SearchDiamond size chart: carat to millimeters for every shape, on a finger and in the ear
From the counter at TraxNYC, 47th Street.
The short answer
- Carat is weight, not size. One carat is 0.2 grams. A 1 carat round is about 6.5 mm across, 2 carats about 8.2 mm, 3 carats about 9.4 mm.
- Twice the weight is not twice the width. A 2 carat round is only about a quarter wider than a 1 carat, because the weight also goes into the depth.
- Shape changes the size you see. At 1 carat an oval is about 8 × 6 mm and a marquise about 10.5 × 5 mm; both look bigger than a 6.5 mm round.
- On the hand: on a size 6 finger a 1 carat round covers about two fifths of the finger's width, a 3 carat more than half.
Between two sizes? Talk to a personal jeweler: call 212-872-9692 or text us, seven days.
Diamond size chart for round diamonds, carat to mm
| Carat | Width face up |
|---|---|
| 0.25 ct | about 4.1 mm |
| 0.5 ct | about 5.2 mm |
| 0.75 ct | about 5.9 mm |
| 1 ct | about 6.5 mm |
| 1.25 ct | about 7.0 mm |
| 1.5 ct | about 7.4 mm |
| 2 ct | about 8.2 mm |
| 2.5 ct | about 8.8 mm |
| 3 ct | about 9.4 mm |
| 4 ct | about 10.3 mm |
| 5 ct | about 11.1 mm |
A well-cut round brilliant, face up. The 0.25, 0.5, 1, 1.5, 2 and 3 carat widths are IGI's size chart; the rest come from the trade's weight formula for a round (width × width × depth × 0.0061) at a 60 percent depth, which gives IGI's widths.
Round diamonds by carat, drawn to one scale
Carat size chart for every shape
| Shape | 0.5 ct | 1 ct | 1.5 ct | 2 ct | 3 ct |
|---|---|---|---|---|---|
| Round | 5.2 | 6.5 | 7.4 | 8.2 | 9.4 |
| Oval | 6.5 × 4.5 | 8 × 6 | 9 × 6.5 | 10 × 7.5 | 11.5 × 8.5 |
| Pear | 7 × 4.5 | 8.5 × 5.5 | 10 × 6.5 | 11 × 7 | 12.5 × 8 |
| Marquise | 8.5 × 4 | 10.5 × 5 | 12 × 6 | 13 × 6.5 | 15 × 7.5 |
| Emerald | 5.5 × 4 | 7 × 5 | 8 × 5.5 | 9 × 6 | 10 × 7 |
| Radiant | 5.5 × 4 | 6.5 × 5.5 | 7.5 × 6 | 8.5 × 6.5 | 9.5 × 7.5 |
| Princess, cushion, Asscher | 4.5 | 5.5 | 6.5 | 7 | 8 |
| Heart | 5 | 6.5 | 7.5 | 8.5 | 9.5 |
About, in mm, length by width: the trade's weight formula for each shape at a typical outline, rounded to the half millimeter. A longer or deeper stone reads differently; a listing's own measurements settle it.
Every shape at 1 carat, drawn to one scale
That is why a long shape looks like more diamond for the carat, while a princess or a cushion sits more compact than a round.
How big a diamond looks on a finger
A size 6 ring finger is about 16.5 mm across, the inside of a size 6 ring; a size 8 is about 18.1 mm.
Round diamonds on a size 6 finger
A 1 carat round covers about two fifths of that width, a 2 carat about half, a 3 carat more than half. On a larger finger the same stone reads a little smaller. Long shapes set along the finger cover more of it: a 1 carat marquise runs about 10.5 mm.
The median center in our engagement rings weighs about 2 carats, counted September 28, 2026. For one stone on a plain band, see our solitaire rings, and for 1 carat in lab-grown or natural, 1 carat engagement rings.
How big a diamond looks in the ear
On a stud listing the carat is often for the pair: 2 Ctw means 1 carat in each ear, about 6.5 mm across. A quarter carat each, about 4.1 mm, is a size you forget you have on; 1 carat or more in each ear shows in conversation.
Our diamond stud earrings guide draws each size on an ear. Shop diamond stud earrings, women's studs or men's diamond studs.
Same carat, different size
Two stones of the same weight can face up at different widths: IGI measured three 1 carat rounds cut to different depths at 6.25, 6.45 and 6.65 mm. A shallow stone looks wider but can leak light; a deep one hides weight underneath.
Read the center carat, not only the total: total carat weight adds every stone, so a halo raises it without changing the center.
Talk to a personal jeweler
Ask to see two sizes side by side before you choose.
Our personal jewelers on 47th Street answer by phone and text seven days, and can hold a stone to a ruler on video.
Questions we hear at the counter
How big is a 1 carat diamond in mm?
About 6.5 mm across for a round. At the same weight an oval is about 8 × 6 mm, a pear about 8.5 × 5.5, a marquise about 10.5 × 5, and a princess about 5.5 mm square.
How many mm is a 2 carat diamond?
About 8.2 mm across for a round, about a quarter wider than a 1 carat; a 2 carat oval is about 10 × 7.5 mm.
How big is a 3 carat diamond?
About 9.4 mm across for a round, more than half the width of a size 6 finger. A 3 carat oval is about 11.5 × 8.5 mm.
Does a higher carat always look bigger?
Heavier, yes. Bigger face up, usually, but the cut depth and the shape move it: IGI measured 1 carat rounds from 6.25 to 6.65 mm.
Keep reading
More answers from the counter: all TraxNYC buyer's guides.
Still deciding
See the two sizes you are between, at the counter or on video.
We are at 64 W 47th St, Booth F-1, New York, NY 10036, on 47th Street since 2004. Monday to Friday 11 am to 6 pm ET, Saturday and Sunday 12 pm to 4 pm ET; around a holiday, text first.
Text or call 212-872-9692, seven days, read our showroom page first, or go straight to engagement rings.