Carat to Millimeter Chart

Carat is weight. Millimeter is size. Two stones of the same carat can differ by a millimeter across, and that difference is visible from across a room. If you are matching an existing piece or ordering a setting, work in millimeters.

Round brilliant

Carat	Diameter	Notes
0.25 ct	4.1 mm	Accent size; too small for a solitaire center
0.50 ct	5.1 mm	Delicate on a small hand
0.75 ct	5.8 mm	Practical everyday size
1.00 ct	6.5 mm	The reference size
1.50 ct	7.4 mm	Noticeably larger without looking costume
2.00 ct	8.1 mm	Statement; consider a lower head
3.00 ct	9.3 mm	Large; band width must increase to balance

Oval

Oval stones are measured length × width. A 1 ct oval is roughly 7.7 × 5.7 mm — it looks bigger than a 1 ct round because the surface area is larger, even though the weight is the same. Ratios between 1.35 and 1.45 look classic; above 1.5 the stone reads narrow.

Pear

A 1 ct pear is about 8.5 × 5.5 mm. Pears face up large because of the point. Watch the bow-tie — a dark band across the widest part — which is most visible in stones with a shallow pavilion.

Emerald and Asscher

Step cuts are deeper for their width, so a 1 ct emerald is about 6.5 × 5 mm — visually smaller than a round of the same weight. This surprises people. If size matters, go up in carat for step cuts.

Cushion

Cushions vary from square to elongated. A 1 ct cushion is roughly 6 × 5.5 mm, but length-to-width ratio matters more than the average. Ask for the actual dimensions of the specific stone.

Why "spread" matters

Spread is how large a stone faces up relative to its weight. A shallow stone has a larger spread and can look bigger for the same carat — at the cost of light return. A deep stone looks smaller but can be better cut. This is why buying by carat alone is a mistake; buy by millimeter plus cut quality.

Matching to a hand

As a rough rule: a stone under 5 mm reads as a band accent; 5–6.5 mm reads as an engagement ring on most hands; 7–8 mm is generous; above 8.5 mm the ring becomes a statement piece and the band needs to be at least 2.2 mm to look balanced.