How to use it
- Choose whether you know the size or the carat weight, then round or fancy cut.
- For a size, enter the diameter, or the length and width of a fancy stone. For a weight, enter the carats and, for fancy shapes, the length to width ratio.
- Check the total depth percentage. The defaults (61% for round, 65% for fancy) are typical values, not a standard; enter the real depth if you have it.
How it works
- Round: carats = diameter² × depth × 0.0061;
- Oval: carats = ((length + width) ÷ 2)² × depth × 0.0062;
- Other fancy shapes: carats = length × width × depth × shape factor.
All sizes are in millimetres; depth is the percentage times the diameter or width. For “size from carats”, the weight grows with the cube of the size, so the calculator takes a cube root.
Example
A round brilliant 6.5 mm across with a 61% depth is 6.5 × 0.61 = 3.965 mm deep (shown as 3.97 mm). Its weight is 6.5² × 3.965 × 0.0061 = 167.5 × 0.0061 = 1.02 ct, or 0.204 g.
The other way round, a 1 ct round brilliant at 61% depth has a diameter of ∛(1 ÷ (0.0061 × 0.61)) = 6.45 mm.
An emerald cut 7 × 5 mm (ratio 1.4) at 65% depth: the factor is 0.00896, the depth 3.25 mm, the weight 7 × 5 × 3.25 × 0.00896 = 1.02 ct.
Tips and common mistakes
- Measure the length along the longest line and the width at right angles to it.
- Two stones of the same carat weight can look very different in size because of their depth.
- For a ring setting, the visible size matters more than the weight; compare the millimetres too.