Diamond Size Chart

Want to know the actual size of a 1 carat diamond on a hand? Use this diamond size chart to convert millimeters (mm) to carat weight for every popular shape. It works for both lab grown and mined diamonds, because they have the same density.

Quick Answers: Diamond Size in mm

1 carat round diamond: about 6.5 mm across.
2 carat round diamond: about 8.0 mm across.
How big is a 4 mm diamond? About 0.25 carat, a quarter carat.
Small melee stones: 1.3 mm is about 0.01 ct, 1.8 mm about 0.025 ct, 2 mm about 0.03 ct.

Moissanite Size to Carat

Moissanite is about 10% lighter than diamond, so it is sold by millimeter size and a diamond equivalent weight. A 6.5 mm round moissanite looks like a 1 carat diamond but actually weighs about 0.88 to 0.9 carat. Use the mm sizes in the chart below for both stones. Not sure which to choose? Read our moissanite vs lab grown diamond guide.

Select your shape below. Ready to shop? Browse lab diamond solitaire rings or moissanite solitaire rings, and check your fit with our ring size chart.