Diamond carat is one of the most misunderstood terms in jewellery. Most people assume it measures size — how big the diamond looks. It doesn't. Carat measures weight. And the difference between those two things is significant when you're trying to buy a diamond that looks right on the ring.
What Is a Carat?
One carat equals 0.2 grams, or 200 milligrams. The term "carat" comes from the carob seed, which was historically used as a counterweight in scales because of its remarkably consistent weight. The unit was standardised internationally in 1907.
Diamonds are also measured in "points" — 100 points equals one carat. So a 0.75ct diamond is sometimes described as "75 points," and a 0.50ct is "50 points" or "half a carat."
Carat vs Size: The Critical Distinction
Two diamonds of identical carat weight can look noticeably different in size, depending on their cut proportions and shape. A round brilliant with a very deep cut concentrates weight in the depth of the stone — it weighs the same as a shallower stone, but looks smaller when viewed from above. A well-cut round brilliant with optimal proportions returns more of its weight to the face-up appearance.
Shape also affects face-up size significantly. An oval diamond appears about 10–15% larger face-up than a round of the same carat weight, because its elongated silhouette covers more surface area of the finger. A pear, marquise, or emerald cut can look even larger. This is one reason elongated shapes are so popular — you get more visible size per dollar spent.
How carat works with the other three Cs
Carat is only one of the 4Cs — cut, colour, clarity, and carat. It's tempting to chase the biggest number, but a larger stone with a poor cut can look duller and smaller than a slightly lighter, well-cut one. For most buyers we recommend fixing your budget, prioritising cut quality first, then choosing the largest carat weight that still leaves room for a good cut. A 1.3ct stone that sparkles beats a lifeless 1.5ct every time.
Common Carat Sizes and What They Look Like
A 0.5ct round brilliant is approximately 5.1mm in diameter. A 1.0ct round is approximately 6.4mm. A 1.5ct round is approximately 7.3mm. These measurements give you a reference point — and when you realise an oval of the same carat weight will measure 7–8mm in length, the size difference becomes tangible.
How many carats is the average engagement ring?
In Australia, the average engagement ring centre stone sits around 1.0 to 1.2 carats, though there is no "right" size — it comes down to the wearer's hand, style, and budget. Because lab-grown diamonds cost far less than mined stones, many couples now choose 1.5ct or larger for the same spend they'd once have put towards well under a carat.
Lab-Grown Diamonds and Carat Value
With lab-grown diamonds costing 50–70% less than natural equivalents, your budget stretches considerably further in terms of carat weight. A $5,000 budget that might buy a 0.8ct natural diamond can often buy a 1.5ct lab-grown — meaning significantly more visible size without more spending.
Browse our lab-grown diamond solitaire rings or learn more about diamond quality before you start shopping.
Browse Una Diamond Rings by Shape
Certain shapes carry their carat weight particularly well. Oval and marquise cuts, for example, look noticeably larger than round brilliant diamonds of the same carat. See them for yourself:
- The Georgina — oval cut (excellent finger coverage)
- The Marina — marquise cut (the most elongating shape)
- The Celina — east-west emerald cut
- The Nova — pear and emerald toi et moi
- Oval cut rings
- Marquise rings
- Browse all ready-to-ship rings →