Diamond Carat Weight: Everything You Need To Know

Diamond Carat Weight: Everything You Need To Know


Diamond carat weight has the most impact on value, however:

Did you know you can’t judge visual size by diamond carat weight (see below) nor even by its physical dimensions? Bright jewelry store lights make all diamonds look big and bright. When removed from spotlights, some diamonds go dark at the edges and seem to shrink before your eyes. Read on to learn how to avoid this.

PriceScope Pointers

  • The same diamond carat weight don’t always look the same size, diamond to diamond.
  • Round brilliant diamonds generally look the largest for their weight.
  • Choosing great cut quality can make your diamond look larger.
  • Most people prioritize diamond carat weight first and sparkle second, reinforcing the pointers above.

Before going on: Check out the PriceScope Diamond Buying Guide

Basics-1 | Diamond Carat Weight Described

One diamond carat equals 200 milligrams. Putting that in perspective, a small paperclip weighs about 600 milligrams, so a paper clip on your finger weighs about the same as a three carat diamond. A carat can also be divided into 100 points, so jewelers call a 1/4 carat diamond a 25-point diamond, a 1/2 carat diamond a 50-pointer, and so on.

Diamond Carat - 25 point, 50 point, 75 point and 99 point diamonds

Diamond carat weight can be deceptive.

Why? Because we usually view diamonds from only one direction – looking down from above. Imagine a 1 pound meatball and a 1 pound pancake. On a scale they weigh the same. But they look different on the tray, because the meatball’s has a deep vertical geometry while the pancake is distributed horizontally.

Spread in Millimeters

This is why it’s important to correlate diamond carat weight with side-to-side spread in millimeters, to ensure a diamond “faces-up” the correct size for its weight. A diamond’s grading report will include its external measurements in millimeters, from which you can determine its spread, side to side.

  • The shallow example below is like the pancake above. It weighs 1.00 carat with external measurements of 6.64-6.66 x 3.80 millimeters. The average of the first two numbers (length and width) are its spread, which becomes 6.65 mm. Diamonds cut too shallow may suffer from brightness reduction in close viewing.
  • The center example is proportionate. It weighs 1.00 carat with given measurements of 6.44-6.46 x 3.80 millimeters, making its average spread 6.45 mm.
  • The right example is like the meatball above. It weighs 1.00 carat with given measurements of 6.24-6.26 x 4.06 millimeters, making its spread 6.25 mm. Diamonds cut too deep often suffer from brightness reduction, and these diamonds are very common in the marketplace. 

Diamond Carat - One carat diamonds spreading 6.65, 6.45 and 6.25 millimeters

Spread and Brightness Work Together

While the shallow diamond above will appear physically larger from above in jewelry store spotlights it’s not necessarily the best choice for those seeking the diamond which will appear the brightest and largest through multiple illumination environments. Read on to see examples of brightness and visual size-appearance.

Diamond Carat Chart: Correct millimeter size per carat

In addition to diamond carat-weight, we have added approximate spread size in millimeters to the chart below. Be sure to correlate the spread of round-brilliant diamonds you’re considering with the given carat weights. If the diamond is physically smaller than it should be for the carat weight, that’s not necessarily a deal-breaker, as long as the price is reduced accordingly and it performs to the standards you’re seeking.

round millimeter sizes

Basics-2 | Diamond Carat Weight: Let’s get practical.

Some people want to balance the Cs carefully. Others want the biggest diamond for their budget. Those seeking the “most bang for the buck” should simply determine the lowest color that looks good to your eye and search for a diamond with “eye clean” SI1 or SI2 clarity. If you don’t mind visible inclusions, you can even drop to I1 clarity.

PriceScope Pointer

Be sure to read about Diamond Cut quality before making a decision. You may not need super ideal cut-quality, but you definitely want to avoid cutting that makes the diamond look smaller, either physically (small spread for weight) or visually (it goes dark at the edges). Diamonds of the same weight don’t always look the same size.

