Free Tool

Diamond Size Chart

See how big a diamond really is. Enter the carat and shape to get its face-up size in millimetres — plus a true-to-scale preview you can calibrate to your screen.

Diamond size

Carat weight

Shape

Approximate face-up size

6.5 mm

Calibrate to your screen

Hold a standard bank/credit card against the bar below and drag the slider until the bar matches the card's long edge (85.6 mm). This makes the diamond render at true physical size.

↔ credit card width (85.6 mm)

Actual size preview

Round · 1 ct

6.5 mm

Sizes are typical for a well-cut stone — a deep or shallow cut of the same carat can look smaller or larger. Calibrate above for true-to-life scale.

Carat to mm diamond size chart

Approximate face-up dimensions (mm) of a well-cut diamond by carat weight and shape.

Carat	Round	Oval	Princess	Emerald	Cushion
0.25 ct	4.1	4.9×3.5	3.5	4.4×3.1	3.7
0.33 ct	4.5	5.3×3.9	3.8	4.8×3.5	4.0
0.50 ct	5.2	6.1×4.5	4.4	5.6×4.0	4.6
0.75 ct	5.9	7.0×5.1	5.0	6.4×4.5	5.3
1.00 ct	6.5	7.7×5.6	5.5	7.0×5.0	5.8
1.25 ct	7.0	8.3×6.1	5.9	7.5×5.4	6.2
1.50 ct	7.4	8.8×6.4	6.3	8.0×5.7	6.6
2.00 ct	8.2	9.7×7.1	6.9	8.8×6.3	7.3
2.50 ct	8.8	10.5×7.6	7.5	9.5×6.8	7.9
3.00 ct	9.4	11.1×8.1	7.9	10.1×7.2	8.4

All measurements in millimetres (mm).

Know the size you want? Estimate its value with the diamond price calculator, or check your ring size before you buy.

Frequently Asked Questions

How big is a 1 carat diamond in mm?

A well-cut 1.00 carat round diamond measures about 6.5 mm in diameter. The same carat weight looks different across shapes — a 1ct oval is roughly 7.7 × 5.6 mm and a 1ct princess about 5.5 mm — because carat measures weight, not size.

Why do two diamonds of the same carat look different sizes?

Carat is a unit of weight. How large a diamond looks face-up depends on its shape and how it is cut. A shallow or elongated cut spreads the weight across a wider top and looks bigger, while a deep cut hides weight below the girdle and looks smaller. That is why cut quality matters as much as carat.

How do I convert carat to millimetres?

Because a diamond’s weight scales with its volume, its width scales with the cube root of carat. Our tool anchors each shape’s dimensions at 1 carat and scales from there, so you can enter any carat weight and see the approximate mm size and a true-to-scale preview.

Which diamond shape looks biggest for its carat?

Elongated shapes such as marquise, oval and pear have a larger face-up area per carat than round or princess cuts, so they appear bigger for the same weight. Marquise typically looks the largest, which is why these shapes are popular for maximizing visual size on a budget.

Is the on-screen preview true to size?

It can be. Screens vary in pixel density, so the tool includes a one-time calibration: hold a standard bank card against the on-screen bar and drag the slider until they match. After that, the diamond preview renders at its real physical size.