Diamond Carat Weight Guide With Diameter Size Chart

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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. But 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 Carat Weight & Diameter Size Chart

0.25 ct 0.5 ct 0.75 ct 1.0 ct
4.0mm diameter 5.1mm diameter 5.8mm diameter 6.4mm diameter
1.25 ct 1.5 ct 1.75 ct 2.0 ct
6.9mm diameter 7.3mm diameter 7.7mm diameter 8.1mm diameter
2.25 ct 2.5 ct 2.75 ct 3.0 ct
8.5mm diameter 8.9mm diameter 9.0mm diameter 9.3mm diameter
3.25 ct 3.5 ct 3.75 ct 4.0 ct
9.5mm diameter 9.75mm diameter 9.9mm diameter 10.1mm diameter
4.25 ct 4.5 ct 4.75 ct 5.0 ct
10.43mm diameter 10.63mm diameter 10.83mm diameter 11.01mm diameter

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

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 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.

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.

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.

Carat and Diameter Correlationdiamond-cut-diagram

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 1.00 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 1.00ct diamond 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).

Cut Grade and Diameter Over Weight

So as you can see, while weight 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?

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