Carat to mm Size Finder
Quick answer
A 1 carat round diamond is about 6.5 mm across, a 2 carat about 8.1 mm. The same weight in a denser stone looks smaller: a 2 ct round sapphire is about 7.7 mm. Pick a material, shape and weight below for any other size.
Assumes typical depth and outline proportions for each shape. Data verified .
Find the size of a stone from its carat weight
What this size finder does
This tool answers the shopper's question: "How big is it?" You give a carat weight and it works out the face-up size in millimetres and inches, then draws the outline to scale next to a ruler and a 1 ct diamond for comparison.
It runs the weight formula backwards. Weight equals size times depth times density times a shape factor, so if you fix the depth as a percent of width and the length to width ratio, there is exactly one size that fits. The tool shows those assumptions and lets you change them.
The two assumptions that matter
Depth percent
A deep stone carries more of its weight underneath, where you cannot see it. A shallow stone spreads the same weight across a wider face. For a 1 ct round diamond, moving from 65% depth to 58% depth takes the diameter from 6.3 mm to 6.6 mm. That is the difference between a stone that "faces up small" and one that "spreads".
Length to width ratio
Elongated shapes can be long and slim or short and wide at the same weight. A 1 ct oval at a 1.30 ratio measures about 7.6 x 5.8 mm. At 1.50 it measures about 8.3 x 5.5 mm. Listings often give the ratio, and you can type it in.
How to use it when shopping
- Enter the weight from the listing. Make sure it is the weight of one stone, not total carat weight for the whole piece.
- Pick the material and shape. If the listing gives exact measurements, you do not need this tool: put them into the mm to carat calculator and check the stated weight is believable.
- Compare the result with stones you already own. Measuring a ring you wear every day is the easiest way to feel what "6.5 mm" really means.
Reading the result
The main figure is the most likely face-up size. The range below it comes from the same uncertainty the weight estimator uses, turned into millimetres. Because size depends on the cube root of weight, a 10% uncertainty in weight becomes only about a 3% uncertainty in diameter. That is why size ranges look narrow next to weight ranges.
The to-scale drawing uses real millimetres in the SVG, so shapes are correct relative to each other and to the ruler. The size on your screen depends on your display, so use the ruler, not your screen, as the reference.
When the estimate will be off
- Native or precision cut colored stones. Many colored gems are cut to save weight, with deep or lumpy pavilions. They face up smaller than the estimate.
- Very shallow stones. Some stones are cut thin to look big. They face up larger than the estimate but show "windows" where light leaks through.
- Moissanite sold by DEW. A "1 ct" moissanite is usually the size of a 1 ct diamond. Use the DEW calculator instead.
For quick reference, see the generated diamond size chart, the moissanite size chart, and charts for sapphire, ruby and emerald.
Related questions
How many mm is a 1 carat diamond?
A well-cut 1 ct round diamond is about 6.5 mm across at a typical 61% depth. Fancy shapes differ: a 1 ct oval is about 8.0 x 5.7 mm and a 1 ct emerald cut about 7.1 x 5.0 mm.
How big is a 2 carat diamond in mm?
About 8.1 mm for a round brilliant at typical depth. It looks larger than a 1 ct stone, but not twice as wide: diameter grows with the cube root of weight.
How big is a 2 carat sapphire?
A 2 ct round sapphire is about 7.7 mm, smaller than a 2 ct diamond at 8.1 mm, because sapphire is denser. Ovals and cushions are the most common sapphire shapes, and the calculator above covers both.
How many mm is a half carat diamond?
A 0.50 ct round diamond is about 5.1 mm across at typical proportions.
Is a 1 carat emerald bigger than a 1 carat diamond?
Yes. Emerald is lighter (SG about 2.72), so a 1 ct round emerald is about 6.9 mm against 6.5 mm for a diamond. Emeralds are usually step cut, which changes the outline again.
Why is a 1 carat CZ so small?
CZ is about 65% denser than diamond, so 1 ct of it takes up less space: about 5.4 mm round, roughly 15% smaller in diameter than a 1 ct diamond.
What depth percent should I use?
Use the default unless you know better. Round brilliants are usually 58 to 65% of their width deep, ovals and pears similar, cushions deeper at 65 to 75%, and step cuts 55 to 68%. A listing that gives total depth percent can be entered directly.
Does a 1 carat stone look the same size in every setting?
The stone is the same size, but the look changes. Thin prongs show more of the girdle, a halo of small stones makes the center look bigger, and a thick bezel can hide up to half a millimetre around the edge.
Sources and method
Weight estimates use the colored-stone shape factors published by the International Gem Society and the Dendritics standard formula, and the diamond trade constants and length to width ratio tables published in jewelry trade references (Esslinger, Omni, Octonus). Specific gravity values are cross-checked against the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect.
Every number on this page is generated by the same formula engine as the calculators. Read the full method, constants and limits and the editorial policy. Data last verified 2026-11-04.
Estimates only. This is not an appraisal, grading report or valuation. A calibrated scale or a lab report is the only real weight.