Faceted Gemstone Weight Calculator

How The Faceted Gemstone Weight Calculator Works

The Faceted Gemstone Weight Calculator is built around a simple idea: for stones of similar cut style, weight is roughly proportional to their volume and density. In practice, this means you can estimate carat weight from the stone’s length, width and depth in millimeters, adjusted by a shape factor and the gem’s specific gravity.

When only calibrated sizes or measurements are available, these formulas help you estimate carat weight for design planning, pricing and comparison, even before the stone is set or weighed on a precision scale.

Step 1: Volume-Based Size Estimate

A faceted gemstone has a three-dimensional shape. A simple volume proxy for commercial cuts is the product of three key dimensions:

Here L is the length, W is the width and D is the depth, all measured in millimeters. For round stones, L and W are both the diameter. This size factor does not give exact volume but captures how volume grows with the main dimensions.

Step 2: Shape Factor For Faceted Cuts

Different shapes and cut styles carve the volume differently. A round brilliant, pear or marquise with the same L × W × D will not weigh exactly the same. To account for this, lapidaries use empirical shape factors. For a reference material such as diamond, the weight in carats can be written as:

The constant k_shape is chosen so that the formula matches typical stones of that shape and cut style. In this calculator, k_shape is calibrated for diamond-like specific gravity (about 3.52).

Step 3: Adjusting For Specific Gravity

Heavier gemstones with higher specific gravity pack more mass into the same volume. To extend the formula from diamond to other gemstones, the calculator scales the result by the ratio of the actual specific gravity to a reference specific gravity SG_ref. The general formula becomes:

In this calculator, SG_ref is set to 3.52, a typical value for diamond. Entering SG = 3.52 leaves the weight unchanged; entering a higher SG slightly increases the estimate, while a lower SG decreases it.

Shape Factors Used In This Calculator

The calculator uses a single, easy estimator mode that supports several common faceted shapes. Typical k_shape values used here (calibrated for diamond-like SG) are:

• Round brilliant: Carats ≈ L × W × D × 0.00610
• Oval: Carats ≈ L × W × D × 0.00590
• Pear: Carats ≈ L × W × D × 0.00570
• Marquise: Carats ≈ L × W × D × 0.00560
• Emerald cut: Carats ≈ L × W × D × 0.00580
• Cushion: Carats ≈ L × W × D × 0.00590
• Princess: Carats ≈ L × W × D × 0.00570

The calculator applies these shape factors internally and then adjusts the result for your specific gravity input.

From Carats To Grams

Once the estimated carat weight is known, converting to grams is straightforward. The carat is defined so that:

So the gram weight is simply:

The calculator applies this exact formula to show both carats and grams for every estimate.

Example: Estimating A Round Brilliant

Consider a round brilliant diamond with measurements L = 6.50 mm, W = 6.50 mm and D = 4.00 mm, and SG ≈ 3.52. Using the round shape factor:

This simplifies to:

A well-cut 1 carat round diamond often has measurements close to this range, which is why these formulas are so useful in everyday practice.

Example: Comparing Two Different Gem Types

Suppose you have two stones with the same shape and dimensions, but different materials. One is diamond with SG ≈ 3.52, the other is sapphire with SG ≈ 4.00. The sapphire’s estimated carat weight will be scaled by:

That means the sapphire would be about 14% heavier in carats than the diamond for the same millimeter dimensions, all else being equal. The calculator applies this SG factor automatically based on your input.

Additional Indicators: Length–Width Ratio And Depth Percentage

In addition to weight estimates, the calculator computes simple proportions that help you understand the cut:

• Length–width ratio: L ÷ W, showing how elongated the stone is.
• Depth percentage: D ÷ W × 100%, expressing depth relative to width.

These numbers are useful when comparing stones of similar carat weight but different outlines or cut styles.

How To Use The Faceted Gemstone Weight Calculator

• Select the gemstone shape that best matches the stone you are estimating.
• Measure length, width and depth in millimeters using a caliper.
• Enter an appropriate specific gravity for the gemstone type.
• Choose how many decimal places you want in the result.
• Click the estimate button to see carat weight, grams, shape factor, SG used and basic proportion metrics.

Limitations And Best Practices

These formulas are approximations based on standard commercial cuts. Real stones can deviate if the table is unusually large or small, the pavilion is very deep or shallow, the girdle is thick, or the specific gravity differs from handbook averages. For high-value stones, always confirm weight on a calibrated scale and treat the calculator as a guide, not a substitute.