Understanding Diamond Carat Weight

Understanding the diamond size chart and which diamond carat size is best for you

Michael Fried
Michael Fried

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Here’s what we’ll cover in this article:

Diamond Size Chart: MM to Carat Weight
What is a Diamond Carat?
Why Does Diamond Carat Weight Matter?
What is the Weight of Half Carat Diamond?
How Large is a Half Carat Diamond?

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 cut light performance

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 a round diamond.

What is a Diamond Carat Size?

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

But you should understand that carat is actually a measurement of weight, not size. One carat is equal to 0.2 grams, or 200 milligrams. So diamond carat size is something of a misnomer, as a diamond’s carat doesn’t tell you exactly how big or small it is – though there is a correlation.

Carat weight vs size

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.

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.

Pro Tip: Carat is just weight. People see size not weight, particularly the face of the diamond. You might think it’s worth getting this 1.50ct diamond from Blue Nile even though it is poorly cut. In truth, this 1.30ct from BN will be far more brilliant and will appear to be the same size (they have the same diameter).

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.

Think You’re A Diamond Pro?

One of these diamonds has a heavier carat weight
but poorer cut. The other is smaller carat weight but
better cut. Can you tell which is which?

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 applies 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.

eye-clean diamond

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

What is Ringo?

Carat and Diameter Correlation

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.

Diamonds cuts and their brilliance
Diamonds cuts and light refraction

How Big is a 1 Carat Diamond?

A one carat round diamond will usually have a diameter of 6.4mm, which comes out to approximately 1/4 of an inch.

How Big 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

And how much does it weight? It 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 Big is a 1/4 Carat Diamond?

A 1/4 carat round diamond will typically have a diameter of 4.0mm, or 0.15 inches.

Do Lab Diamonds Weight the Same As Natural Diamonds?

Yes, lab-grown diamonds weigh the same, and are measured the same in terms of weight, as natural or Earth-grown diamonds.

Lab diamonds are exactly the same, chemically and physically, as diamonds formed naturally in the Earth. They are graded the same way as well, across the 4 C’s, which includes carat weight.

That means everything in this article, such as how carat weight translated to actual carat size, applies to diamond sizes for both lab grown and natural diamonds.

Diamond sparkle graphic

Need Help Choosing The Right Diamond Size?

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?

If you need help searching for a diamond, feel free to contact our experts. We have years of expertise in the diamond industry, and we can assist you with any questions that come up during your search, related to diamond carat size or anything else. We’ll help you judge whether a higher carat diamond is really worth the money, so you can come out with the perfect diamond for your budget.

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What we love about them:
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  • Lifetime Warranty
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  • Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
  • Provide insurance appraisal
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