Diamond Carat to MM Converter
Convert diamond carat weight to an approximate millimeter size by shape, or enter length and width to estimate carats. Handy for comparing online listings, engagement ring stones, earrings, and upgrade options.
Convert diamond size
Choose a diamond shape, pick a direction, and get an instant practical estimate.
A 1.00 ct round diamond is commonly around 6.5 mm across, though exact measurements vary by cut.
How to use the diamond size converter
- Choose the diamond shape, because each shape carries weight differently.
- Use carats to millimeters when comparing listing sizes.
- Use millimeters to carats when you already know the stone measurements.
- Treat the result as a shopping estimate, then confirm with the jeweler's certificate or listing measurements.
Quick diamond carat to mm chart
| Carats | Round | Oval | Princess |
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Carat weight is not the same as visible size
Two diamonds with the same carat weight can have different millimeter measurements. A deep stone may look smaller from the top, while an elongated oval, pear, or marquise can look larger because its weight is spread over more length.
Diamond size conversion FAQ
Is 1 carat always 6.5 mm?
No. A 1 carat round diamond is often around 6.5 mm, but the real measurement depends on cut proportions. Fancy shapes use length and width instead of one diameter.
Which diamond shape looks largest for its carat weight?
Marquise, oval, pear, and emerald cuts often look larger face-up than round or princess cuts at the same carat weight because they have more length.
Can I estimate carats from length and width?
Yes, this tool can estimate carat weight from millimeter measurements, but it cannot know depth, girdle, or exact cut. Use it for comparison, not certification.