Basics-3 | Real world information

Bright jewelry store lights show diamonds at their best when it comes to carat-weight appearance. A one-carat diamond will never look larger than it does when seen under a seller’s spotlights. The real test comes when the diamond leaves the store and goes into normal lighting situations.

For example, these stones both look big under bright lighting. But in normal lighting the 0.80 carat stone remains bright from edge to edge, while the 1.00 carat stone seems to shrink before your eyes.

Diamond Carat - 0.80 and 1.00 carat examples in bright and normal lighting
Actual Photos

In fact, the 0.80 carat stone seems visually larger than 1.00 carat, even though it’s physically smaller.

The difference is cut-quality: If the diamond is cut with proportions and angles which promote edge to edge brightness, it will continue to look large in many illumination scenarios. If not, it will ‘leak light’ through the bottom, going dark at the edges and looking small away from jewelry store spotlights. Depending on cut quality, diamond carat weight can vary in size appearance from room to room.

Basics-4 | Can I choose Diamond Carat Weight online with confidence?


  1. Carat weight is standardized. It’s the only truly objective component of the 4Cs as it refers to how heavy the diamond is on a scale. You can look at Diamond Grading Reports (aka certificates) to find the diamond’s spread in millimeters and be sure it has the proper physical size for its weight.
  1. Run the diamond’s primary measurements through our patented, proprietary Holloway Cut Advisor for details about its physical spread and performance predictions.
  1. Ask the Seller Directly: If you have questions beyond what a diamond grading report and supplemental information can communicate, pick up the phone, chat or email and get in touch with the vendor. It’s in their best interest to communicate transparently with you. After all, they don’t want the expense of shipping a diamond, only to have it returned on their dime.
  1. Get quick answers to any question now: Ask our community of unbiased independent helpers.
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Intermediate Diamond Carat Weight


Analysis-1 | Prices jump at key weights.

Staple diamond carat weights such as 0.50ct, 0.75ct and 1.00 are key weights (sometimes called “magic sizes”) because prices seem to magically leap up at those sizes.

All other things equal, there is negligible visual difference between a diamond of 0.99 carats and a diamond weighing 1.00 carat. Someone would be hard-pressed to sort them correctly when asked. But the price difference between 0.99 carats and 1.00 carat is significant. Why? Supply and demand. People desire to hit these key weights. Psychologically, owning a 1.00 carat diamond seems to have more value than a 0.99 point diamond, based on how many people pay the going price.

Diamond carat weight chart showing key weight increases:

Diamond Carat - Chart showing key carat weight price jumps

Analysis-2 | Different shapes spread differently.

Fancy shapes tend to have more weight and less side-to-side spread. This is one reason round-brilliant diamonds tend to cost more than fancy shapes of the same carat weight.

Diamond Carat - One carat Round, Cushion and Princess cut diamonds, profile view

Deep shapes like cushions and princess cuts frequently carry more weight in the body than round brilliants, so they will look physically smaller from the top.

Diamond Carat - One carat Round, Cushion and Princess cut diamonds, top view

Shallow shapes like emerald cuts, marquise and ovals can bring aesthetic benefit in terms of spread, but may not have the same dispersion or contrast potential as deeper shapes.

Analysis-3 | The rule of 0.999.

International carat weight rounding is not the same as what you learned in school. Diamond weights are rounded up from the third decimal point only when it is a nine. This is to permit some small latitude in favor of the diamond-cutter for calibration differences between scales.

0.995ct = 0.99ct
0.998ct = 0.99ct
0.999ct = 1.00ct

Analysis-4 | “Total carat weight” (ctw)

The term “total carat weight” refers to the sum of all diamonds in a piece of jewelry.

A halo ring with a 0.75 carat center stone and 0.25 carats in small surrounding stones can be advertised as having 1.00 carat total weight or “1ctw.” Likewise, a bracelet with twenty 0.10 carat diamonds may be advertised as a 2.00 carats total weight bracelet, or “2ctw.” Total carat weight is the collective weight of all diamonds in that piece of jewelry.


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