Tool
How big is a one carat diamond?
Carats are weight, not size. Convert between the two for eleven shapes, and see how much of the face-up size a shape gives back.
The reference shape. Everything else is compared to it.
What is on the certificate or the receipt.
Measures about
An estimate for a normally cut stone. Two stones of the same weight can differ by a few tenths of a millimetre depending on how deep they are cut, so treat this as close rather than exact.
Carat
1.00 ct
100 points
Face-up size
6.44 × 6.44 mm
Depth
3.96 mm
Area seen
32.5 mm²
looking down at it
At this weight, drawn to scale
All eleven shapes at 1.00 ct, on the same scale. This is the comparison retailers’ shape charts never show, because every stone in those is drawn at whatever size fits the grid.
Round brilliant
6.4 × 6.4
Oval
8.0 × 5.7
Princess
5.5 × 5.5
Cushion
6.0 × 5.5
Emerald cut
7.2 × 5.1
Asscher
5.7 × 5.7
Radiant
6.7 × 5.2
Pear
8.7 × 5.6
Marquise
10.4 × 5.3
Heart
6.6 × 6.6
Trillion
7.4 × 7.4
Why doubling the carats does not double the size
0.25 ct
4.1 mm wide
0.5 ct
5.1 mm wide
1 ct
6.4 mm wide
2 ct
8.1 mm wide
3 ct
9.3 mm wide
The arithmetic behind that row
Weight grows with the cube of the size. Going from 1 ct to 2 ct multiplies the weight by two and the width by the cube root of two, which is about 1.26 — so a stone that costs a great deal more looks around a quarter wider. It is the single most useful thing to know before comparing two quotes.
Next
Weigh it, pick the karat, and see the value of the metal at the live gold price — then what a buyer is likely to hand over, which is a different number.
Sterling, coin, Britannia and fine silver at the live silver price — including why small silver items are often worth less than the trouble of selling them.
PT950, PT900 and PD950 at live prices. Both metals are denser than gold, which is why the weight surprises people.