Why does the same carat weight look different across shapes?

Carat is a unit of mass (0.2 g), not size. Different cuts distribute that mass differently — an elongated marquise spreads weight across a larger face than a compact round brilliant of the same carat weight.

How accurate are these formulas?

The formulas use industry-standard average proportions. Actual dimensions vary by ±5–10% depending on cut quality, depth percentage, and the individual cutter's choices. Use this tool for estimates and verify exact measurements with a jeweler's gauge.

What is a "well-cut" depth for a round brilliant?

GIA ideal range is 59–62.5% of the diameter. This tool uses 61.5% as a typical reference depth.

Can I use this for lab-grown diamonds?

Yes. Lab-grown diamonds have the same crystal structure and density as natural diamonds, so the carat-to-mm relationships are identical.

My diamond measures differently than the calculator shows — why?

Real diamonds vary. A very deep cut packs more mass into a smaller diameter; a shallow cut yields a larger face but may sacrifice brilliance. The tool shows average industry proportions, not a universal guarantee.

Diamond Carat to MM Converter

Name: Diamond Carat to MM Converter
Availability: InStock
Author: Nham Vu

Convert diamond carat weight to physical millimeter dimensions (and back) for round, princess, oval, emerald, cushion, marquise, pear, and heart shapes.

Converter

Diamond Shape

Conversion Direction

Carat Weight

Result

Shape Round Brilliant

Diameter 6.5 mm

Width —

Est. Depth 4.0 mm

Est. Carat 1.00 ct

Round Diamond Size Reference

Industry-average diameters for well-cut round brilliants.

Carat	Diameter (mm)	Depth (mm)

Summary

Convert diamond carat weight to physical millimeter dimensions (and back) for round, princess, oval, emerald, cushion, marquise, pear, and heart shapes.

How it works

Select a diamond shape from the dropdown (round, princess, oval, emerald, cushion, marquise, pear, or heart).
Choose whether to convert carat weight → mm dimensions or mm → carat weight.
Enter the carat weight or diameter in millimeters.
Results appear instantly with a proportional size visualization and depth estimate.
The reference table at the bottom shows standard sizes for common carat weights.

Use cases

Verify that an online listing's stated carat weight matches the listed mm dimensions.
Estimate how large a diamond will look on a ring before purchasing.
Compare two diamonds of different shapes at the same carat weight.
Back-calculate approximate carat weight from a photograph's measured diameter.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu