Two stones of identical weight can differ visibly in size. Use this to compare shapes at a fixed weight, or weights at a fixed apparent size.
Diamonds
Carat to Millimetre Estimator
Carat is weight, not size. This estimator converts a carat weight into the approximate face-up diameter for each common shape, so you can compare what you will actually see.
Estimates assume well-proportioned cutting. A deeply cut stone of the same weight will look smaller face up, and a shallow one larger — sometimes by a noticeable margin.
How to use it
- Enter the carat weight you are considering.
- Choose the shape, or compare several at once.
- Read the approximate face-up measurement in millimetres.
Before relying on the result
- Assumes standard proportions; individual stones vary.
- Elongated shapes look larger than round stones of the same weight.
- Does not account for setting, which changes apparent size considerably.
How the tool works
Each shape has a typical weight-to-diameter relationship at standard proportions. The estimator applies that relationship and reports a range rather than a single figure.
Data & methodology
- Data version
- PLACEHOLDER v0
- Conversion data status
- Active
- Last reviewed
- July 14, 2026
- Status last refreshed
- July 14, 2026
Methodology
PLACEHOLDER METHODOLOGY — replace with the real basis for the size tables and the date checked.
Frequently asked questions
Why do two one-carat stones look different sizes?
Because carat measures weight. A stone cut with a deep pavilion carries more of its weight below the girdle, where you cannot see it, so it looks smaller face up than a well-proportioned stone of the same weight.
Which shape looks largest for its weight?
Elongated shapes — oval, marquise, pear — generally look larger face up than a round stone of the same weight, because more of the material sits in the visible outline.