Precious Stone Weight Calculator

How The Precious Stone Weight Calculator Works

The Precious Stone Weight Calculator estimates carat weight from the visible dimensions of a gemstone. By combining length, width, depth, shape and specific gravity, it uses practical formulas that reflect how real faceted stones are cut and how dense different gem materials are.

This approach is useful when a stone is already set in jewelry and cannot be removed from its mounting to be weighed on a scale. It is also helpful for planning, design work and educational comparisons between different gems.

The Core Idea: Volume × Density → Weight

Physically, weight is volume multiplied by density. For gemstones, density is usually expressed as specific gravity, which compares the material’s density to that of water. Dimensions in millimeters give a geometric volume, and specific gravity scales that volume into a gram and carat estimate.

The calculator simplifies this by using shape factors that approximate how much of a bounding box a typical cut occupies, then multiplies by specific gravity to convert volume into weight.

General Carat Estimation Formula

For many faceted gems, the estimated carat weight in millimeters can be written in a compact form. Let L be length, W be width, D be depth, SG be specific gravity and K be a shape factor. All dimensions are in millimeters. The general formula is:

Here 3.52 is the typical specific gravity of diamond. Shape factors are originally calibrated for diamond cuts, so scaling SG by SG ÷ 3.52 adjusts the estimate for heavier or lighter materials while keeping the same proportions.

Round Brilliant Formula

For round brilliant cuts, length and width are both equal to the diameter d. A widely used approximation for diamonds is:

In the calculator, this is incorporated into the general formula by taking L = d, W = d and K = 0.0061 for the round shape. For other materials, the estimate is scaled by SG ÷ 3.52 to respect density differences.

Shape Factors For Common Gem Cuts

Different shapes fill their length–width–depth bounding box differently. To reflect this, the calculator uses distinct shape factors K for each cut:

• Round brilliant: K ≈ 0.0061
• Oval: K ≈ 0.0062
• Emerald cut: K ≈ 0.0061
• Pear: K ≈ 0.0059
• Marquise: K ≈ 0.0057
• Cushion: K ≈ 0.0065
• Princess: K ≈ 0.0069

These factors are calibrated to typical well-cut stones. Real stones can deviate from these averages if they are deeper, shallower or have unusually thick or thin girdles.

Specific Gravity And Material Differences

Specific gravity describes how heavy a material is compared to water. Since a carat is a unit of weight, two stones with identical dimensions but different specific gravities will have different carat weights. The calculator uses approximate specific gravity values such as:

• Diamond: SG ≈ 3.52
• Ruby and sapphire: SG ≈ 4.00
• Emerald and aquamarine: SG ≈ 2.72
• Amethyst: SG ≈ 2.65
• Garnet group: SG ≈ 3.8
• Topaz: SG ≈ 3.53
• Zircon: SG ≈ 4.65
• Cubic zirconia: SG ≈ 5.8

You can choose a gemstone type to pre-fill a typical specific gravity, then refine it manually if you have a more precise value for your particular sample.

From Carats To Grams And Milligrams

Once the estimated carat weight is known, grams and milligrams follow directly from the definition of a metric carat. One carat equals 0.2 grams, and one gram equals 1000 milligrams:

The Precious Stone Weight Calculator applies these formulas after the carat estimate so that carats, grams and milligrams always remain internally consistent.

Worked Example: Round Brilliant Diamond

Consider a round brilliant diamond with an average diameter of 6.5 mm and a depth of 4.0 mm. Using the round formula with SG = 3.52, the calculator applies:

The squared diameter is 6.5 × 6.5 = 42.25, so:

Grams can then be estimated as:

Worked Example: Oval Sapphire

Now consider an oval sapphire with length 8.0 mm, width 6.0 mm, depth 4.5 mm and SG ≈ 4.0. Using the general formula with an oval shape factor K ≈ 0.0062:

The bounding box volume factor L × W × D is 8.0 × 6.0 × 4.5 = 216. The diamond-calibrated estimate is:

Scaling for SG gives:

How To Use The Precious Stone Weight Calculator

• Measure the stone’s length, width and depth in millimeters. For round stones, use the average of multiple diameter measurements.
• Select the stone’s shape so the calculator can apply the appropriate shape factor.
• Choose a gemstone type to pre-fill specific gravity, or select custom and enter your own SG value.
• Set the decimal precision you prefer for the results.
• Press the estimate button to see carat, gram and milligram values, along with the shape factor and SG used.

Limitations And Best Practices

Dimension-based carat estimation is an approximation. It assumes standard proportions and average girdle thickness. Stones that are unusually deep, shallow or irregular will not follow the formulas exactly. When precision matters, always confirm with a scale measurement.

However, for many practical tasks such as design planning, comparing stones, estimating stock, or evaluating set gems, the Precious Stone Weight Calculator offers a fast and transparent way to convert measurements into realistic carat estimates.