The carat became the unit of diamond measurement in 1907. Before that, gemstone evaluation varied between countries, and standards changed depending on where you were in the world.
Carat (ct.) refers to the unique unit of weight measurement used exclusively to weigh gemstones and diamonds. Carat weight is often confused with visual size even though it is a measurement of weight. You cannot actually see carat weight with the naked eye. Several factors affect the weight of a stone, such as density, the formulation of the jewel, and the shape.
A stone that is visually substantial does not indicate a high carat count. Depending on the shape and type of gemstone being weighed, the weight will visually show itself differently. For example, a 1.00 ct. round diamond will measure around 6.5mm, and a 1.00 ct. round sapphire will measure around 6.0mm. This is due to the varying density of different gemstones.
A phrase to look for when shopping is Total Carat Weight or TCW. Total carat weight represents the total weight of all diamonds or other gemstones in a piece of jewelry, when more than one stone is used. Sets of jewelry, or jewelry sold in pairs, are typically sold using TCW. Diamond solitaire earrings, for example, are usually quoted in TCW, indicating the combined weight of the diamonds in both earrings.
Actual Diamond Size on Size 6 Hand
Our Diamond Size Chart shows what different diamond shapes look like depending on carat weight. Each image shows what it looks like when viewing the diamond from the top down.
The diamonds in our Diamond Size Chart may appear to be different sizes, but the carat weight is still the same. For example, the 1-carat radiant diamond looks smaller than the 1-carat pear diamond, yet they are the same carat weight.
View Diamond Size Chart
The term carat comes from the carob seed. In the past gem merchants used carob seeds to help weigh gemstones and diamonds because the seeds were uniform in weight and size.
Today, a carat equals about the weight of a paperclip - 0.2 grams, or 200 milligrams. Carat weight is a uniform measurement worldwide, regardless of the measurement system a country typically uses.
When determining the carat of a diamond, first weigh the diamond and ensure you use grams as the unit of measure. After you figure out how much the diamond weighs, divide that number by 0.2 and that is the carat weight. Most modern jewelers do not calculate carat weight by hand - they use a specific scale to weigh gemstones. These scales are created exclusively for measuring carat weight.
Gemologists and jewelers also use a point system to refer to diamond carat weight. Here is how it works:
And so on.
Carat size is influential, but it is not the only aspect of a diamond to consider. Carat weight is only as prominent to the buyer as they want it to be. There is not one specific carat weight that is universally considered the best, it’s subjective and depends predominantly on what qualities matter to each person.
The 4 Cs are in no particular order, but each affects the value and beauty of a diamond. When shopping for the perfect diamond, you should consider the person receiving it. If the recipient prefers sparkly eye-catching jewelry, you may want to focus on cut, clarity, and color rather than carat. If the visual size is what matters to the recipient, then carat weight and cut should be focal points.
Carat weight is one of the most crucial factors in determining the price of a diamond. Once cut, color, and clarity grade have been determined, the carat weight of a diamond can be easily established to fit within a budget.
Larger diamonds are much more valuable because they are discovered in nature much less frequently than small diamonds. Because of that, diamonds are not linearly priced. Meaning, a one carat diamond costs more than two half-carat diamonds of the same quality. The general rule of thumb around diamond pricing is that double the carat weight means it will cost four times more. So, purchasing jewelry that features multiple diamonds with a lower carat weight can be more cost effective than purchasing jewelry that features one diamond with a higher carat weight.
The most popular carat weights are 1.25 ct and 1.0 ct. But if you want to cut down on cost, try finding a slightly lower carat diamond with a top-notch cut. A high-quality cut can help make a diamond appear larger.
Some diamonds fall within categories that jewelers refer to as “under-sizes,” which means the diamonds weigh just below a cutoff weight. The cutoff weights are 0.50 ct., 0.75 ct., 0.90 ct., 1.00 ct., 1.50 ct., and 2.00 ct. Under-sized diamonds are more difficult to find as a diamond cutter will choose to sacrifice beauty in order to cut a diamond with a weight that reaches one of the cutoff weights, or “magic numbers” as they are known in the industry. That said, seemingly microscopic carat weight differences impact cost significantly. So, looking for an under-size diamond could help save money.
This diagram illustrates the typical size relationship between diamonds of increasing carat weights. While the measurements below are typical, every diamond is unique.
Request a free ring sizer, print a free ring sizing guide at home, or view our international size chart in our Ring Size Guide.
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