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Diamond Size Chart: Carat Weight Buying Guide (MM to Carat)

Bottom Line Recommendation:

Choose the lowest color and clarity that still look great to the naked eye, and then choose the largest diamond carat weight that still fits within your budget. Some salespeople might try to push “only the best” on you. Keep in mind that if you follow that route, you may end up with a diamond like this one from Blue Nile for your $5,000 budget. Yet this gorgeous diamond from Blue Nile will look just as perfect as the previous diamond, yet it’s 25% larger for the same price.

Another issue we will address is the law of diminishing returns. For example you may think this diamond from Blue Nile is a great deal compared to this diamond from Blue Nile. After all, it’s 4% larger and only $100 more. In reality, there is no person on earth who can tell the difference in size between these two diamonds without using a scale. It’s $100 down the drain. 

You are best off asking one of our experts to guide you through this process. They will be able to help you find the balance between “getting the biggest diamond” and “not wasting money to gain carat size that won’t be noticeable.” You can also refer to our diamond size chart below.

Always keep in mind the cut quality, since a poorly cut heavier diamond can look smaller than an excellently cut lighter diamond. For example, this 1 carat diamond from Blue Nile has a terrible cut with a 66.7% depth. Its diameter is only about 6.10mm. On the other hand, this beautifully cut 1 carat diamond from Blue Nile has an Ideal Cut with an easily noticeably larger diameter of 6.5mm. 

Diamond Size Chart: MM to Carat Weight

A diamond size chart can help you determine what might be the right carat weight for you by providing a mm to carat comparison. The diamond size chart is meant to be a reference point, as diameters of diamonds vary based on cut quality. 

In the diamond size chart below, we display the diamond’s diameter mm to carat weight based on averages of round brilliant diamonds. For example, a round diamond’s diameter of 9.00mm to carat is 2.75ct on average.

Round MM SizeRound Carat Weight
4.0mm diameter0.25 ct
5.1mm diameter0.5 ct
5.8mm diameter0.75 ct
6.4mm diameter1.0 ct
6.9mm diameter1.25 ct
7.3mm diameter1.5 ct
7.7mm diameter1.75 ct
8.1mm diameter2.0 ct
8.5mm diameter2.25 ct
8.9mm diameter2.5 ct
Round MM SizeRound Carat Weight
9.0mm diameter2.75 ct
9.3mm diameter3.0 ct
9.5mm diameter3.25 ct
9.75mm diameter3.5 ct
9.9mm diameter3.75 ct
10.1mm diameter4.0 ct
10.43mm diameter4.25 ct
10.63mm diameter4.5 ct
10.83mm diameter4.75 ct
11.01mm diameter5.0 ct

*Sizing of diamonds is based on averages for a brilliant round diamond. Consult with a professional member of our Diamonds.Pro team for more details.

To imagine the diamond sizes even better, take a look at this simple image comparison to a US quarter:

size vs carat weight round cut

*This graphic shows mm to carat for various carat weights of round diamonds.

What is a Diamond Carat?

According to Wikipedia, the term diamond carat comes from the Greek word meaning “fruit of the carob tree.”

In ancient times, the seeds of the carob pod were used as standards for weight measurements, since the size of their seeds were almost always identical.  This provided a natural standard of measurement.

In 1907 at the Fourth General Conference on Weights and Measures, it was agreed that a modern diamond carat should equal precisely 200 milligrams.  This is technically known as a metric carat, and is the standard diamond carat weight that is used universally today.

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Why Does Diamond Carat Weight Matter?

The truth is, it doesn’t matter.  Think about it.  When you look at a diamond set in a ring, what does your eye see?  Does your eye see weight, or does your eye see physical size?  Obviously, your eye can only see physical dimensions—light doesn’t carry weight information back to your eye.

For example, you may not see the difference between this G color diamond from James Allen and this H color diamond. But you will definitely see the difference between those one carat diamonds and this 1.50ct diamond from James Allen.

If you’re not sure how a certain carat size may look on your finger, check out James Allen’s Carat Weight Tool. They show different diamond sizes on a woman’s hand using a simple sketch.

And if you think about it even more, your eye also doesn’t see the total physical size of the diamond, because most of it will be covered in jewelry.  All you can really see when you look at a diamond is the size of its surface area on the top of the diamond, as displayed above in our diamond size chart.

