Diamond Carat to MM Size Chart
See how big a diamond of a given carat weight is in millimeters for ten popular shapes, or estimate the carat weight from measured length and width. Size outlines compare your stone with common round sizes.
Last updated:
Empty = length ÷ typical ratio
Empty = typical for the shape
Depth as % of width. Empty = typical
How to use the Diamond Carat to MM Size Chart
- Choose a direction: carat to mm, or mm to carat.
- Pick the shape, for example round brilliant or oval.
- Enter the carat weight, or the measured length and width in millimeters.
- Optionally adjust the length-to-width ratio and depth % to match a specific stone.
- Press Convert and compare the outline with the round reference sizes and the chart.
Diamond carat to mm chart
Approximate face-up size for well-proportioned stones, calculated with the same formulas as the converter (typical ratio and depth for each shape).
| Carat | Round | Princess | Oval (1.4:1) | Cushion | Emerald (1.4:1) | Pear (1.55:1) |
|---|---|---|---|---|---|---|
| 0.25 | 4.1 mm | 3.5 mm | 5.1 × 3.6 | 3.6 mm | 4.4 × 3.1 | 5.5 × 3.6 |
| 0.50 | 5.1 mm | 4.4 mm | 6.4 × 4.6 | 4.6 mm | 5.5 × 3.9 | 6.9 × 4.5 |
| 0.75 | 5.8 mm | 5.0 mm | 7.3 × 5.2 | 5.2 mm | 6.3 × 4.5 | 8.0 × 5.1 |
| 1.00 | 6.4 mm | 5.5 mm | 8.0 × 5.7 | 5.7 mm | 6.9 × 4.9 | 8.8 × 5.6 |
| 1.50 | 7.4 mm | 6.3 mm | 9.2 × 6.6 | 6.6 mm | 7.9 × 5.7 | 10.0 × 6.5 |
| 2.00 | 8.1 mm | 6.9 mm | 10.1 × 7.2 | 7.2 mm | 8.7 × 6.2 | 11.0 × 7.1 |
| 3.00 | 9.3 mm | 7.9 mm | 11.6 × 8.3 | 8.3 mm | 10.0 × 7.1 | 12.6 × 8.1 |
The formulas behind the sizes
Carat weight depends on volume, so it is estimated from the three dimensions in millimeters and a factor for each shape:
- Round:
carat = diameter² × depth × 0.0061 - Oval:
carat = length × width × depth × 0.0062 - Princess: factor 0.0083; cushion and asscher 0.0080; radiant 0.0081; heart 0.0059
- Emerald: 0.0080 at 1:1 rising to 0.0092 at 1.5:1, 0.0100 at 2:1 and 0.0106 at 2.5:1
- Pear: 0.00615 at 1.25:1 down to 0.00575 at 2:1; marquise: 0.00565 at 1.5:1 up to 0.00595 at 3:1
To go from carats to millimeters the converter assumes a typical length-to-width ratio and depth, and solves for the width: width = ∛(carat ÷ (factor × ratio × depth%)). Example: 1 ct round with 61.5% depth gives ∛(1 ÷ (0.0061 × 0.615)) = 6.44 mm.
Tips when comparing diamond sizes
- Look at mm, not just carats: two 1 ct ovals can differ by half a millimeter in length depending on cut. Grading reports list measurements as length × width × depth.
- "Magic sizes": prices jump at 0.5, 0.9, 1.0, 1.5 and 2.0 ct. A 0.9 ct round (about 6.2 mm) looks nearly the same as 1 ct (6.4 mm).
- Finger size changes the look: the same stone looks larger on a slim finger; check your band size with the Ring Size Converter.
- Convert carat weight to grams with the Carats to Grams converter (1 ct = 0.2 g).
Frequently asked questions
How big is a 1 carat diamond in mm?
A well-cut 1 carat round brilliant is about 6.4 to 6.5 mm across. A 1 carat princess is about 5.5 mm square, an oval about 8 × 5.7 mm, and an emerald cut about 7 × 5 mm.
How is carat weight estimated from mm?
Gemologists use formulas such as carat = diameter² × depth × 0.0061 for rounds, and length × width × depth × a shape factor (about 0.0062 for ovals, 0.0083 for princess, 0.0080–0.0106 for emerald cuts depending on the ratio) for fancy shapes. All measurements are in millimeters.
Does a 2 carat diamond look twice as big as 1 carat?
No. Weight grows with volume, so doubling the carats increases the diameter by only about 26% (from roughly 6.5 to 8.1 mm for rounds) and the face-up area by about 59%.
Which diamond shape looks biggest for its weight?
Elongated shapes such as marquise, oval and pear spread their weight over a larger face-up area and look bigger than a round of the same weight. Princess and asscher cuts carry more weight in their depth, so they look smaller.
Why can two diamonds of the same carat weight differ in size?
Cut proportions matter. A deep stone hides weight below the girdle and looks smaller from the top; a shallow one looks bigger but may sparkle less. A thick girdle also adds weight without adding face-up size.
How accurate are these sizes?
They are estimates for typical proportions, usually within about 5–10% of real stones. A grading report lists the exact measurements; use those when you have them.
How many grams is a carat?
One metric carat is exactly 0.2 grams, and it is divided into 100 points, so a 0.75 ct stone is 75 points or 0.15 g.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.