A carat is 0.2 grams, a weight, yet what you see on a hand is the face-up size in millimetres. Two stones of the same weight can look quite different depending on cut and proportions. Here every stone is drawn at its true size on a finger as wide as your ring size, so you can judge it the way it will look.
How the sizes are worked out
Gemmologists estimate weight from measurements. For a round brilliant the formula, often credited to GIA, is diameter squared times depth times 0.0061. At a typical 61.5% depth a 1 carat round is about 6.4 mm across, a 0.5 ct about 5.1 mm and a 2 ct about 8.1 mm.
Fancy shapes use length times width times depth times a shape factor that changes with the length-to-width ratio. We use published factors at stated default proportions: oval 1.35 : 1, emerald 1.4 : 1, pear 1.6 : 1, marquise 2 : 1, cushion 1.1 : 1, radiant 1.25 : 1 and square princess. Real stones vary, so treat each size as typical, not exact.
Round: ct ≈ ز × depth × 0.0061 (mm) · fancy: ct ≈ L × W × depth × k
Which cuts look biggest
Elongated cuts spread their weight over a longer outline. At 1 carat a marquise is about 10.5 mm long and a pear 8.9 mm, against 6.4 mm for a round, and both cover more of the finger. Princess and cushion cuts are deeper, so a 1 carat princess faces up at about 5.6 mm.
Weight grows with the cube of size, so doubling the carat weight adds only about a quarter to the width: 1 ct round is 6.4 mm, 2 ct is 8.1 mm. That is why a 2 carat stone does not look twice as large.
Moissanite, sapphire and emerald
Other stones have different densities. Moissanite (specific gravity about 3.21) is roughly 9% lighter than diamond (3.52) at the same size, so a 6.5 mm round moissanite weighs about 0.94 ct by density, against about 1.03 ct for a 6.5 mm diamond. Retailers usually quote 0.88 ct, because moissanite is also cut a little shallower, and sell it by “diamond equivalent weight”.
Sapphire and ruby (corundum, about 4.00) are about 14% heavier, so a 1 carat sapphire faces up smaller than a 1 carat diamond. Emerald (about 2.72) is about 23% lighter, so a 1 carat emerald looks larger. The stone switch in the visualiser redraws the size for each.
Seeing it on your hand
Measure your ring size first, or pick it with the finger chips, so the finger is drawn at its real width. Hold your hand beside the screen with the stone drawing between your knuckles, or lay a loose stone on the glass to identify its weight from its size.