Gemstones

Diamond Carat Weight Calculator

Estimate diamond carat weight from millimeter dimensions for round and fancy shapes with optional girdle and shape adjustments.

MM to Carat Round & Fancy Points & Grams Reference Table

Diamond Carat Weight Estimator (MM → Carat)

Use diameter or length/width/depth in millimeters to estimate carat weight for common diamond shapes.

This tool estimates weight from dimensions. Real diamonds vary by cut proportions (depth, pavilion “belly”, girdle thickness) and measurement method. For exact carat weight, use a grading report or a scale.
Round formula uses average diameter squared × depth × a round adjustment factor. Small diameter and depth differences can change the estimate meaningfully.
Conversions: 1 carat = 0.2 grams and 1 carat = 100 points. Points are commonly used in jewelry pricing (e.g., 75 points = 0.75 ct).

Why a dimension-based diamond weight is an estimate

When a gemstone cannot be weighed loose, an approximate weight can be derived from measured length, width, depth, shape, and an empirical factor. That estimate depends on the assumed geometry and diamond density. It is useful for initial jewellery planning, but it is not equivalent to the mass recorded by a calibrated balance for a loose stone.

Consider two diamonds with the same face-up length and width. If one is cut with greater depth, it can contain more material and therefore weigh more, even though the stones appear similar from above. Shape matters too: an oval, round brilliant, and emerald cut cannot safely share a single volume coefficient without adjustment.

Use accurate millimetre measurements and confirm the calculator’s supported cut type and assumptions before comparing estimated weights. Mountings may hide girdle or pavilion dimensions, creating further uncertainty. A laboratory report or direct measurement is preferable for valuation, insurance, or trade documentation.