You are best off asking one of our experts to guide you through this process. They will be able to help you find the balance between “getting the biggest diamond” and “not wasting money to gain carat size that won’t be noticeable.”

For a round diamond, that’s the diameter, and for other shapes, it’s a function of length and width.  For simplicity’s sake, for the rest of this article, I will refer to diameter, but what I am writing applied equally to non-round shapes.

Eye-Cleanliness is Paramount

Even though this article is about carat size, eye-cleanliness is an important part of picking out a diamond. In our opinion, a consumer’s goal should be to find the cheapest (in regards to clarity; other factors matter as well) “eye-clean” diamond you can find. We use “eye-clean” to describe diamonds that may have inclusions if you look at them with a magnifying glass (or microscope or loupe), but the typical person can’t see the inclusion with their naked eye.

We have recently developed Ringo, a patented artificial intelligence model, that can examine videos of diamonds and determine if they are eye-clean. Ringo will also filter for other parameters like making sure the diamond is well-cut, doesn’t have fluorescence issues and will match the style setting you choose.

    Ringo Approved Eye Clean Diamonds

  • James Allen

    Carat Weight 0.70

    Color H

    Clarity VS1

    Check Price
  • James Allen

    Carat Weight 0.71

    Color G

    Clarity SI1

    Check Price
  • Blue Nile

    Carat Weight 0.70

    Color G

    Clarity SI2

    Check Price
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Carat and Diameter Correlation

 Diamonds cuts and their brilliance

Diamonds cuts and their brilliance

Now, obviously, there is a direct correlation between the diamond carat weight and the diameter of a diamond.  It’s mathematically impossible for a 0.05ct diamond to have more surface area than a 1 carat diamond.

But because cut qualities can vary greatly, there is a vast amount of variance in the range of possible diameters for any given carat weight.

In my experience at Leo Schachter, a 1 carat diamond ring can have a diameter that ranges from 5.60mm (a horrifically ugly deep diamond) all the way to about 6.60mm (a shallow “flat” diamond).

Take a look at these three diamonds. They are all the same exact carat weight – 1.20ct.

Deep cut diamond

Deep Cut

Shallow cut diamond

Shallow Cut

Ideal cut diamond

Ideal Cut

The first diamond we found on James Allen is very deeply cut. You can see the diamond is very dark in the middle and has a diameter of 6.73mm.

The second diamond from James Allen is very shallow and has a diameter of 7.03. This diamond looks glassy and won’t have much sparkle in natural light.

The third diamond from James Allen looks exactly like a round diamond should look. The proportions are perfect and the diamond will be very brilliant.

Weight of Half Carat Diamond

A half carat diamond weighs 100mg (milligrams) or one tenth of a gram. A half carat diamond weighs exactly half of a one carat diamond. Half carat diamonds are also referred to as ½ carat diamonds, .5 diamonds, 0.5ct diamonds, or 50 point diamonds. 

Examples of beautiful half carat diamonds include this round brilliant from James Allen and this cushion cut diamond from Blue Nile. See our diamond size chart above for mm to carat for a half carat diamond.

How Large is a Half Carat Diamond

A half carat diamond is usually 5.1mm in diameter. That is 1/5th of an inch. To compare that, a one carat diamond is 6.4mm. So while the one carat is double the weight, the face of the half carat diamond is only 21% smaller.azctual size of half a carat round diamond

Cut Grade and Diameter Over Weight

So as you can see, while weight, as one of the 4 C’s,  should matter to you because it will help determine how much you are going to pay, what you should really be focusing on is the diamond’s cut grade and diameter.

After all, wouldn’t you rather have a perfectly cut 0.90ct Brian Gavin Signature cut diamond that is 6.2mm and looks fantastic yet is cheaper than a 1.00ct diamond that is only 6.1mm and looks terrible?

About the author
Michael Fried
Mike learned the diamond business from the ground-up at Leo Schachter Diamonds - one of the world's top diamond manufacturers. He has been recognized as a diamond industry expert by Time, PeopleMoney, The Daily Mirror, NerdWallet, The Times Herald, Yahoo Finance Australia, The Art of Charm, The Washington Diplomat, The Next Web, and more.
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