.5 mm | 2.29 ct | 2.19–2.38 ct | 229 |
| 9 mm | 2.71 ct | 2.50–2.91 ct | 271 |
| 10 mm | 3.72 ct | 3.75–3.78 ct | 372 |
| 11 mm | 4.95 ct | 5.00 ct | 495 |
| 12 mm | 6.43 ct | 6.43 ct | 643 |
Trade charts are Stuller's (May 2025) and Walt's Jewelers', read between their rows where a size isn't listed. Each size links to its own page.
Common mistakes
- Typing the depth % as millimetres. A certificate's "61.8%" is a percentage. The depth in mm is in the measurements line, such as 6.45–6.48 × 3.99 mm.
- Measuring over the prongs. On a set stone, measure the girdle, not the claws holding it.
- Ignoring the girdle. The International Gem Society suggests adding 3–4% for a thick girdle and 5–6% for a very thick one. Cartwright notes that small stones need the larger corrections.
- Treating melee charts as exact. Under about 2 mm, trade charts run heavier than the formula, and small stones are bought by sieve and parcel weight. Use the diamond sieve size chart for those.
When a calculator stops being enough
Estimating one stone takes seconds. Following thousands of stones from rough through planning, sawing, bruting and polishing, with the weight recorded at every step, is where registers and spreadsheets break down. We build diamond manufacturing software for Surat units that records each packet's weight at every process. If you also price finished pieces, try the gold jewellery price calculator.
Sources
- Mark T. Cartwright, "Zeroing in on diamond weight calculations", Jewellery Business, 1 July 2013: the round formula and girdle corrections.
- International Gem Society, "How to Estimate Gem Weight in a Jewelry Setting", updated 26 April 2023: girdle and bulge corrections.
- Diamond Gemology, formula for calculating the weight and Leveridge gauge weight tables: oval, emerald, marquise and pear factors.
- Omni Calculator, diamond weight calculator: princess, cushion, asscher, radiant and heart factors.
- Stuller, Millimeter to Diamond Carat Weight Conversion Chart, May 2025, and Walt's Jewelers' diamond size chart: typical sizes.
FAQ
Questions
How do I convert diamond mm to carat?
Multiply the measurements by the shape's constant. For a round: diameter × diameter × total depth × 0.0061. A 6.5 mm round that is 3.96 mm deep weighs about 1.02 ct. For fancy shapes use length × width × depth × the shape factor, for example 0.0062 for an oval.
What size is a 1 carat diamond in mm?
A well-cut 1 ct round is about 6.4–6.5 mm across: Stuller's chart lists 6.4 mm, Walt's Jewelers 6.5 mm, and the formula gives 6.45 mm at 61% depth. Stuller lists a 1 ct oval at 8 × 6 mm, a princess at 5.5 mm square and a cushion at 5.6 mm square.
How accurate is a mm to carat calculator?
Within a few per cent for a well-cut stone measured carefully. The appraiser Mark Cartwright puts his own error at 3–5% for diamonds under 7 mm. A thick girdle, a bulging pavilion or a deep cut makes the real stone heavier, so weigh loose stones when the price depends on it.
What if I don't know the depth?
Leave it blank. The calculator assumes 61% of the diameter for a round, typical of a well-cut stone, and a middle value for fancy shapes: 60% of the width for oval, pear, marquise and heart, 65% for cushion, emerald, radiant and asscher, and 70% for princess. Moving a round from 59% to 63% depth changes its weight by about 7%.
How many cents make one carat?
100. Jewellers in India often say cents (or points): a 50 cent diamond is 0.50 ct and a 10 cent stone is 0.10 ct. One carat is also 0.2 g, so five carats make a gram.
Can I use this for gemstones other than diamond?
Only for diamond, natural or lab-grown, which have the same density. Moissanite is lighter and cubic zirconia heavier than diamond at the same size, and coloured gems vary widely, so they need a formula that uses the gem's specific gravity.
Why do mm to carat charts disagree?
Each chart reflects the cut of the stones its maker sells. For a 2.5 mm round, Stuller lists 0.06 ct and Walt's Jewelers 0.05 ct; for 9.3 mm both list 3 ct. When it matters, measure the real depth and use the calculator instead of a chart.
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