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Diamond carats explained

Various diamond carats

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There are many factors to consider when purchasing a diamond engagement ring, and the diamond carat weight is a major one. 

In this article, we'll explore what a diamond carat is and include some tips on what to look for when choosing a diamond for your engagement ring. 

What is a diamond carat?

In the context of diamonds and gemstones, carats are a unit of weight.

Before 1907, one carat weighed slightly different amounts in different countries, but all were roughly equivalent to the mass of one carob seed, from which the word "carat" is derived. However, in 1907 the metric carat was adopted, with one carat or 1ct set at 0.2g or 200mg. Metric carats are divided into one hundred points, such that a 1ct diamond weighs 100 points.

Weighing a 1.9ct diamond on digital scales
Weighing a 1.9ct diamond on digital scales

Diamonds can be cut and polished to any carat weight, so while you often see them at 0.5ct or 0.7ct or 1ct, you're just as likely to find them at 0.44ct or 0.63ct or 0.87ct.

Confusingly, the term "carat" has a second completely different meaning when used to describe gold (for example, 9ct gold, 18ct gold and 24ct gold). When used in this context, carat is a measure of purity, not weight (24ct gold being pure, 18ct gold being 75% pure, and 9ct gold being 37.5% pure)

The relationship between carats and size

diamond size
Measuring diamond diameter on a diamond gauge

While the weight of a diamond increases with its size, the relationship between carats and size is not straightforward.

Weight is a function of volume. Double the volume of the diamond, and you will double its carat weight. However, generally speaking, a 2ct diamond does not look twice as big as a 1ct diamond.

This is because, when we look at diamonds, we tend to assess the face-up size, and this does not increase in a linear way with volume and carat weight.

For example, a textbook 1ct round brilliant cut diamond is about 6.5mm in diameter, whereas a 2ct round brilliant cut diamond is typically about 8mm in diameter. So although the diamond's weight (and volume) has doubled, the diameter has increased by less than 25%.

Another complication is that some diamond shapes look bigger than others for a given carat weight. A 1ct oval cut diamond, for example, will tend to look significantly larger than a 1ct round brilliant cut diamond. This is because "face-up" ovals tend to have larger surface areas than rounds, which have more of their carat weight in the bottom (pavilion) of the stone.

Likewise, an emerald cut diamond is more shallow than a round diamond, so its remaining carat weight results in a larger crown. The crown is the top of the diamond you see when it's face-up. 

Diamond carat size charts

While two diamonds of the same shape and carat weight rarely have precisely the same dimensions, diamonds end up being cut with reasonably consistent proportions, meaning we can use charts like the one below to get an approximate idea of the dimensions of a diamond at any given carat weight.

Chart of diamond carat sizes by shape

We can use this same principle to provide an indication of how different carat sizes appear on the typical hand. 

Chart of diamond carat sizes on a hand
Chart of diamond carat sizes on a hand

Diamond carats and price

Unsurprisingly, as the carat weight increases, so does the price of the diamond. However, the price does not increase in a linear way. As larger diamonds are both more scarce and desirable than smaller ones, prices increase exponentially with size. This means, for example, that a 2ct diamond is much more than twice the price of a 1ct diamond of the same quality.

Prices also change in a stepped manner, increasing noticeably as you move from 0.49ct to 0.5ct or from 0.89ct to 0.9ct, for example. As you might expect, the particularly strong demand for 1ct diamonds means there is a significant step up in price between 0.99ct and 1ct.

However, for those looking for a deal, this provides an opportunity. Knowing that to the naked eye, a 0.99ct diamond looks identical in size to a 1ct diamond, the savvy buyer can pick up a relative bargain by looking for stones just below one carat, for example.

Does carat size matter?

It is not uncommon to want the biggest diamond you can get with your budget. However, while size matters, we advise our customers against significantly compromising quality in order to get a bigger stone. A slightly smaller diamond of a higher quality will often have a greater "wow factor" than a larger one of lower quality. A 2ct diamond that is discoloured and dull will be less appealing to most people than a 1.5ct diamond that is bright white and sparkles brilliantly.

Loose diamonds

Do lab grown diamonds weigh the same as natural diamonds?

Lab grown diamonds (sometimes referred to as lab created diamonds, cultured diamonds or synthetic diamonds) are real diamonds grown in a laboratory environment. They should not be mistaken for diamond simulants, such as cubic zirconia and moissanite, which are white gemstones that look similar to diamonds but are not the real thing.

Lab grown diamonds have exactly the same chemical, physical, and optical properties as mined diamonds. This means that the carat weight of lab grown diamonds is identical to that of mined diamonds.

Diamond carat buying tips

A diamond engagement ring is a very significant purchase and needs careful consideration. Choosing the right carat weight for the diamond is one of the key decisions you'll need to make. Here are some of our top tips to help you get this right:

  • Consider hand size. A smaller carat weight will appear more prominent on a smaller finger.
  • Consider diamond shape. Some, such as oval and emerald cuts, appear bigger than round brilliant cuts or cushions cuts of the same carat weight.
  • For any given budget, the more you prioritise diamond size, the more you'll need to compromise on quality. Make sure you leave enough in your budget to get a colour, clarity and cut that you're happy with.
  • Consider looking at carat weights just below the round numbers. A 0.95ct carat diamond will look virtually identical in size to a 1ct diamond but will cost quite a lot less.
  • The band size and settings can make a diamond appear bigger or smaller. For example, a thin band with a halo setting can make the central diamond appear larger.
2000 B21 Gem Diamond


Which carat is best for a diamond?

There is no such thing as a best carat weight. It is a personal decision which will be guided by your preferences, priorities, and budget. But, whatever carat weight you decide is right for you, don't neglect the quality of the stone in terms of colour, clarity and cut.

If you don't have a preference for a particular diamond shape, then you may wish to choose an oval or emerald cut. These are often priced slightly lower than round brilliant cuts, meaning you can get a bigger stone for your budget. In addition, they tend to look bigger for any given carat weight than a round diamond.

A trade-off is that oval and emerald cut diamonds show more colour than round diamonds, so you may want to go for a higher colour grade.

What is 1 carat worth?

The price of a diamond depends not only on its carat weight but also on a number of other factors, the most important being colour, clarity and cut. This means that two different 1ct diamonds can have very different values.

How big is a one-carat diamond?

A typical 1ct round brilliant cut diamond is about 6.5mm in diameter. Other diamond shapes (fancy cut diamonds) will have different dimensions.

The bottom line

Carat weight, alongside the colour, clarity and cut, is just one of the four main factors (the 4Cs) to consider when choosing a diamond. We recommend that our clients take a balanced approach across each of these factors in order to find their perfect diamond.

If you're looking to purchase a diamond engagement ring, one of our experienced experts will be delighted to answer any questions and help you source the perfect diamond for your ring.

David Rhode
Together with Tim Ingle, David created Ingle & Rhode to offer a better alternative to the traditional luxury brands. Since 2007, we’ve provided our customers with genuinely ethical engagement rings, wedding rings and fine jewellery – free from conflict diamonds, dirty gold and child labour. With more than 16 years experience in the jewellery industry, David has deep expertise in diamonds, gemstones and jewellery design and manufacturing.