Diamond Carat Weight: Everything You Need To Know

Diamond Carat Weight – Basics

Carat weight has the most impact on value but you can’t judge a diamond’s visual size by its weight (see below) nor even its physical dimensions. Bright jewelry store lights make all diamonds look bright and big. When removed from spotlights, many diamonds go dark at the edges, seeming to shrink before your eyes. Read on and learn how to avoid this.

PriceScope Pointer: Great cut quality can make your diamond look larger.

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.

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.

Meatball and pancake seen in profile and from above

Same weight but different physical spread

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.

Physical Spread

One carat diamonds spreading 6.65, 6.45 and 6.25 millimeters

  • The shallow example is like the pancake in the first illustration. It weighs 1.00 carat but measures 6.64-6.66 x 3.80 millimeters. The average of the first two numbers (length and width) are its spread, which is 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 and measures 6.44-6.46 x 3.80 mm, making its spread 6.45 mm.
  • The right example is like the meatball in the first illustration. It weighs 1.00 carat and measures 6.24-6.26 x 4.06 mm, making its spread 6.25 mm. Diamonds cut too deep often suffer from brightness reduction, and these diamonds are very common in the marketplace. 

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.

Do you want proper spread and great brightness combined? Check out the diamond brands offered by our exceptional, vetted vendors, such as Whiteflash A CUT ABOVE®, James Allen True Hearts and Astor by Blue Nile™.

Diamond Carat Charts

Be sure any round brilliant diamond you are considering has proportionate spread for its carat weight: A diamond’s spread is the average of the first two measurements in millimeters (length and width).

Carat Weight and Size: Under 1.00 Carat

Carat Weight MM Size
0.103.00
0.203.75
0.304.30
0.404.75
0.505.10
0.605.45
0.705.75
0.806.00
0.906.20

Printable

Proportionate Spread: 1 to 10 Carats

Open and print this PDF to see the actual size round brilliant diamonds should be for their weight. Be sure to set your print dialogue to USA Letter Size Paper (8.5″ x 11″) scaled to 100%, or it may not print at the correct size.

diamond carat - printable PDF  printable PDF

You can correlate the spread of round-brilliant diamonds you’re considering with the carat weights given above. If the diamond is physically smaller than stated 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.

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 before making a decision. You may not need super ideal cut, but you will want to avoid cutting that makes the diamond look smaller, either physically (small spread) or visually (its edges go dark). 

Basics-3 | Lighting | Visual-Spread

Real world information: Visual Spread

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.

0.80 and 1.00 carat examples in bright and normal lighting
Actual Photos (enlarged)

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: The 0.80 carat example was cut with proportionate angles which promote edge to edge brightness, looking large in many illumination scenarios. The 1.00 carat example was cut with wide (deep) angles which increased its carat weight but cause it to ‘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 actually vary in size appearance from room to room.

Basics-4

Can I choose carat weight online with confidence?

YES.

  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.

Get fast answers to any question: Ask our community of unbiased independent helpers.

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Intermediate Diamond Carat Weight

Analysis

diamond carat weight - scales

Analysis-1 | Key-Weights

Prices jump at key weights.

Staple diamond carat weights such as 0.50ct, 0.75ct and 1.00 are key weights (sometimes called “magic weights”) because prices seem to magically increase 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:

Chart showing key carat weight price jumps

Shop smart and save money: Diamonds just under key weights, like 0.95 carats or 1.40 carats, represent big savings – just be sure they’re cut well and have proper spread. Blue Nile, James Allen and Whiteflash can all help find those “magic” diamonds in the rough. Call or chat now, and get help today.

Analysis-2 | Shapes

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.

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 | CTW

“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.

Get fast answers to any question: Ask our community of unbiased independent helpers.

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