Carat is the measure people ask about most and understand least, because it sounds like size when it is really weight. This guide explains what a carat is, how big a 1 carat diamond actually looks, and why two diamonds of the very same carat can look like different sizes on the hand.
Carat is a diamond's weight, not its size. One carat is 0.2 grams, and a well cut 1 carat round diamond measures about 6.5mm across the top. Because carat measures weight, two diamonds of the same carat can look different sizes depending on their shape and how they are cut.
What is a carat?
A carat is the unit used to weigh diamonds and gemstones, and one carat is exactly 0.2 grams, or 200 milligrams. It is written as ct, and it is divided into 100 points, so a half carat diamond is 50 points, or 0.50ct, and a quarter carat is 25 points. The word comes from the carob seed, which was once used as a small, even counterweight on a jeweller's scale before the measure was standardised. The key thing to hold onto is that a carat tells you what a diamond weighs, not how big it looks.
How big is a 1 carat diamond?
A well cut 1 carat round diamond measures about 6.5mm across the top, a little wider than a pea and close to the width of a standard pencil eraser. That is the face-up size, the part you actually see once it is set. The exact figure moves a little with the cut, so a shallower stone can spread to around 6.6mm while a deeper one shows closer to 6.3mm even though both weigh a full carat. In other shapes a 1 carat diamond looks quite different again, which we come to below.
Diamond size chart
This chart gives the approximate face-up width of a round brilliant diamond at common carat weights. The figures are averages for a well cut stone and are a reliable guide rather than an exact rule.
| Carat | Round diameter (approx.) |
|---|---|
| 0.25ct | 4.1mm |
| 0.50ct | 5.2mm |
| 0.75ct | 5.8mm |
| 1.00ct | 6.5mm |
| 1.50ct | 7.4mm |
| 2.00ct | 8.1mm |
Why weight and size are not the same thing
Because carat is weight, a heavier diamond is not automatically a larger looking one, and this catches a lot of people out. Two things break the link between the number and the look.
The first is cut. A diamond cut too deep carries some of its weight hidden below the surface, where it adds carats but nothing to the face-up size, so it can weigh a full carat yet look smaller than a well cut stone of the same weight. A well cut diamond puts its weight where you can see it, which is one more reason cut is worth prioritising, as our guide to the 4Cs explains.
The second is that weight and size do not rise together. Doubling a round diamond from 1 carat to 2 carats does not make it look twice as big. It makes it only about 27 percent wider, going from roughly 6.5mm to 8.1mm, and around 60 percent larger in face-up area. This is why a 2 carat diamond costs far more than twice a 1 carat but does not look twice the size, and why stepping just under a round number, say 0.90ct instead of 1.00ct, can save a good deal for a difference you can barely see.
How size changes by shape
Shape changes how large a diamond looks at the same weight, because each shape spreads its carat differently across the top. Elongated shapes carry more of their weight where you see it, so they look larger than a round or a square of the same carat. The chart below shows the approximate dimensions of a 1 carat diamond in each shape.
| Shape (1 carat) | Approx. dimensions |
|---|---|
| Round | 6.5mm |
| Princess | 5.5mm |
| Cushion | 5.5mm |
| Oval | 7.7 x 5.7mm |
| Emerald | 7.0 x 5.0mm |
| Pear | 8.0 x 5.5mm |
| Marquise | 10.0 x 5.0mm |
You can see it clearly in the numbers. A 1 carat marquise stretches to about 10mm long, while a 1 carat princess sits nearer 5.5mm across, yet both weigh exactly the same. If looking larger for the carat matters to you, an elongated shape does more with the same weight. Our guide to diamond shapes covers the character of each.
What carat size should you choose?
The right carat is the one that looks right to you and sits within your budget, not a number to chase for its own sake. It helps to think about where the piece will be worn. For diamond studs worn every day, around half a carat each is the most popular size, present without being showy, while a full carat each makes more of a statement. For a pendant that sits quietly at the collarbone, a smaller stone often looks best, and for a piece meant to catch the eye, more carat earns its place.
Choose the look rather than the number. Because shape and cut change how large a diamond appears, a well cut elongated stone can look bigger than a heavier one that is cut too deep. Buying just under a round weight, like 0.90ct rather than 1.00ct, also stretches the budget for a size difference the eye cannot really see.
Once you have a feel for size, the next step is balancing it against the other qualities, which is covered in how to choose a diamond, part of our wider guide to diamonds. You can see everyday carat sizes at work in our diamond studs, and a run of matched stones in a diamond tennis bracelet.
