Cabochon Weight Calculator

Cabochon Weight Calculator – How The Formula Works

Cabochons are gemstones with a domed top and flat or slightly curved bottom, often cut in oval, round or freeform shapes. Because many cabochons are mounted in settings where direct weighing is inconvenient, jewelers and lapidaries often estimate weight from measurements. This Cabochon Weight Calculator automates the standard estimation formula and converts the results into carats, grams and milligrams.

The calculator is built around the widely used cabochon weight relationship that combines dimensions, specific gravity and a shape factor into a single, easy-to-use expression.

Core Cabochon Weight Formula

Let L be the cabochon length in millimeters, W the width in millimeters, H the height (or dome height) in millimeters, SG the specific gravity of the gemstone and K a cabochon shape factor. A practical estimating formula for cabochon weight in carats is:

In this calculator, the constant 0.002 plays the role of the combined shape and unit conversion factor K. It folds together three ideas:

• Cabochon shape is approximated by a standard geometric factor.
• Volume in cubic millimeters is converted into grams.
• Grams are converted into carats using 1 carat = 0.2 grams.

The result is a compact expression you can use directly with millimeter measurements and specific gravity values.

From Dimensions To Volume

A perfect rectangular solid of size L × W × H has volume:

A cabochon does not fill this entire rectangular box. Its domed surface removes material compared to a full block, so an effective shape factor less than 1 is used. Rather than working with a separate factor, the 0.002 constant incorporates an average cabochon profile into the overall conversion.

Using Specific Gravity In The Formula

Specific gravity SG is the ratio of gemstone density to the density of water. In gemology, typical values include:

• Quartz and Amethyst: SG ≈ 2.65
• Opal: SG ≈ 2.15
• Garnet: SG ≈ 3.6–4.2 (often approximated as 3.8)
• Tourmaline: SG ≈ 3.0–3.2
• Turquoise: SG ≈ 2.7–2.8

In the formula, SG scales the estimated mass to match the density of the specific gemstone. Higher SG values produce heavier weights for the same set of dimensions.

Converting From Carats To Grams And Milligrams

Once the carat weight is estimated, unit conversions are straightforward. The definitions are:

Let C be the estimated cabochon weight in carats. Then:

The Cabochon Weight Calculator applies these formulas to present the result in all three units at once, which is useful for comparing against lab scales or invoicing requirements.

Step-By-Step Example

Consider an oval cabochon with the following measurements:

• Length L = 10 mm
• Width W = 8 mm
• Height H = 4 mm
• Gemstone: quartz with SG ≈ 2.65

Plugging these into the formula:

First compute the rectangular volume approximation:

Then multiply by SG and the constant:

The calculator then shows the same result in grams and milligrams:

Interpreting The Constant 0.002

If you break the estimation into smaller pieces, the structure looks like this:

• Approximate cabochon volume from dimensions
• Convert volume to mass using density and metric relationships
• Convert mass in grams to carats

The constant 0.002 is chosen so that for typical cabochon proportions and common specific gravities, the final carat estimate aligns with real-world measurements. It acts as a combined conversion and shape factor for domed cab stones.

How To Use The Cabochon Weight Calculator

• Measure the cabochon’s length, width and height in millimeters with calipers.
• Select the gemstone type from the dropdown to auto-fill a typical specific gravity, or enter your own SG value.
• Choose how many decimal places you want in the result.
• Click the estimate button to compute approximate carat, gram and milligram weights.
• Use the estimate as a guide for pricing, inventory or planning, and confirm with a scale when possible.

Limitations And Best Practices

The cabochon weight formula provides an estimate, not an exact measurement. Real cabochons differ in dome height, back curvature and girdle shape. To get the best use from the calculator:

• Measure dimensions at the widest and highest points for consistency.
• Use gemstone-specific SG values rather than a single generic number when available.
• Remember that more extreme domes or shallow cuts may produce larger deviations.
• Treat calculated weights as approximations and verify against a precise scale for final certification.