How Big Is a 1 Carat Diamond? Carat to mm by Shape

Carat is the number on the paperwork, but millimetres are what you see on the hand. When people ask how big a 1 carat diamond is, they usually mean: will it look the way I imagine on a real finger, and is 2 carats worth the step up? Here are the answers in millimetres for all eight shapes in our designer.

Carat is weight, not size

A carat is a unit of weight: 1 ct is 0.2 g, so five carats make a gram. It divides into 100 points, so a 0.50 ct stone is a fifty-pointer.

Weight becomes size through the cut: a deep 1.00 ct round can face up at about 6.2 mm, a shallow one at about 6.6 mm with less brightness. Ask for the measurements as well as the carat; our guide to the 4Cs explains why cut matters.

Diamond size chart: carat to mm

Face-up sizes from our designer, length × width in millimetres, length along the finger. Real stones vary with their cut, so read each figure as "about".

Carat	Round	Oval	Cushion	Pear	Marquise	Emerald	Princess	Radiant
0.50 ct	5.2	6.1 × 4.5	4.8	6.7 × 4.4	8.3 × 4.1	5.6 × 4.0	4.4	5.2 × 4.2
0.75 ct	5.9	7.0 × 5.2	5.5	7.7 × 5.0	9.5 × 4.7	6.4 × 4.5	5.0	5.9 × 4.8
1.00 ct	6.5	7.7 × 5.7	6.0	8.5 × 5.5	10.5 × 5.2	7.0 × 5.0	5.5	6.5 × 5.3
1.50 ct	7.4	8.8 × 6.5	6.9	9.7 × 6.3	12.0 × 6.0	8.0 × 5.7	6.3	7.4 × 6.1
2.00 ct	8.2	9.7 × 7.2	7.6	10.7 × 6.9	13.2 × 6.6	8.8 × 6.3	6.9	8.2 × 6.7
3.00 ct	9.4	11.1 × 8.2	8.7	12.3 × 7.9	15.1 × 7.5	10.1 × 7.2	7.9	9.4 × 7.6

The designer covers every weight from 0.30 to 3.00 ct in steps of 0.05 ct.

Why 2 carats is not twice the size of 1

A diamond grows in length, width and depth at once, so each dimension follows the cube root of the weight. Double the carat and the stone gets about 26% longer and wider: a round goes from 6.5 mm at 1 ct to 8.2 mm at 2 ct and 9.4 mm at 3 ct. In face-up area, 2 ct covers about 59% more of the finger than 1 ct.

The rule also works in your favour: a 0.90 ct round is about 6.3 mm, only 0.2 mm smaller than a 1.00 ct. Prices across the market tend to jump at weights such as 1.00 and 1.50 ct, so ask for a quote on both.

Why elongated shapes look bigger per carat

Much of how big a stone looks comes from its length along the finger, so a longer outline reads as bigger.

• Marquise. About 10.5 mm long at 1 ct, 4 mm longer than a round.
• Pear. About 8.5 × 5.5 mm, roughly 30% longer.
• Oval. About 7.7 × 5.7 mm, 18% longer.
• Emerald and radiant. About 7.0 × 5.0 and 6.5 × 5.3 mm: close to a round in length, but narrower.
• Cushion and princess. The shortest along the finger, about 6.0 and 5.5 mm, because more of their weight sits in their depth and corners.

Try a 1 ct oval solitaire in yellow gold, then switch shapes at the same weight. Our diamond shapes guide covers how each one wears.

Your finger changes the picture

A stone looks big or small against the finger under it. A 6.5 mm round covers about 39% of the width of a UAE size 52 finger (about 16.5 mm inside diameter, close to a US 6), about 44% of a size 46 and about 33% of a size 62.

So a 1 ct round looks as generous on a size 52 as a 1.3 ct does on a size 57 (about 18.1 mm, close to a US 8). Unsure of the size? Our ring size guide shows how to measure.

The setting changes it too

• Solitaire. The stone alone, close to the chart.
• Halo. In our designer, the frame of small diamonds takes a 1.00 ct round from about 6.5 mm to about 8.8 mm across, the width of a 2.50 ct round on its own. Real halos vary.
• Three Stone. Side stones add width across the finger.
• Pavé and Marquise Shoulders. Diamonds on the band, or the eight marquise diamonds on the shoulders of our signature setting, add sparkle without changing the centre stone.

Compare a 1 ct round solitaire with the same stone in a halo. Our settings guide covers the trade-offs.

See it on a hand before you choose

The designer draws the ring at true scale on a UAE size 52 finger, in four skin tones. On a slimmer finger the stone will look a little larger than on screen, and on a broader one a little smaller. Add your ring size before you send the design, or choose "Not sure yet" if you do not know it.

Questions we hear at the counter

How big is a 1 carat diamond? About 6.5 mm across for a round, 7.7 × 5.7 mm for an oval and 10.5 × 5.2 mm for a marquise.

How much bigger does 2 carats look than 1? About 26% longer and wider, with about 59% more face-up area: clearly bigger, but not double.

Is 1 carat big enough? There is no correct size. On a slim finger a 0.75 ct round, about 5.9 mm, looks generous; on a broad hand, an oval or pear adds length at the same weight.

Does lab-grown or natural change the size? No. Cut to the same proportions, a lab-grown and a natural diamond of the same weight face up at the same size.

At Mazimo. Design the ring on a hand at true size in our ring designer and send it to us on WhatsApp for the price, or come and see stones and settings at Al Manal Center, Deira, a few minutes' walk from the Gold Souk. Every ring is made in 18K gold, hallmarked in Dubai, and delivered insured across the UAE at no charge.