Round Cut Diamonds Size Charts in MM Guide [2025]

Jan 15, 2025
Round Cut Diamonds Size Charts in MM Guide [2025]

Round Diamonds Carat Size Chart in MM

The Round Cut Diamond is the most ideal and best known cut shape. If we go by history, the first diamond engagement ring was a round cut diamond. For its charming design that can take on any look, round cut diamond engagement rings are best known. So much more beautiful than a simple square diamond, the round diamond shape allows the diamond to sparkle to its own beauty.

Based on size

A fancy length shaped diamond is generally perceived to be larger and broader to the naked eye than a similar carat weight round or square shaped diamond.

Based on length and width

These shape measurements directly tell you what kind of diamond you are looking at. Well cut diamonds of carat weight are a typical phenomenon.

Based on the crown area

This is the crown area (the size of the diamond themselves, how big is the diamond itself when viewing it from top down). In its top view, a marquise shape would look visually larger than a round shape.


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How is diamond size measured?

Carat weight is measured in diamond size, a unit of 0.2 grams. Nevertheless, size is now more precisely and indeed more correctly measured by its dimensions, i.e., length, width and depth in millimeters. contains diamonds, and a diamond's diameter (at the top of the diamond, just before the crown) can be used to estimate their apparent size (especially when comparing two like diamonds).

How is diamond size measured?

Carat weight is an industry standard method to measure diamond size and measures how much diamond you get for your dollar;. Nevertheless, dimensions are more correctly defined in size than their actual size, which in millimetres are its length, width and depth. But the size (diameter) of a diamond (top of the diamond), can give you an idea of the apparent size (i.e. when comparing two similar diamonds).

What’s the distinction between ‘total carat weight’ and ‘carat weight’?

Carat weight means the weight of one diamond, however, total carat weight (TCW) is the cumulative weight of all diamonds of one piece of jewellery like a ring for example a centre diamond and small diamonds used.

Carat vs Millimeters Weight Measurements

For jewelry shoppers, it’s important to know the difference between millimeters and carat weight. Gemstone dimensions are in millimeters (mm) and weight in carats.


Carat weight impacts how a diamond appears, but it doesn’t actually give a true depiction of size. The weight of a carat can mean two diamonds of the same carat weight could look different depending on how the weight is distributed. An example is round diamonds of the same carat weight as elongated fancy shaped diamonds with long dimension and shallow depths may appear larger.


Other gemstones have different densities, so same sized diamonds may look bigger or smaller depending on the other material’s density. 


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MM to Carat Conversion Charts

The conversions mm to carat are approximate, not exact. Below are estimated diamond millimeters to diamond carat weights. The mm to carat weight conversions for the other gemstones will vary from this specifically based on their own specific density.  


As a single diameter, round and perfectly square gemstones are measured by one dimension such as “8mm” as its equal dimension of where they are measured by. Two length and two width measurements are listed in millimetres for other shapes. The length to width ratio is a guide as to the specific look that gemstones in cushion cuts will give from square to rectangular depending on the ratio. 

Round MM to Carat Conversion

Rounds are the most popular because their diameter or circles are uniform, and they convert easily.


Approximate MM Diamond Size

Approximate Diamond Carat Weight

4mm

0.25ct

5mm

0.5ct

6.3mm

1ct

7mm

1.25ct

7.3mm

1.5ct

8mm

2ct

8.7mm

2.5ct

9.3mm

3ct

9.7mm

3.5ct

10.2mm

4ct

10.6mm

4.5ct

11mm

5ct


  • Is it better to have a bigger lower quality diamond, or a smaller higher quality diamond?

  • In some cases, however, it is mainly a matter of personal preference.

  • while others prefer a smaller, high-quality diamond with better brilliance and fire. Finding the right balance that suits your taste and budget is essential.

What is a round diamond cut called?

A round diamond cut is known as a “round brilliant cut.” This cut is specifically designed to enhance the diamond’s brilliance and sparkle, making it the most popular and well-known diamond shape.


That is why the most famous solitaire diamond engagement ring more often than not comes with a round cut shape. The round shape diamond comes into prominence known as ‘Diamond Design’. 


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