Carat size chart: diamond mm by shape and how each looks on your finger

A well-cut 1 carat round diamond measures about 6.4 mm across, and a 2 carat about 8.05 mm, going by the averages of excellent cut stones listed for sale. At 1 carat, a typical oval is about 8.1 x 5.4 mm, a pear 9.1 x 5.5 mm, a marquise 10.4 x 5.2 mm and an emerald 6.7 x 5.0 mm, while princess and cushion cuts run 5.5 to 5.8 mm on a side (DiamDB). Your finger changes the look: a 1 carat round covers 43% of the width of a size 4 finger and 35% of a size 8, using standard ring diameters. Shop by the millimeters on the grading report, since carat measures weight.

Carat to mm chart for eight shapes

GIA defines a metric carat as 200 milligrams, so any size chart is a model of how a typical stone of that weight is cut. This one uses DiamDB's models of well-cut stones at the length to width ratio shown for each shape. Sizes are length x width in millimeters. A single number means a round or square stone.

Shape (ratio)	0.5 ct	1 ct	1.5 ct	2 ct	3 ct
Round	5.2	6.5	7.4	8.2	9.4
Oval (1.50)	6.4 x 4.3	8.1 x 5.4	9.3 x 6.2	10.2 x 6.8	11.7 x 7.8
Emerald (1.35)	5.3 x 3.9	6.7 x 5.0	7.7 x 5.7	8.4 x 6.2	9.6 x 7.1
Pear (1.66)	7.2 x 4.3	9.1 x 5.5	10.4 x 6.2	11.4 x 6.9	13.1 x 7.9
Marquise (2.00)	8.2 x 4.1	10.4 x 5.2	11.9 x 6.0	13.1 x 6.5	15.0 x 7.5
Cushion (1.00)	4.6	5.8	6.7	7.3	8.4
Radiant (1.00)	4.6	5.8	6.6	7.3	8.3
Princess (1.00)	4.4	5.5	6.3	6.9	8.0

Width grows much more slowly than weight. Doubling a round from 1 to 2 carats adds about a quarter to its diameter, from 6.40 to 8.05 mm in DiamondScreener's averages, and about 58% to its face-up area.

DiamDB's cushion and radiant rows are square; rectangular radiants run longer and narrower, as the next table shows. Our oval vs round comparison measures how much more surface an oval shows from above.

Real stones measure differently from the charts

The size guides that rank for this search don't agree with each other. The table puts DiamDB's 1 carat figures next to the size guides from VRAI and Brilliant Earth, and next to real stones from Annys, an Australian jeweler that groups GIA and IGI graded stones by face-up size and publishes the median weight for each size, from 630,306 stones in an August 30, 2026 snapshot. The last column shows the size whose median weight is closest to 1 carat. VRAI's ranges are rounded to 0.1 mm.

Shape, 1 ct	DiamDB model	VRAI chart	Brilliant Earth chart	Real stones (median weight)
Round	6.5	6.4 to 6.6	6.3	6.4 (1.01 ct)
Oval	8.1 x 5.4	8.3 to 8.5 x 5.6 to 5.8	8.0 x 5.5	7.9 x 5.6 (1.00 ct)
Emerald	6.7 x 5.0	7.2 to 7.4 x 4.9 to 5.1	7.0 x 4.5	6.5 x 4.7 (1.00 ct)
Pear	9.1 x 5.5	9.1 to 9.3 x 5.5 to 5.7	9.0 x 5.5	8.6 x 5.5 (1.00 ct)
Marquise	10.4 x 5.2	10.4 to 10.6 x 5.0 to 5.2	10.5 x 5.0	10.0 x 5.3 (1.04 ct)
Cushion	5.8	6.2 to 6.4 x 5.9 to 6.1	5.7	5.5, square (1.01 ct)
Radiant	5.8, square	6.6 to 6.8 x 5.2 to 5.4	5.5	7.0 x 5.0 (1.04 ct)
Princess	5.5	5.5 to 5.7	5.5	5.5 (1.04 ct)

Rounds are predictable. Every source puts a 1 carat round between 6.3 and 6.6 mm, and DiamondScreener's 2,271 excellent cut 1 carat rounds averaged 6.40 mm with a standard deviation of 0.05 mm. DiamondScreener notes that many charts overstate diameter for the weight.

Elongated shapes are where charts mislead. Real 1 carat ovals, emeralds and pears run 0.4 to 0.9 mm shorter than VRAI's chart. The real 1 carat oval, at 7.9 x 5.6 mm, has a ratio near 1.41, rounder than the 1.45 to 1.5 the three charts draw.

One outline can also hide a lot of weight. Annys lists a typical range of 0.92 to 1.20 carats for a 7.9 x 5.6 mm oval. GIA explains the cause: a diamond cut too deep carries weight hidden below the girdle that you pay for and won't see once it's set.

