Carat is specifically a measure of a diamond’s weight, and by itself may not accurately reflect a diamond’s size.

We tend to evaluate diamond size by viewing it from the top because that is how diamonds are presented to us when set into a ring.

To understand diamond size, carat weight should be considered in conjunction with two other criteria:

– Distance in millimeters across the top of the diamond.
– Diamond’s cut grade.

Carat is the term used to describe the weight of any gemstone, including diamonds. Although the definition of a carat has changed over time, since 1913 the international standard has been 200 milligrams, or 1/5 of a gram. Often, jewelers describe carats in 1/4 increments.

In jewelry pieces with more than one diamond, the carats may be described in terms of total carat weight (TW). This is the combined total weight of all the stones in the piece.

Diamonds can range in size from a fraction of a carat to several carats. Given the rarity of large stones, however, the price increases rapidly with size; therefore, a single 2-carat diamond will cost much more than two 1-carat diamonds. Very large diamonds with good color and clarity are very rare.

Expect to pay a premium for stones that are above a full carat weight. For example, a .95 carat diamond will cost a bit more than a .90 carat stone.

A diamond’s cut grade should also be considered because, as we noted in the cut grade section, when a diamond is cut with the proper proportions, the maximum amount of light (or sparkle) is returned out of the top of the diamond. Thus, when a diamond is well cut, the light reflected out of the top makes it appear larger. In addition, much of the weight of a poorly cut diamond, for example, may be “hidden” in the base of the diamond, making the diamond appear smaller than its carat weight would imply.

It is therefore possible to have a diamond of a lower carat weight, but higher cut grade, that appears larger than a diamond with a larger carat weight, but poor cut.

Once you’ve selected your cut, color, and clarity grade, it’s easy to determine the carat weight of diamond that will fit within your budget.

Much as there are 100 pennies in a dollar, a one-carat diamond is comprised of 100 points. Hence, 50 points is equal to 1/2-carat, and so on.

This chart illustrates how diamonds of different carat weights look when set in a ring. Note that a 2-carat diamond does not appear to be twice the size of a 1-carat diamond when viewed from the top.

Because round brilliant cuts follow exact standards, you can make a good estimate of the carat weight of the stone based on the stone’s diameter. The following chart compares the relative sizes of stones and describes how much a round brilliant diamond of a certain size is likely to weigh. This chart is for educational purposes and represents a guideline for diameter and carat weights. It is not representative of other cuts or shapes. It is also not applicable to colored gemstones which have a different density from diamonds.

 

Weight of each diamond (in carats)		Size in MM
From	To	From	To
.005	.009	1.0	1.2
.010	.019	1.2	1.6
.020	.029	1.6	1.9
.030	.039	1.9	2.1
.040	.049	2.1	2.4
.050	.059	2.4	2.6
.060	.069	2.6	2.7
.070	.079	2.7	2.8
.080	.089	2.8	2.9
.090	.099	2.9	3.0