Diamond Carat Calculator

Estimate a diamond's carat weight from its length, width, and depth in millimeters, using the trade-standard shape formula (D² × depth × 0.0061 for round brilliants, L × W × depth × a shape constant for fancy cuts).

Quick Facts

Round formula
Carat ≈ D² × depth × 0.0061
D is the average of the smallest and largest girdle diameter, in millimeters.
1 carat
= 200 milligrams (0.2 g)
Carat is a unit of mass, not size — cut proportions affect how large a given carat weight looks.

Your Results

Calculated
Estimated carat weight
-
Using the shape's estimation formula
Footprint (L × W)
-
Table-view area used in the formula
Depth percentage
-
Depth ÷ average width or diameter
Length-to-width ratio
-
1.00 = square/round outline

Ready

Choose a shape, enter length, width, and depth in millimeters, then press Calculate.

Understanding the Diamond Carat Calculator

A diamond's carat weight is normally read straight off a jeweler's scale. But when a stone is already mounted in a ring or pendant, it usually cannot be weighed directly. Jewelers and appraisers get around this with a trade-standard estimated weight formula: measure the stone's length, width, and depth with calipers (in millimeters), then multiply those dimensions by a constant that depends on the diamond's shape. This calculator applies that formula.

The formula

For a round brilliant, the formula is:

Estimated carat weight = D² × depth × 0.0061

where D is the average girdle diameter in millimeters (the mean of the smallest and largest diameter, since few round stones are perfectly circular) and depth is the total height from table to culet, also in millimeters.

Fancy shapes replace D² with the length-times-width footprint and use their own constant:

  • Round brilliant: D² × depth × 0.0061
  • Princess: L × W × depth × 0.0083
  • Cushion: L × W × depth × 0.0080
  • Emerald: L × W × depth × 0.0080
  • Asscher: L × W × depth × 0.0080
  • Oval: L × W × depth × 0.0062
  • Marquise: L × W × depth × 0.00565
  • Pear: L × W × depth × 0.0060
  • Heart: L × W × depth × 0.0059

These constants are averages built from typical, well-proportioned cuts of each shape. A stone cut unusually shallow, deep, or "spread" wide relative to its weight will estimate less accurately — the formula assumes ordinary proportions.

Depth percentage and proportions

Alongside the weight estimate, it helps to check depth percentage: depth divided by the average diameter (or width), shown as a percent. For round brilliants, a depth of roughly 58% to 63% of the diameter is typical of a well-proportioned cut; figures well outside that band often signal either an unusual cut or a measurement mistake. The length-to-width ratio (length ÷ width) is another quick check — close to 1.00 means a round or square outline, while a higher ratio means a more elongated stone, which is expected for shapes like oval, marquise, and pear.

Common reference sizes

As a rough guide, well-proportioned round brilliants of these approximate carat weights typically measure close to these diameters: 0.50 ct ≈ 5.2 mm, 1.00 ct ≈ 6.5 mm, 1.50 ct ≈ 7.4 mm, and 2.00 ct ≈ 8.2 mm. Actual diameters vary with cut depth, so use these as a sanity check, not an exact lookup.

Practical context

This formula is an estimate, not a substitute for a certified weight. Whenever possible, prefer the carat weight printed on a gemological lab report (GIA, AGS, or similar) or measured on a calibrated scale after the stone is unmounted. Use this calculator when neither is available — for a quick appraisal of mounted jewelry, a private sale, or an inherited piece.

Frequently Asked Questions

How is diamond carat weight estimated from measurements?
Measure length, width, and depth in millimeters with calipers, then multiply by a constant that depends on shape. For a round brilliant: average diameter squared × depth × 0.0061. Fancy shapes use length × width × depth × their own constant, since their outlines are not circular.
What is the standard round brilliant formula?
Estimated carat weight = D² × depth × 0.0061, where D is the average of the smallest and largest girdle diameter in millimeters. A typical 1 carat round brilliant measures about 6.5 mm across with roughly 4.0 mm of depth.
Why does depth percentage matter?
Depth percentage (depth ÷ average diameter or width, as a percent) is a standard proportion check. Round brilliants cut too shallow or too deep relative to their diameter tend to lose brilliance, even though the weight formula still returns an estimate. A depth around 58%-63% of the diameter is typical of a well-proportioned round brilliant.
Is this estimate as accurate as weighing the diamond on a scale?
No. It is a widely used trade approximation based on average cut proportions, not a direct measurement of mass. Cut quality, girdle thickness, and faceting symmetry all shift the actual weight away from the estimate. A measured weight from a scale, or the weight on a gemological lab report, is always more accurate than this calculator's estimate.

Related calculators