Convert gemstone millimeter measurements to estimated carat weight. Instant conversion tables for round, oval, pear, emerald, and princess cut stones.
Converting between millimeter measurements and carat weight is one of the most common tasks in gemology and jewelry shopping. Jewelers describe stones by carat weight while shoppers often think in terms of physical size—and the relationship between the two isn't intuitive because weight scales with volume (cubic) while apparent size is roughly area (squared).
This mm-to-carat conversion calculator provides instant lookups for common gemstone sizes and shapes. Enter a measurement in millimeters and get the estimated carat weight, or enter a carat weight and find the expected dimensions. The tool supports round, oval, pear, emerald, princess, cushion, and marquise cuts.
The conversion is specific to gem species because density varies—a 6.5mm round stone weighs 1.00 carat if it's a diamond but 1.14 carats if it's a sapphire and 0.77 carats if it's an emerald. This calculator accounts for mineral density so you get accurate results regardless of gemstone type.
The comprehensive reference tables show mm-to-carat conversions at a glance, making this tool invaluable for jewelers, online shoppers comparing stones described in different units, and anyone who needs to quickly translate between physical measurements and the weight-based pricing system used industrywide.
Use this calculator when you want to translate millimeter measurements into a weight estimate quickly, especially when listings mix size-based and carat-based descriptions. It is useful for shopping, appraisal work, and comparing stones across shapes without relying on a separate chart for each one. That makes it easier to compare visible size with expected weight before you buy or list a stone.
Round: carats ≈ diameter² × depth_ratio × 0.0061 × (SG/3.52). Standard depth ratios by shape: Round 61.5%, Oval 62%, Emerald 65%, Pear 62%, Princess 73%. SG = specific gravity of the gemstone.
Result: ~1.25 carats
A 7.0mm round diamond with standard depth proportions (61.5% depth) estimates to approximately 1.25 carats.
The mm-to-carat conversion assumes standard cut depth proportions. **Round Brilliant:** 59–62.5% total depth (ideal ~61.5%). **Oval:** 58–65% depth. **Emerald/Step Cut:** 61–67% depth. **Pear:** 58–65% depth. **Princess:** 68–78% depth (deeper due to square shape). **Marquise:** 58–65% depth. **Cushion:** 65–75% depth.
Stones cut outside these ranges will deviate from the conversion. A "spread" stone (shallow cut) will appear larger for its weight but may sacrifice brilliance. A deep stone will appear smaller but may have better light return.
3.0mm ≈ 0.10ct, 3.5mm ≈ 0.15ct, 4.0mm ≈ 0.25ct, 4.5mm ≈ 0.35ct, 5.0mm ≈ 0.50ct, 5.5mm ≈ 0.65ct, 6.0mm ≈ 0.80ct, 6.5mm ≈ 1.00ct, 7.0mm ≈ 1.25ct, 7.5mm ≈ 1.50ct, 8.0mm ≈ 2.00ct, 8.5mm ≈ 2.40ct, 9.0mm ≈ 2.75ct, 9.5mm ≈ 3.25ct, 10.0mm ≈ 3.75ct, 11.0mm ≈ 5.00ct.
In jewelry marketing, carat weight is the dominant metric because larger stones are rarer and more valuable per carat. However, two diamonds of the same carat weight can differ dramatically in visual size depending on cut. A well-cut 0.90ct diamond can appear as large as a poorly cut 1.00ct stone while costing 10–15% less. Always compare physical measurements alongside carat weight.
No, it's an estimate based on standard proportions. Actual weight depends on the specific depth and cut quality. Well-cut stones follow the standard closely; deep or shallow cuts deviate.
Because gems have different densities. Diamond has SG 3.52, sapphire 4.0, emerald 2.72. Denser gems weigh more at the same dimensions.
Use a Leveridge gauge or digital calipers. For round stones, measure the diameter. For fancy shapes, measure the length (longest) and width (perpendicular to length).
A well-cut 1-carat round brilliant diamond is approximately 6.5mm in diameter. Poorly cut stones may be slightly larger or smaller for the same weight.
Yes. Lab-grown diamonds have the same density as natural diamonds, so the conversion is identical.
Most published charts are accurate for well-cut stones to within ±5%. This calculator adds density correction for non-diamond gems, improving accuracy further.