Round brilliant conversion chart
| Carat weight | Diameter | What it reads as |
|---|---|---|
| 0.25 ct | 4.1 mm | Accent stones, small halos |
| 0.33 ct | 4.4 mm | Common side stone |
| 0.50 ct | 5.2 mm | Reads as a proper solitaire |
| 0.75 ct | 5.9 mm | Noticeably bigger than a half |
| 0.90 ct | 6.2 mm | The value pick, see below |
| 1.00 ct | 6.5 mm | The benchmark |
| 1.25 ct | 6.9 mm | Visible step up |
| 1.50 ct | 7.4 mm | Substantial on most hands |
| 1.75 ct | 7.8 mm | Large |
| 2.00 ct | 8.2 mm | Statement size |
| 2.50 ct | 8.8 mm | Very large |
| 3.00 ct | 9.4 mm | Covers most of the finger width |
| 4.00 ct | 10.4 mm | Rare in everyday wear |
| 5.00 ct | 11.0 mm | Exceptional |
These assume good proportions. A deep cut stone of the same weight measures smaller across the top, sometimes by 0.3 mm or more, which is visible when two rings sit side by side.
Why doubling the carats does not double the size
Weight tracks volume, and volume tracks the cube of the diameter. Which means to double the weight you only need to increase the width by about 26 percent.
In practice: a one carat stone is 6.5 mm and a two carat stone is 8.2 mm. You paid roughly three to four times as much for 1.7 mm. That is worth knowing before you upgrade, because the difference on a hand is real but far smaller than the price difference suggests.
The 0.90 carat argument
A 0.90 carat stone measures 6.2 mm. A full carat measures 6.5 mm. That is three tenths of a millimetre, which is less than the thickness of three sheets of paper, and it is invisible on a finger.
Prices, meanwhile, jump hard at 1.00 because that is where demand clusters. Buying just under a round number is the most reliable way to save money in this entire category without giving up anything you can see.
Cut quality changes apparent size
Two stones can both weigh exactly 1.00 carat and measure 6.5 mm and 6.2 mm. The second one was cut deep, hiding weight in its belly below the girdle. You paid for a full carat and you are wearing something that looks like a 0.85.
This is one more reason cut grade is the C that matters most. It controls brilliance and apparent size at the same time.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
How wide is a 1 carat diamond?
Why is the millimetre jump so small between weights?
Does the chart work for fancy shapes?
Should I shop by millimetre instead of carat?
Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.