Diamond Carat Weight Calculator

Diamond Carat Weight Calculator – How The Formulas Work

Carat is the standard unit of weight for diamonds and most gemstones. One carat equals 0.2 grams. When you cannot weigh a diamond directly, you can estimate carat weight from its millimeter measurements and an approximate shape formula. This Diamond Carat Weight Calculator applies those formulas for popular diamond shapes.

The key idea is to approximate the diamond’s volume from length, width and depth, then multiply by the density of diamond to convert volume into weight. Each shape has a slightly different volume factor, which is built into the constants used in the formulas.

Base Relationship Between Volume, Density And Carats

In general form, the estimated carat weight can be written as:

For many commercial rules of thumb, this is simplified to a product of measured dimensions times a constant, written as:

or, for round stones where the diameter is the same in every direction, as:

The constant k absorbs the geometric shape factor, the conversion from cubic millimeters to grams and from grams to carats.

Round Diamond Carat Formula

Round brilliant diamonds are usually described by two diameters and a depth, for example 6.50 × 6.47 × 3.95 mm. The average diameter is:

A commonly used estimator for round brilliant carat weight is:

All dimensions are in millimeters. The constant 0.0061 reflects the geometry of a round brilliant cut and the density of diamond. The Round tab in this calculator applies this formula directly to the values you enter.

Princess And Square Diamond Formula

Princess and square diamonds have four equal or nearly equal sides and a deep pavilion. A typical volume-based estimator is:

Here Length, Width and Depth are in millimeters. For a perfectly square princess cut, Length and Width are very similar, so the formula is dominated by side length and depth. The Princess tab uses this formula with your measurements.

Emerald, Cushion And Radiant Diamond Formula

Emerald, cushion and radiant cuts are all quadrilateral shapes with chamfered or rounded corners. Their volume factors are similar to princess cuts, so the same constant is often used:

The Emerald, Cushion and Radiant tabs each ask for Length, Width and Depth in millimeters and apply this formula. Differences in crown height, pavilion angles and girdle thickness will still cause some variation from the estimated weight.

Oval, Pear And Marquise Diamond Formula

Oval, pear and marquise diamonds are elongated shapes with curved outlines. They share similar overall volume profiles and are often modeled with a slightly larger constant than square shapes:

The Oval, Pear and Marquise tabs use this formula with your inputs. The slightly higher constant 0.0062 compensates for the elongated, oval-like geometry of these fancy shapes.

Dimension Units And Consistency

The formulas assume that Length, Width, Depth and Diameter are all measured in millimeters. Because diamond grading reports and jeweler’s calipers typically use millimeters, you can copy measurements directly into the calculator. Using different units, such as inches, would give incorrect results unless you convert them to millimeters first.

Step-By-Step Example For A Round Diamond

Consider a round brilliant with reported measurements 6.50 × 6.47 × 3.95 mm. The average diameter is:

Using the round formula:

Compute the intermediate factor:

Then multiply by the constant:

The calculator performs these steps internally and shows both the estimated carat weight and the Diameter² × Depth factor it used.

Example For A Fancy Shape Diamond

Suppose you have an oval diamond with Length = 8.00 mm, Width = 6.00 mm and Depth = 3.80 mm. Using the oval formula:

First multiply the dimensions:

Then apply the oval constant:

The Oval tab runs exactly this calculation, displaying both the volume factor and the final carat estimate.

How To Use The Diamond Carat Weight Calculator

• Choose the tab that matches your diamond shape.
• Enter length, width and depth (or average diameter for round) in millimeters.
• Select the desired number of decimal places for the estimated carat weight.
• Click the estimate button to compute carat weight and see the underlying dimension product.
• Compare results across shapes or adjust dimensions to explore how small changes affect carat weight.

Understanding The Limits Of Estimation

While the formulas used in this calculator are standard in the jewelry trade, they are still approximations. Real diamonds can deviate from “typical” proportions in several ways:

• Thick or very thick girdles add weight without increasing face-up size.
• Unusually deep stones hide weight in the pavilion.
• Very shallow stones spread more weight across the top view.
• Non-standard crown heights and pavilion angles change the volume.

Because of these factors, the estimated carat weight may be slightly lower or higher than the laboratory-reported carat weight for a specific stone. The calculator is best used for quick estimates, comparisons and learning how dimensions relate to carat weight.