How each shape looks on size 4, 6 and 8 fingers

A ring's inside diameter is a fair stand-in for the width of the finger at its base. Angara's ring size chart gives 14.9 mm for a US size 4, 16.5 mm for a 6 and 18.1 mm for an 8. Divide a stone's width by that number and you get the share of the finger it covers from side to side. The table uses Annys's real-stone sizes, each with a median weight within 0.04 carat of 1 or 2 carats, set with the long side running down the finger. Each cell shows 1 ct / 2 ct.

Shape	1 ct size (mm)	2 ct size (mm)	Size 4 (14.9 mm)	Size 6 (16.5 mm)	Size 8 (18.1 mm)
Round	6.4	8.0	43% / 54%	39% / 48%	35% / 44%
Oval	7.9 x 5.6	9.9 x 7.1	38% / 48%	34% / 43%	31% / 39%
Pear	8.6 x 5.5	10.9 x 7.0	37% / 47%	33% / 42%	30% / 39%
Marquise	10.0 x 5.3	12.7 x 6.6	36% / 44%	32% / 40%	29% / 36%
Emerald	6.5 x 4.7	8.4 x 6.0	32% / 40%	28% / 36%	26% / 33%
Radiant	7.0 x 5.0	8.8 x 6.3	34% / 42%	30% / 38%	28% / 35%
Cushion	5.5	7.2	37% / 48%	33% / 44%	30% / 40%
Princess	5.5	6.9	37% / 46%	33% / 42%	30% / 38%

A 2 carat round covers more than half of a size 4 finger. A 2 carat emerald covers 40% of the same finger, less than a 1 carat round does. Elongated shapes put their weight into length: a 1 carat marquise covers less width than a 1 carat round but runs 10.0 mm down the finger, 56% longer than the round is wide.

A size 8 finger is 21% wider than a size 4, so every stone covers about 18% less of it. Most stores show rings on a size 6, which Frank Darling calls the US average, and its rule of thumb is that a 1 carat looks like a 0.90 on a size 8 and a 1.10 on a size 4. Measured by width, the gap is wider. On a size 8 a 1 carat round covers 35%, close to the 36% that an average 0.8 carat round (5.93 mm) covers on a size 6. On a size 4 it covers 43%, which takes a 1.4 carat round (7.15 mm) on a size 6 and a 1.8 carat (7.77 mm) on a size 8, using DiamondScreener's averages.

If you don't know your size, DreamRing's ring sizer gives an approximate one from a ring you already wear or a strip of paper. Have a jeweler confirm it before you order.

Why some shapes look bigger for their weight

GIA's shape guide says marquise diamonds appear larger than other shapes of the same weight, and that ovals, pears, emeralds and rectangular radiants make the finger look longer. It also says princess cuts may look slightly smaller than other square shapes because their deep pavilions hold much of their weight, and that cushions can look smaller for the same reason.

Annys's real stones bear this out. Near 1 carat, princess and cushion cuts measure 5.5 mm on a side, against 6.4 mm for a round.

Depth tells you where the weight went. DiamDB's buying guides put normal depth at 59 to 65% for ovals, pears and marquises, 59 to 70% for cushions and radiants, and 64 to 75% for princess cuts. Above those, more of the stone sits below the girdle.

Decision rules

• Pick a width first, then a carat weight. Choose the share of your finger you want from the table above and multiply by your finger width. 40% of a size 6 is 6.6 mm. Stone pages at Rare Carat and Blue Nile list each diamond's measurements, so you can shortlist by width.
• Size 4 or 5 and wanting 1.5 carats or more: a 1.5 carat round (7.32 mm) covers 49% of a size 4 and 47% of a size 5. If that feels wide, an oval, pear or emerald of the same weight puts the size into length.
• Size 8 or larger: a 1.4 carat round (7.15 mm) covers 40% of a size 8, close to the 39% a 1 carat covers in size 6 store photos.
• Tight budget: buy just under a magic size. GIA names 1.00, 1.50 and 2.00 carats as magic sizes and says stones at or just above them cost more per carat. A 0.90 carat round averages 6.17 mm, 0.23 mm narrower than a 1.00, or 37% of a size 6 finger against 39%.
• Buying a round: lean on the cut grade. GIA graders compare carat weight to diameter as one input to the grade, so a high cut grade doubles as a check on hidden weight.
• Buying a fancy shape: check the numbers yourself. The round is the only shape with a GIA cut grade. Grading reports list a fancy shape's measurements as length x width x depth, so compare the first two against the real-stone sizes above and the depth against the ranges in the previous section. A 1 carat oval well under 7.9 x 5.6 mm is spending its weight somewhere you can't see.