Diamond Carat Weight Calculator
Enter your diamond dimensions in millimeters and choose a shape to get an estimated carat weight. The calculator uses GIA-standard industry formulas with an optional girdle thickness correction. It works for 12 diamond shapes including round brilliant, oval, princess, emerald, cushion, asscher, heart, marquise, pear, trillion, radiant, and baguette.
Formula
Worked example
A round brilliant measuring 6.5 mm diameter and 4.0 mm depth with a medium girdle (1%): 6.5 x 6.5 x 4.0 x 0.0061 x 1.01 = 1.043 ct. An oval measuring 7.7 x 5.7 x 3.7 mm with a 1.35:1 ratio uses SF = 0.00625 (closest ratio bracket): 7.7 x 5.7 x 3.7 x 0.00625 x 1.01 = 1.013 ct.
How diamond carat weight is calculated from measurements
A carat is a unit of mass equal to 200 milligrams (0.2 grams). Because most mounted diamonds cannot be placed on a scale, jewelers estimate weight from measurements taken with a gauge or calipers. The industry-standard formula multiplies the stone's key dimensions in millimeters by a shape factor (SF) derived from average density and the geometry of each cut style. For round brilliants the formula is: diameter squared times depth times 0.0061 (the round brilliant shape factor) times a girdle-thickness correction. For all other shapes it is: length times width times depth times the shape factor for that cut, times both the girdle correction and an optional weight-correction factor for irregular proportions. The shape factor varies because different cuts fill space differently: an asscher or princess cut is denser (SF around 0.0083) because its square outline and step facets retain more material than the tapered wings of a marquise (SF around 0.0058). Emerald and radiant cuts also use length-to-width ratio to pick the best SF because wider stones have a different profile than elongated ones of the same cut. This calculator implements ratio-interpolated factors from the GIA and Palmieri weight-estimation system for all 12 shapes.
Girdle thickness, depth percentage and accuracy
The girdle is the narrow band around the widest part of the diamond separating crown from pavilion. A thicker girdle adds mass that is invisible from above but affects estimated weight. The girdle correction factor (GTF) is typically 1-6%. Most well-cut stones carry a thin-to-medium girdle (GTF of 1%); very thick girdles, sometimes used to protect fragile points on pear or marquise shapes, can add 6% or more. Depth percentage (depth divided by average diameter, expressed as a percent) tells you how the stone will look relative to its weight. A round brilliant with 59-62% depth is considered ideal: it returns maximum light to the eye and its face-up diameter matches the expected size for its carat weight. A deep stone (depth above 63%) hides weight in the pavilion, making it look smaller than its carat weight implies. A shallow stone (below 57%) looks large but may sacrifice brilliance. This calculator displays the depth percentage so you can judge both the weight estimate and the proportions at a glance.
How diamond weight affects value
Diamond pricing is highly non-linear in weight. Because large rough diamonds are far rarer than small ones, the price per carat jumps sharply at psychologically and commercially important thresholds: 0.50 ct, 0.75 ct, 1.00 ct, 1.50 ct, and 2.00 ct. A 0.99 ct stone and a 1.01 ct stone are nearly identical in size, but the 1.01 ct stone commands a meaningfully higher price per carat. Buyers who want maximum value sometimes choose a well-cut stone just below a major threshold, getting a face-up size close to the milestone at a lower cost per carat. The length-to-width ratio also influences perceived value. Ovals in the 1.3-1.5 range and pears in the 1.5-1.75 range are considered well-proportioned by most buyers. Elongated stones can look elegant on the finger and may appear larger face-up than a round of the same carat weight. Marquise and pear shapes typically gain about 10% more face-up area per carat than a round brilliant of the same weight.
Metric units and carat conversion
The carat was standardized internationally in 1907 at exactly 0.200 grams (200 milligrams). One carat is divided into 100 points, so 0.50 ct is also called a 50-point diamond. The metric system is universal in the gem trade: diamonds are always weighed in carats regardless of country, and dimensions are always given in millimeters. To convert carats to milligrams multiply by 200; to convert to grams multiply by 0.2. Equivalently, a 1-carat diamond is 0.007 ounces (avoirdupois) or 0.006430 troy ounces, though neither unit is used in the trade.
