Diamond Weight Calculator

A carat is a unit of mass — 0.2 grams exactly — not a unit of size. Two diamonds with identical measurements can weigh different amounts if one is cut deeper than the other. This is why two rings described as having a 1-carat stone can look noticeably different in size: a shallower cut spreads more surface area for the same weight, while a deeper cut hides mass below the setting where it adds neither sparkle nor visual presence.

The estimation method works by approximating the diamond as a simple geometric solid — a cone, cylinder, or rectangular prism depending on cut shape — then multiplying by a specific gravity constant of 3.52 and a correction factor that accounts for faceting geometry. The round brilliant formula, for example, squares the average diameter, multiplies by depth, then multiplies by 0.0061. That constant encodes both the specific gravity and the average volume ratio of a well-proportioned round brilliant cut versus a perfect cylinder.

Each cut shape has its own correction factor because the pavilion geometry differs significantly. Marquise and pear cuts have tapered ends that remove considerable volume compared to their bounding box dimensions. Princess cuts are squarer and retain more mass. Emerald cuts have cropped corners and a flatter crown, which is why the formula accounts for their rectangular geometry differently. These correction factors are derived from measured populations of actual cut diamonds rather than pure geometry — which is also why they carry a margin of error.

Use this calculator when you have physical access to a diamond but no certificate, when you need a quick sanity check against a stated carat weight before committing to a purchase, or when pricing estate jewelry for resale where paying for a full grading report is not economical. It is also appropriate when comparing stones of different cuts to understand how size and weight trade off — a 1.00 ct round and a 1.00 ct oval look different in size even though they weigh the same.

Do not rely on this estimate when the transaction price exceeds a few hundred dollars without also obtaining an independent grading report. The formula cannot detect treatments, inclusions, or unusual specific gravity variations from trace elements. Lab-grown diamonds have the same specific gravity as natural diamonds, so this formula applies equally to both — but moissanite has a specific gravity of 3.21, and cubic zirconia runs at 5.6 to 6.0, meaning this calculator will underestimate moissanite weight and dramatically underestimate CZ weight if applied to simulants.

Also avoid using this calculator as a substitute for direct weighing when the stone is loose. If you have the stone in hand and access to a precise scale, a direct weight measurement is always more accurate than any formula estimate.

The most common mistake is measuring the diameter of the setting rather than the diamond. Prong-set rings have metal extending beyond the stone's edge, sometimes by 0.5–1.0 mm per side, which adds 1–2 mm to the apparent diameter and dramatically inflates the carat estimate. Always measure from girdle edge to girdle edge, not from metal tip to metal tip.

A second frequent error is confusing depth percentage with total depth. If a grading report lists depth as 61.4%, that is a percentage of the diameter — not a millimeter measurement. Entering 61.4 into the depth field when it should be around 3.9 mm for a 6.5 mm stone will produce an estimate more than 15 times too large. The depth field in this calculator always expects millimeters.

A subtler mistake is applying the round brilliant formula to all shapes. Fancy cuts have meaningfully different correction factors — using the round formula on an oval stone overestimates weight by about 10–15% because the oval's tapered ends hold less mass than a cylinder of the same dimensions suggests. Always select the correct cut shape before trusting the estimate.

The general form of the weight estimate is: Carats = L x W x D x CF, where L is length in millimeters, W is width in millimeters, D is total depth in millimeters, and CF is the cut-specific correction factor.

For round brilliants, the formula uses the square of the average diameter instead of separate length and width values: Carats = ((L + W) / 2)^2 x D x 0.0061. The correction factor 0.0061 encodes both the specific gravity of diamond (3.52 g per cubic centimeter) and a volume reduction factor of approximately 0.62, reflecting that a round brilliant occupies roughly 62% of the volume of the cylinder that would enclose it.

Depth percentage — shown as a secondary output — is calculated as (D / W) x 100. For round brilliants, an ideal depth percentage falls between 59% and 62.5%. Stones below 55% or above 66% are generally considered poor cuts regardless of other quality factors. The length-to-width ratio indicates the proportional outline of fancy shapes: a marquise between 1.75:1 and 2.25:1 is considered classic, while an oval between 1.30:1 and 1.60:1 represents the mainstream preference. These ratios matter in resale because stones outside preferred proportions are harder to price and sell.

The correction factors used in volume-based weight formulas assume the diamond conforms to average cut proportions. In practice, the crown height, pavilion angle, and girdle thickness all shift the effective volume relative to the formula's geometric assumption. A thick girdle alone can add 2–3% mass without changing visible surface area at all — diamonds cut to retain rough weight sometimes carry girdles graded 'very thick' or 'extremely thick' precisely because it inflates stated carat weight while degrading optical performance. When measuring a stone that appears smaller than its weight suggests, check the girdle width: the formula will underperform on girdle-heavy stones.