Diamond Size Chart: Carat Weight Buying Guide (MM to Carat)
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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.
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 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
| 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.
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 with using a simply 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.
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.
Carat and Diameter Correlation
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.
Deep Cut
Shallow Cut
Ideal Cut
The first diamond is very deeply cut. You can see the diamond is very dark in the middle and has a diameter of 5.47mm.
The second diamond is very shallow and has a diameter of 5.77. This diamond looks glassy and won’t have much sparkle in natural light.
The third diamond looks exactly like a round diamond should look. The proportions are perfect and the diamond will be very brilliant.
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?
James Allen is the leader in online diamond sales. Their imaging technology is at the point where it's almost the same as inspecting a diamond with a jeweler's loupe. Their customer experience is best in class, and they have recently upgraded their non-engagement jewelry inventory as well. And for a limited time, take advantage of their 25% Off pre-Black Friday Sale!- No questions asked returns within 30 days of shipment. James Allen will send you a paid shipping label to return the ring.
- Lifetime Warranty
- Free International Shipping
- Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
- Provide insurance appraisals
- One free resizing within 60 days of purchase
- Free ring inscriptions
- Best-in-class high quality imagery of all diamonds in stock
- 24/7 Customer Service
- Best-in-class packaging
Blue Nile is the largest and most well known internet jewelry seller. They have the industry-leading largest available exclusive online inventory. Their high quality images are catching up to James Allens' and their prices are amazing. - No questions asked returns within 30 days of shipment. Blue Nile will send you a paid shipping label to return the ring.
- Lifetime Warranty
- Free Shipping
- Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
- Provide insurance appraisal
- One free resizing within the first year of purchase
- High quality images of about half of their diamonds
- 24/7 Customer Service
- 100% credit towards future upgrades (must be at least double in value)
- Best in class fulfillment
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