Round brilliant: diameter to approximate carat weight
| Diameter (mm) | Approx. carats | Typical use |
|---|---|---|
| 3.0 | 0.10 | Side stone / melee |
| 3.8 | 0.20 | Side stone |
| 4.0 | 0.25 | 1/4 ct equivalent |
| 4.5 | 0.36 | Accent stone |
| 5.0 | 0.50 | 1/2 ct equivalent |
| 5.5 | 0.66 | Small solitaire |
| 5.75 | 0.75 | Three-quarter ct range |
| 6.5 | 1.00 | 1 ct solitaire |
| 7.0 | 1.30 | Popular solitaire |
| 7.3 | 1.50 | Halo center stone |
| 7.75 | 1.75 | Statement ring |
| 8.0 | 2.00 | 2 ct solitaire |
| 8.8 | 2.50 | Large solitaire |
| 9.4 | 3.00 | Investment grade |
Standard industry reference for ideal-cut round brilliants (depth 59-62%). Actual weight varies with depth and girdle.
Frequently asked questions
What is a diamond carat?
A carat (abbreviated ct or sometimes kar) is a unit of mass equal to exactly 200 milligrams, or 0.2 grams. It is the universal unit of weight for gemstones. The word comes from the carob seed, which was historically used as a counterweight on scales. One carat is divided into 100 points, so a 0.50 ct diamond is a "50-point" diamond. Carat weight is one of the four key quality factors (the 4Cs: cut, color, clarity, and carat weight) and is the primary driver of a diamond's size and a major factor in its price.
How accurate is an estimated weight from measurements?
The formulas used by this calculator are the same GIA-derived coefficients used by professional appraisers and jewelers to estimate weight for mounted stones where you cannot use a scale. For a well-proportioned diamond with typical dimensions, accuracy is usually within 5-10% of the actual weight. Accuracy decreases for stones with unusual depth, very thick or very thin girdles, large culets, asymmetric outlines (especially pear and heart), or heavily worn corners. For a certified weight, only a grading report from GIA, AGS, or a similar laboratory is authoritative.
Which diamond shape weighs the most for the same face-up size?
Step cuts like asscher and emerald tend to be heavier per face-up millimeter because their flat table and less dramatic pavilion retain more depth proportional to their outline. Round brilliants and cushion cuts carry a moderate depth. Elongated shapes such as marquise, oval, and pear appear larger face-up for a given carat weight because their outline covers more area than a round of identical mass. A 1-carat oval typically measures about 8x5.5 mm while a 1-carat round is roughly 6.5 mm in diameter.
What is the depth percentage and why does it matter?
Depth percentage is the stone's total depth divided by its average diameter (or average width for non-rounds), expressed as a percentage. For round brilliants, the ideal range is 59-62%. Stones in this range return maximum light to the eye, giving the best brilliance. Stones cut deeper than 63% hide weight below the girdle, making them look smaller than their carat weight suggests. Stones shallower than 57% look large but may show a dark center (the "fish-eye" effect). This calculator shows depth percentage so you can immediately see whether the stone falls in the optically efficient range.
What is a good length-to-width ratio for oval, pear, or marquise diamonds?
The most popular ratios by shape are: oval 1.3-1.5 (classic) or up to 1.7 (elongated trend), pear 1.5-1.75 (balanced) or up to 2.0 (elongated), marquise 1.75-2.25 (traditional), and emerald 1.3-1.5 (square-ish) to 1.7 (elongated). Personal preference matters most. Elongated stones tend to appear larger on the finger and create a slimming visual effect. The length-to-width ratio affects the shape factor used in the weight formula, which this calculator accounts for automatically.
What is the difference between carat and karat?
Carat (ct) is a unit of mass used for gemstones: 1 carat = 0.200 grams. Karat (k or kt) is a unit of gold purity: 24 karat is pure gold, 18 karat is 75% gold, and so on. The words share a common etymology (the carob seed) but measure entirely different things. A diamond in a ring might be described as "1 ct in 18k gold," where ct refers to the gem weight and k to the gold purity.
How do I measure a diamond that is already set in a ring?
Use a millimeter gauge or digital calipers to measure the visible dimensions (diameter for a round, length and width for other shapes). Depth is harder to measure on a set stone: you can estimate it from the profile if the setting allows, or use the average depth percentage for that cut style as a reference (59-62% for round). This gives an approximate estimate. Some gemological tools are designed for measuring mounted stones, and most professional appraisers use dedicated millimeter gauges that let them measure between the prongs.