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How big is a one carat diamond? Our diamond sizing guide (Ct to mm)

No two diamonds are exactly alike, even in ways you might not expect. These subtle differences can make knowing what size diamond to use in your ring a little tricky, but we’re here to help.

Choosing a diamond that’s the perfect size for your engagement ring is one of the most important decisions you’ll make in the ring design process. So we’ve made a simple comparison chart to help you get a better picture of what you can expect from your diamond.

This is what a round brilliant diamond looks like next to a standard sized pencil, ranging from 0.60ct to 3ct. We’ve also included a diamond size conversion from carat to mm (carats to millimetres).

While this chart only covers round brilliant diamonds, it can be useful for visualising the size of the other fancy diamond cuts as well.

Notice that a 0.60 carat round diamond is approximately 5.35mm in diameter, while a 1 carat diamond is 6.4mm in diameter.

So, even though the 1 carat diamond is almost double the carat weight of the 0.60 carat diamond, it’s only about 20% larger when viewed from the top-down. The same goes for other diamond shapes. But why is that? What is a ‘carat’ anyways?

What exactly is a ‘carat’?

Diamonds are measured in ‘carats’. Rather than a measurement of the diamond’s width, length, or scale, carats measure how much a diamond weighs. We use this unit of measurement because each diamond has an ever so slightly different cut (and therefore proportions) than every other diamond—even a diamond of the same shape. 

And when you’re dealing with diamonds of completely different shapes—such as a cushion diamond vs a princess diamond, or a marquise vs a round—when it comes to dimension, they’ll appear totally different. You can learn more about diamond shapes here.

As such, we measure diamonds according to their weight in order to get an accurate idea of how big they really are. Because two diamonds with the same top-down surface area won’t always have the same mass. One diamond may be deeper than the other, or one might have a cut that makes it bottom- or top-heavy when viewed from the side. 

But a diamond’s value and character is about so much more than just its size! We always recommend checking out our Diamond Basics page to get a full breakdown of what you need to know when it comes to technical diamond knowledge and ring design.

carat weight

Try our printable diamond carat and band size guide

Want to see what different carat weights, diamond shapes, and band widths look like on your hand? Download and print this PDF and you’ll be able to cut out to-scale outlines of virtually every diamond size and shape. You can mix and match them on different ring band sizes to get a better idea of what your ring will look like in real life.

What is the average size of an engagement ring diamond centre?

A one carat diamond is often seen as a benchmark diamond size when it comes to engagement rings. But in truth, there’s no concrete reason why one carat is the best size for an engagement ring. We always let our customers know that because of the reputation it’s garnered over the years, a one carat diamond will actually fetch a cost premium due to its popularity. So we recommend thinking hard about the other aspects of your overall design and then picking a diamond size that serves your ring the best.

We hope our carat weight sizing guide makes it easier than ever to shop for a diamond engagement ring online, but of course our team of diamond experts is here to help you find your perfect diamond or gemstone.

Our team is on hand to search through a portfolio of thousands of diamonds we’ve sourced from our suppliers around the world. With some simple guidance, we’ll make it easy for you to find ‘the one’.

If you want to cut out the guesswork and choose the best diamond for your design, your price point, and your love, then get in touch with our team today.

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