TL;DR: Two diamonds of the same carat weight can have noticeably different millimetre dimensions depending on cut depth and shape, meaning carat alone doesn't tell you how large a stone will actually look on the finger. Comparing the length and width in millimetres gives a far more reliable sense of face-up size.
Why Carat Weight Alone Is an Incomplete Measure
Carat measures weight, not size — and because diamonds of the same weight can be cut to different depths and proportions, two stones with an identical carat figure can present quite differently on the finger. Relying on carat weight alone to imagine how a stone will look is a genuinely common source of surprise when a stone arrives.
How Depth Affects Face-Up Size
A diamond cut too deep carries more of its weight low down, away from where it's visible from above, meaning it can look smaller face-up than a shallower-cut stone of the same carat weight. This is exactly why cut quality matters more than carat weight alone — a well-proportioned stone shows more of its weight where it's actually visible.
Why Shape Changes the Comparison Entirely
Elongated shapes — oval, marquise, pear — spread the same carat weight across a larger visible surface area than a round brilliant does, meaning a 1ct oval will generally measure larger in millimetres than a 1ct round. Comparing carat weight across different shapes without accounting for this gives a genuinely misleading impression of relative size.
Always Ask for Length and Width in Millimetres
Rather than relying on carat weight to imagine size, ask for the specific length and width measurements (and length-to-width ratio for fancy shapes) from the certificate. This gives you an actual, comparable figure rather than an assumption based on weight alone.
Using This When Comparing Stones
If you're deciding between two stones — even the same shape and carat weight — comparing their exact millimetre dimensions alongside the carat figure tells you which will genuinely look larger face-up, rather than assuming identical carat weight means identical visual size.
Choosing by Real Dimensions at À Vie Diamonds
We provide exact millimetre measurements for every stone we shortlist, not just carat weight. Get in touch to discuss stone options based on real dimensions.
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Frequently Asked Questions
Why do two diamonds of the same carat weight look different sizes? Cut depth and shape both affect how much of the stone's weight is visible from above — a deeper cut or a different shape can produce a genuinely different face-up size at the same carat weight.
What should I ask for instead of just carat weight? The exact length and width in millimetres from the certificate, which gives a far more reliable sense of actual visible size than carat weight alone.
Last reviewed: July 2026 · À Vie Diamonds