Carat is weight — 0.2 grams — not size. Two one-carat stones can face up visibly different depending on cut depth and shape. Buy spread (the face-up measurement), not just the number on the certificate.
The price cliffs
Prices jump at the magic numbers: 0.50, 0.75, 1.00, 1.50, 2.00. A 0.92-carat round costs 10–18% less than a 1.00 and is indistinguishable on the hand — the single best trick in diamond buying.
Spread by shape
Per carat, ovals, pears, and marquises face up 8–15% larger than rounds; Asschers and princesses face up smaller. If size-per-dollar drives you, elongated brilliant shapes win every time.