Diamond Weight Calculator

How to use this diamond weight calculator

What this calculator does

This calculator estimates the carat weight of a loose diamond from its measured dimensions and a shape-specific empirical factor. It is most useful when you know the millimeter measurements but do not have a calibrated gemstone scale available. The result is an approximation of mass, not a substitute for laboratory weighing, identification, valuation, or grading. Carat is a weight unit rather than a visual-size unit: the Gemological Institute of America explains that one metric carat equals 200 milligrams, and diamonds with the same carat weight can still have different face-up dimensions.

When to use it

Use the estimate to sanity-check a loose-stone listing against stated measurements, compare similarly proportioned diamonds before an in-person inspection, approximate the weight of a stone that cannot conveniently be placed on a scale, or explore how deeper, wider, or thicker-girdled proportions change estimated mass. For buying, selling, insurance, or grading decisions, confirm the weight from a trustworthy report or direct scale measurement.

How to calculate

1. The calculator opens with a complete demonstration: a pear-shaped diamond measuring 6 mm by 4 mm by 3 mm, with a 2% girdle adjustment and 5% shape correction. Its XLSX workbook is ready immediately.
2. Select Diamond shape. Enter Length, Width, and Depth in millimeters. For Round, the Length field becomes Diameter and Width is not used.
3. If appropriate, enter a Girdle adjustment and Shape correction. Both are optional percentages; a blank optional field is treated as 0%.
4. Read the live Weight result and the calculation breakdown. Use Download Excel to export the current validated inputs and results as an OOXML workbook.
5. Reset clears the demonstration values and calculated content. Download Excel is then disabled until a complete valid set of required inputs is entered again.

Input guide

Diamond shape is required and accepts one of twelve listed outlines. The selection determines the empirical coefficient; for emerald, marquise, pear, and radiant shapes, the coefficient also depends on the nearest listed length-to-width ratio. Example: Pear. A common mistake is treating “shape” as a cut-quality grade; GIA distinguishes a diamond's outline from the craftsmanship and light-performance meaning of diamond cut.

Length is required, in millimeters, and accepts a positive decimal from 0.01 to 1000 using a period as the decimal separator; grouped US-style thousands are also accepted. Example: 6. For Round it represents diameter. A larger length generally increases estimated weight in direct proportion, except the round formula uses diameter squared, so diameter changes have a stronger effect. Do not enter centimeters, inches, scientific notation, or decimal-comma values.

Width is required for non-round shapes, in millimeters, with the same format and range as Length. Example: 4. Increasing width increases the estimate and can also change the nearest ratio-based coefficient for emerald, marquise, pear, or radiant stones. Width is intentionally not used for a round stone.

Depth is required, in millimeters, from 0.01 to 1000. Example: 3. It is the table-to-culet height, so a deeper stone yields a larger dimensional weight estimate. For round brilliant terminology, GIA describes total depth and average girdle diameter in its diamond anatomy reference. Do not substitute a depth percentage unless you have first converted it to an actual millimeter depth.

Girdle adjustment is optional and accepts 0% to 25%. Example: 2%. It multiplies the geometric estimate by 1 plus the entered percentage. A thicker girdle can retain more weight, but this simple allowance is not a cut-grade judgment. Shape correction is optional and accepts -20% to 50%. Example: 5%. It adjusts for non-standard outlines or pavilion features; positive values raise the estimate and negative values reduce it. The round formula does not use this correction, so the field is disabled for Round.

Output guide

Weight is the primary estimated mass in carats, displayed to three decimals. It is driven by the selected shape, dimensions, girdle adjustment, and, where applicable, shape correction. A higher result means greater estimated mass, not necessarily better cut, higher quality, or a proportional increase in price. The header pills labeled Shape, Dimensions, and Estimate repeat the active outline, compact millimeter measurements, and current carat estimate for quick scanning. The Base estimate is the dimensional weight before percentage adjustments. Length ÷ width shows the aspect ratio used for the variable-factor shapes; a value of 1.00 is square or circular in outline, while higher values are more elongated. Formula factor is the empirical shape coefficient applied to the dimensions. In the breakdown table, Stage names each calculation step, Factor applied shows the coefficient or multiplier used at that step, and Estimated weight reports the resulting carat value. All of these are estimates derived from the same current model.

Worked example

The demonstration Pear is 6 × 4 × 3 mm. Its length-to-width ratio is 1.50, so the pear factor is 0.00600. The base estimate is 6 × 4 × 3 × 0.00600 = 0.432 carats. Applying the 2% girdle factor gives 0.432 × 1.02 = 0.44064 carats. Applying the 5% shape correction gives 0.44064 × 1.05 = 0.462672 carats, displayed as 0.463 ct. The downloadable workbook carries the unrounded canonical numeric values and formats them for readability.