Diamond Carat Size Chart: MM by Shape

5 days ago

2 min read

Carat is a measure of weight, not size: one carat is 0.2 grams. Two stones of the same carat weight can look noticeably different on the hand depending on shape and how the stone is cut. These are the approximate face-up dimensions to expect.

Round brilliant, by carat

0.25 ct — about 4.1 mm
0.50 ct — about 5.2 mm
0.75 ct — about 5.9 mm
1.00 ct — about 6.5 mm
1.25 ct — about 6.9 mm
1.50 ct — about 7.4 mm
2.00 ct — about 8.2 mm
2.50 ct — about 8.8 mm
3.00 ct — about 9.4 mm
4.00 ct — about 10.2 mm
5.00 ct — about 11.0 mm

Notice how slowly the diameter grows. Doubling from 1 to 2 carats adds only about 1.7 mm, because weight grows with volume while the face grows with area.

One carat, across shapes

Same weight, different footprint:

Round — 6.5 mm
Princess — about 5.5 mm square
Cushion — about 5.5 mm
Asscher — about 5.5 mm
Emerald — about 6.5 × 4.5 mm
Radiant — about 6.2 × 4.8 mm
Oval — about 7.7 × 5.7 mm
Pear — about 8.5 × 5.5 mm
Marquise — about 10.0 × 5.0 mm
Heart — about 6.5 × 6.5 mm

Marquise, pear and oval look largest per carat because more of the weight sits in the face rather than the depth. Princess, cushion and asscher look smallest for the same reason in reverse.

Two carats, across shapes

Round — 8.2 mm
Oval — about 9.3 × 7.0 mm
Emerald — about 8.5 × 6.0 mm
Pear — about 10.5 × 7.0 mm
Marquise — about 12.0 × 6.0 mm
Cushion — about 7.5 mm

Why this matters when buying

Buy the look, not the number. If a stone that reads large is the goal, an elongated shape gets you there for less money than a round of the same apparent size.

Depth can hide weight. A poorly proportioned stone carries weight below the girdle where nobody sees it. Two 1 carat rounds can differ by 0.4 mm face-up, which is visible.

Hand size changes perception. The same stone reads larger on a size 5 finger than a size 9. If you wear a smaller size, you can go lower in carat for the same effect.

These figures are approximate and vary with cut proportions. Always check the millimetre dimensions on the certificate rather than relying on carat alone.