Visual tool
Diamond Size Visualizer
Carat is weight, not size. A 2-carat diamond looks bigger than a 1-carat — but only ~25% wider, not "twice as big." See it side by side.
1.00ct round brilliant · approximately 6.5mm across the top
The numbers above are face-up diameters for a well-cut round brilliant. Other shapes vary:
- · Princess: similar diameter, square outline
- · Oval: ~10% wider, ~15% longer than a round of the same carat
- · Marquise: ~25% longer, ~10% narrower than a round of the same carat
- · Emerald: ~5% wider in length, narrower across the width
Rule of thumb: a 1ct stone fills the gap between two finger knuckles on most hands.
Carat is weight, millimetres are size
A carat is 200 milligrams — a unit of mass, not of dimension. Two stones of identical weight can face up at noticeably different widths because a cutter chooses where the weight goes: deep and heavy below the girdle, or spread wide across the top. The millimetre reading is what a wearer sees, and it is the number to compare against a finger, a hand photo, or another stone.
A well-proportioned round brilliant of one carat measures roughly 6.4 to 6.6 millimetres across. The same weight in an oval or a marquise covers more area because the outline is elongated, while a princess or an emerald concentrates it into a squarer footprint. That is why "which looks bigger" has no single answer: it depends on the outline as much as on the scale.
Weight also has price thresholds attached. Per-carat pricing steps up at half-carat marks, so a 0.98 carat and a 1.01 carat look identical on the hand while crossing a price boundary. Buying just below a threshold — 0.90 or 1.40 — is the standard way to keep the size and lose the jump.
Reading the visualizer
The circle above draws the top-down diameter of a round brilliant at typical proportions. Treat it as a ruler rather than a photograph: pick a carat, read the millimetres, then hold that number against your own finger width or against a ring you already wear. The shape list underneath gives the equivalent numbers for the elongated cuts, which all look longer than a round of the same weight.
Settings change perceived size more than most people expect. A halo adds visible diameter around the stone, a thin band makes the centre look larger by contrast, and an elongated shape reads longer along the finger than a round of the same carat. A taller setting shows more of the stone in profile, which helps in photographs and from the side, but does not change the face-up width at all.
Frequently asked questions
How many millimetres is one carat?
About 6.4 to 6.6 millimetres across for a well-cut round brilliant. Other shapes of the same weight differ: an oval or marquise runs longer, a princess is closer to a round in width but square in outline, and a deep-cut stone can weigh more while measuring narrower.
Is a bigger carat always better?
No. Cut comes first, then face-up size. A stone cut deep enough to hold weight below the girdle looks smaller than a slightly lighter stone that spreads its weight across the top, and the deeper stone usually costs more for the privilege.
Do certain settings make a diamond look bigger?
Yes. A halo adds a border of small stones, a slim band increases the contrast with the centre, and elongated shapes read larger along the finger. None of these change the millimetre measurement; they change how the eye reads it.
Where should I buy just under a price threshold?
The thresholds sit at the half-carat marks. Stones at 0.90, 1.40 and 1.90 carats are the usual targets: the size difference against the next step up is invisible on the hand, while the per-carat price difference is not.