Carat Weight vs Diamond Size: What You Actually See

When people ask how big a diamond is, they usually answer with its carat. Carat is a unit of weight, not of size. Understanding the difference helps you get the look you want for your budget.

What a carat is

One carat equals 0.2 gram. Weight is divided into 100 points, so a 50-point diamond weighs half a carat. The word carat in diamonds is unrelated to the karat that measures gold purity. See tola, gram and masha for more on units.

How weight relates to size

For a well-cut round diamond, the diameter seen from above is roughly:

• 0.25 carat: about 4.1 mm
• 0.50 carat: about 5.1 mm
• 0.75 carat: about 5.8 mm
• 1.00 carat: about 6.5 mm

Weight rises faster than diameter. Doubling the weight does not double the width. Going from 0.5 to 1 carat adds about 1.4 mm in width.

Why two stones of one carat can look different

• Shape: an oval or marquise of one carat has a larger face-up area than a round of the same weight, because its weight is spread over a longer outline.
• Cut: a stone cut deep holds weight in its lower part and looks smaller from above. A well-proportioned stone shows more of its weight in its face.
• Setting: thin prongs, a halo or a bezel change how large a stone looks.

The measurements on the certificate tell you the stone's size in millimetres. See how to read a diamond certificate.

The magic numbers

Prices rise sharply at whole and half carat marks, such as 0.50 and 1.00, because demand is high at these weights. A stone of 0.95 carat can look almost the same size as a 1.00 carat stone yet be priced noticeably lower. Whether that matters to you is personal, but it is a well-known way to spend less for a similar look.

Total carat weight in jewellery

In a piece with several stones, such as a pendant with a cluster or earrings, the weight listed is often the total carat weight of all the stones together. A pair of 0.25-carat studs is 0.50 carat total, but each stone is the size of a quarter-carat. See total carat weight in diamond jewellery.

Choosing the size

• Think about the hand or ear it will be worn on, and the clothes you wear.
• Compare size in millimetres, not only carat.
• Weigh the trade-off between size and quality, as covered in how to set a diamond jewellery budget.

Common questions

Is a bigger diamond always better? Not if it is poorly cut. A well-cut smaller stone can look more brilliant than a larger one with weak proportions.

What is a carat in grams? One carat is 0.2 gram, so five carats weigh one gram.

Why do stated sizes in photos look so large? Close-up images magnify the stone. Always check the millimetre measurements.

See it for yourself

A diamond looks different in a photograph than on a hand. You can book a visit to compare sizes side by side, or browse our diamond earrings where the stone weights are listed on each page.