How do you estimate carat weight from millimetres?

For a round brilliant stone, multiply the girdle diameter in millimetres by itself, multiply by the total depth in millimetres, then multiply by a shape factor of 0.0061 for diamond. A 6.5 mm diameter stone that is 4.0 mm deep works out at about 1.03 carats. For other gems the factor is scaled by their specific gravity.

How many grams is one carat?

One carat equals exactly 0.2 grams, or 200 milligrams. So a 1.03 carat stone weighs about 0.206 grams. Carat is a measure of weight, not size, so two stones of the same carat weight can look different sizes depending on how they are cut.

Why does specific gravity change the result?

Two stones cut to the same shape and dimensions weigh different amounts if they are denser. Corundum such as ruby and sapphire is denser than diamond, so a same-size sapphire weighs more carats. The calculator scales the diamond factor by each stone's specific gravity to reflect this.

Carat Weight Calculator

This carat weight calculator estimates how much a round faceted gemstone weighs from its measured size, which is useful when a stone is still in a setting and cannot be put on a scale. You enter the girdle diameter and the total depth in millimetres, pick the stone type, and it returns the estimated weight in carats along with the equivalent in grams and milligrams. The method is the one jewellers use for a quick check: for a round brilliant diamond the weight is the diameter squared, times the depth, times a shape factor of 0.0061. That factor already includes diamond's density, so for other stones the calculator scales it by specific gravity, because a ruby or sapphire of the same size weighs more than a diamond while an emerald weighs less. Remember that carat is a measure of weight rather than width, so a shallow-cut stone can look large for its carat weight and a deep-cut stone can look small. The result is an estimate only: the real weight depends on girdle thickness, the exact cut proportions and the true specific gravity of the material, so treat it as a guide and confirm with a proper scale or a valuation where value matters. It pairs well with our engagement ring budget tool.

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Stone type

Girdle diameter

Total depth

1.03 ct

estimated carat weight

In grams0.206 g

In milligrams206 mg

Specific gravity3.52

A 6.5 mm round stone 4.0 mm deep at specific gravity 3.52 weighs about 1.03 ct (0.206 g).

This is an estimate for round brilliant stones. Actual weight varies with girdle thickness, cut proportions and the true specific gravity. Confirm on a scale for anything that matters.

How it works

For a round brilliant stone, weight in carats is the girdle diameter squared, times the total depth, times a shape factor. The standard factor for diamond is 0.0061, which already bakes in diamond's specific gravity of about 3.52 and a small allowance for the girdle. For any other stone the calculator keeps the same geometry but replaces the density, multiplying by the ratio of that stone's specific gravity to diamond's. Because one carat is exactly 0.2 grams, the gram and milligram figures follow directly. The formula assumes a normal round cut: very shallow, very deep or fancy-shaped stones will read high or low, so use it as a first estimate rather than a final answer.

Worked example

You have a round brilliant diamond measuring 6.5 mm across the girdle and 4.0 mm deep. The weight is 6.5 x 6.5 x 4.0 x 0.0061, which is 42.25 x 4.0 x 0.0061, giving about 1.03 carats. In metric that is 1.03 x 0.2, or roughly 0.206 grams, which is 206 milligrams. Swap the stone to a sapphire and the higher specific gravity of 4.00 lifts the same dimensions to about 1.17 carats, because the denser material packs more weight into the identical size.

Related calculators

Engagement Ring Budget Calculator: set a sensible ring budget.
Density Calculator: mass, volume and density.
Grams to Ounces Calculator: convert small weights.
Weight Calculator: general weight conversions.
Kilograms to Ounces Converter: metric to imperial weight.

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