What Is a Carat?
One metric carat equals exactly 0.200 grams or 200 milligrams. The word “carat” derives from the carob seed, historically used as a counterweight by gem traders. Carat weight is measured to two decimal places (0.01 ct = one “point”).
Carat vs. Size: Why They Differ
Because density is constant (3.52 g/cm³ for diamond), a heavier diamond is physically larger — but the shape in which that mass is distributed varies by cut. A deep-cut stone carries weight in its base (pavilion), hiding it beneath the setting. A well-proportioned stone spreads mass toward the top (crown) where you see it.
Approximate Diameter by Carat (Round Brilliant)
- 0.50 ct — ~5.1 mm
- 0.75 ct — ~5.8 mm
- 1.00 ct — ~6.4 mm
- 1.50 ct — ~7.4 mm
- 2.00 ct — ~8.1 mm
- 3.00 ct — ~9.3 mm
Shapes That Look Larger Per Carat
Elongated shapes (oval, pear, marquise, emerald) spread their mass over a larger surface area. An oval of 1.00 ct often measures 7.7 × 5.7 mm — appearing 10–15% larger face-up than a round of the same weight. This is why ovals and pears are such popular choices for those wanting maximum visual impact.
Smart Buying Strategy
Buy just below the magic thresholds (0.49 ct, 0.96–0.99 ct, 1.45–1.49 ct) for savings of 10–20% on identical-looking stones. Combine with an Excellent cut to maximise apparent size through light performance.