Diamond Weight Calculator
Enter the shape and measurements of a loose diamond to get an estimated carat weight, depth percentage, length-to-width ratio, and a cut-quality note. The tool supports 12 popular shapes and uses the same GIA-calibrated industry coefficients that gemologists rely on when a scale is not available. Results are typically within 5-10% of the actual carat weight.
How the diamond weight formula works
Gemologists estimate a loose diamond's weight without a scale by using shape-specific coefficients derived from the density of diamond (3.52 g/cm3) and average proportions for each cut style. For a round brilliant, the formula is: Diameter x Diameter x Depth x 0.0061 x Girdle Factor. For every other shape the formula becomes: Length x Width x Depth x Adjustment Factor x Girdle Factor x Correction Factor. The adjustment factor accounts for how much of the bounding box a particular shape actually fills - a trillion, for example, has three pointed corners that remove a lot of material, so its factor (0.0057) is lower than a chunky cushion (0.00815). Shapes like emerald, radiant, marquise, and pear use length-to-width ratio tables instead of a fixed factor because their proportions vary widely in the market.
Depth percentage and length-to-width ratio
Depth percentage is the depth of the stone divided by its average width (or diameter for rounds), expressed as a percentage. It is one of the most important cut quality indicators: a round brilliant with a depth below 57% leaks light through the pavilion and looks glassy; one above 63% appears smaller face-up than its weight suggests because so much mass is hidden below the setting. The length-to-width ratio tells you the visual outline of the stone. A round should be very close to 1.00. An oval looks best between 1.30 and 1.60, a marquise between 1.70 and 2.15, and a pear between 1.45 and 1.75. Both metrics are calculated automatically when you enter dimensions.
Girdle thickness and weight correction
The girdle is the thin edge between the crown (top) and pavilion (bottom). A thicker girdle adds real weight that does not appear in the face-up size, so it inflates the carat count relative to the visible spread. The base coefficients assume a thin-to-medium girdle, which GIA considers ideal for most shapes. If you are measuring a stone with a visibly thick girdle, selecting slightly thick, thick, very thick, or extra thick adds 2-9% to the estimate. The weight correction field handles less common features: wide, high wings on a pear or marquise can add 1-10%, significant pavilion bulge on an emerald or radiant 1-18%, and a large culet (the bottom facet) 1-2%.
Accuracy and when to use a scale
The dimension-based formula is accurate to within 5-10% for well-cut rounds and 10-15% for step cuts and fancy shapes. The main sources of error are measurements taken over the setting instead of the bare stone, asymmetrical shapes, non-standard depth profiles, and extreme girdle variation around the circumference. The estimate is reliable enough for insurance replacement quotes, appraisal approximations, and selling valuations when the stone cannot be removed from its setting. For legal, grading, or purchase-decision purposes, always use a calibrated milligram scale on the unmounted stone, or rely on a GIA or AGS grading report.
Typical carat weight by shape and millimetre size
| Shape | Diameter / Length | Approx. carat weight |
|---|---|---|
| Round | 4.1 mm | 0.25 ct |
| Round | 5.2 mm | 0.50 ct |
| Round | 6.5 mm | 1.00 ct |
| Round | 8.2 mm | 2.00 ct |
| Princess | 4.4 mm | 0.50 ct |
| Princess | 5.5 mm | 1.00 ct |
| Oval | 7.7 x 5.7 mm | 1.00 ct |
| Cushion | 5.5 mm | 1.00 ct |
| Emerald | 7.0 x 5.0 mm | 1.00 ct |
| Marquise | 10.0 x 5.0 mm | 1.00 ct |
| Pear | 8.5 x 5.5 mm | 1.00 ct |
| Heart | 6.5 mm | 1.00 ct |
Approximate weights for well-cut diamonds at standard proportions. Actual weight varies with exact depth, girdle, and cut style.
Frequently asked questions
What is a carat?
A carat is the standard unit of weight for gemstones. One carat equals exactly 0.2 grams or 200 milligrams. The name comes from the carob seed, which was historically used as a counterweight on gem scales because of its remarkably consistent weight. Carat weight is often abbreviated as "ct" and should not be confused with "karat" (kt), which refers to the purity of gold.
How accurate is this diamond weight estimator?
For round brilliants measured on a bare stone, the formula is typically within 5% of the actual scaled weight. For fancy shapes (oval, pear, marquise, emerald, etc.) the margin rises to 10-15% because those shapes vary much more in depth profile and shoulder width. You can improve accuracy by averaging two or three diameter readings across the stone, using the correct girdle option, and applying the weight correction for any unusual features.
Why does my diamond's size seem larger than its carat weight?
Carat is a unit of weight, not size. Two 1-carat diamonds of different shapes can look very different face-up because they distribute their weight differently. A shallow diamond spreads weight outward, making it look larger but leaking more light. A deep stone concentrates weight below the setting where you cannot see it. Oval and marquise shapes in particular can look 10-15% larger face-up than a round of the same carat weight.
What is depth percentage and why does it matter?
Depth percentage is the stone's depth (from table to culet) divided by the average width, expressed as a percentage. For a round brilliant, 59-62.5% is the sweet spot where most incoming light reflects back through the table and scintillates. Below 57% the stone looks flat and glassy; above 63% it looks smaller than its weight suggests. Every shape has its own ideal range, shown in the cut-quality note this calculator returns.
What does the length-to-width ratio tell me?
The L/W ratio describes the visual outline of a non-round diamond. An oval at 1.35 looks compact and almost round; one at 1.65 looks elegantly elongated. Marquise rings are traditionally 1.85-2.00. There is no single "correct" ratio, but each shape has a classic range that most buyers prefer. This calculator flags when your stone falls outside that conventional range.
How do I measure a diamond that is set in a ring?
Measure length and width across the top of the stone with a millimetre gauge or digital calipers. For depth, estimate by subtracting the apparent height of the setting from the total height of the ring profile at the stone. A set diamond measurement carries a larger error than a bare-stone measurement, so treat the estimate as a rough range. If accuracy matters, have a jeweller remove the stone.
Which shapes tend to look largest for their carat weight?
Oval and marquise cuts typically look 10-15% larger face-up than a round of the same weight because their elongated outline covers more surface area. Pear and heart shapes are similar. Princess and cushion cuts sit in the middle. Asscher, emerald, and other step cuts often look smaller than their carat weight because of their deep pavilions and smaller tables.