diamond anatomy ritanidiamond anatomy ritani

Diamond Measurements: What You Should Know Before You Buy

6 Minute Read

HomeDiamond AdviceThe 4 Cs of Diamonds - CaratsDiamond Measurements: What You Should Know Before You Buy

IGS may receive customer referral fees from the companies listed in this page.

If you're shopping for the perfect diamond, think beyond the carats. A diamond's measurement is more than just carats-consider its shape, cut, and length-to-width ratio as well. With a little research, it's easy to find a diamond where all of these factors work together to maximize the diamond's beauty. 

Carats and Diamond Size

Compare the size and weight of the asscher-cut diamonds (on the left) to the round cut diamonds (on the right). A 1-carat asscher-cut diamond is 5.5 mm across, while a 1-carat round-cut diamond is 6.4 mm across. The vintage-style asscher cut diamonds have a smaller surface area, so they will appear smaller than round diamonds, despite weighing the same. From Ritani.

When it comes to a diamond's measurements, carats and size are two different things. A diamond's size is how large the diamond will look when viewed from above, while carats describe how much the diamond weighs. One carat is equal to 0.2 grams, so a five-carat diamond weighs one gram. 

A diamond's size is described in millimeters (mm). These measurements are roughly comparable to carats. A 1-carat round diamond is typically 6.5 mm, while a 1.25-carat round diamond is 6.8 mm. For square-cut diamonds like princess, cushion, and asscher, 1-carat is typically 5.5 mm and a 1.25 carat is 6 mm. 

Compare these two 1.01-carat diamonds. On the left is an oval-cut diamond, 7.96 mm long and 5.6 mm wide. On the right, a marquise-cut diamond that is 9.83 mm long and 5.05 mm wide. Both diamonds are the same weight, but they look like very different sizes. From James Allen

Fancy shaped diamonds like oval, emerald, pear, marquise, and radiant are typically described by their length. One carat emerald and radiant diamonds are 7 mm long, while 1-carat oval and pear diamonds are 7.7 mm long. Marquise diamonds are long and thin in comparison-a 1-carat marquise diamond is 10 mm and a 1.25-carat is 11 mm long. 

Length to Width Ratio

A 0.82-carat emerald-cut diamond from Blue Nile. The length to width ratio of 1.41 gives this emerald-cut diamond its classic rectangular shape. 

Length to width ratio describes the proportions of a diamond-whether it's perfectly round, square, oval, or rectangular. The ratio calculated by dividing a diamond's length by its width. The 0.82-carat emerald-cut diamond shown above is 6.24 mm long and 4.41 mm wide, so it has a length to width ratio of 1.41. 

The ideal length to width ratio varies depending on the diamond's shape. The length and width of round and square diamonds should be almost identical, giving them a length to width ratio close to 1.0. Rectangular and oval-shaped diamonds should be longer than they are wide, with length to width ratios ranging from 1.25 to 2. 

Each diamond shape has a range of ideal length to width ratios. Diamonds within these ranges will be the most proportional and elegant:  

  • Round: length to width ratio of 1.0 - 1.05
  • Princess: length to width ratio of 1.0 - 1.04 
  • Cushion: length to width ratio of 1.0 - 1.08 
  • Emerald: length to width ratio of 1.3 - 1.4
  • Oval: length to width ratio of 1.25 - 1.5
  • Pear: length to width ratio of 1.45 - 1.75 
  • Marquise: length to width ratio of 1.85 - 2.0 

Diamond Proportions and Appearance

Anatomy of a diamond. From Ritani.

The depth and table percentage are two other measurements that affect a diamond's brilliance and sparkle. Depth percentage is a diamond's depth (or height) divided by its diameter. 

Diamonds that are short and wide have a low depth percentage and are considered to be shallow. Shallow diamonds may appear larger from above, but they don't reflect light well, giving them a dark appearance.

Diamonds with a high depth percentage are tall and wide. Deep cut diamonds often have a dull appearance, as light escapes out of the bottom of the diamond. Make sure your diamond has the maximum sparkle and brilliance by choosing a diamond within these depth percentage ranges:

  • Round: 59 - 63%
  • Princess: 68 - 74% 
  • Cushion: 61 - 68%
  • Emerald: 61 - 68%
  • Oval: less than 63%
  • Pear: less than 68%
  • Marquise: 58 - 62% 

A diamond's table is the flat facet on the top of the diamond. When light enters a diamond, the table refracts the light and sends it to the rest of the diamond's facets, making the diamond sparkle. The table percentage is calculated by dividing a diamond's table width by its diameter.  

The ideal table percentage varies depending on the diamond's shape. Light won't properly reflect off diamonds with table percentages that are too high or too low, resulting in a dull, lackluster diamond. Look for a diamond within the following table percentage ranges:

  • Round: 54 - 57%
  • Princess: 69 - 75% 
  • Cushion: less than 68%
  • Emerald: 61 - 69%
  • Oval: 53 - 63%
  • Pear: 53 - 65%
  • Marquise: 53 - 63% 
princess cut anatomy blue nile

A 0.70-carat princess cut diamond with non-ideal proportions. The length to width ratio is 1.07, so the diamond is slightly rectangular instead of perfectly square. The table percentage is 79% (higher than the ideal range of 69 - 75%) and the depth percentage is 72%. From Blue Nile

A diamond's girdle is the belt between the crown and the pavilion. Girdle thickness is the height of this belt and ranges from "extremely thin" to "extremely thick":

  • Extremely Thin
  • Very Thin
  • Thin
  • Medium
  • Slightly Thick
  • Thick
  • Very Thick
  • Extremely Thick

Look for a diamond in the thin to slightly thick range. Girdles that are too thin could chip or become deformed by the prongs holding the diamond in a ring. Girdles that are too thick can don't reflect light as well and hide weight, making the diamond appear smaller than what's normal for its carat size. 

asscher cut blue nile
Top and side view of a 0.71-carat asscher diamond with a very thick girdle. Avoid diamonds with very thick or extremely thick girdles, as the girdle will hide weight, making the diamond appear smaller than its carat size. From Blue Nile.

Carats and Cost

Two-carat diamonds command a shockingly high price. On the left, the 1.005-carat H color VS1 ideal cut round diamond is $7,660.00. On the right, the 2.09 carat IF ideal cut round diamond is $97,846. From Brian Gavin

Carat has a huge influence on a diamond's cost. The larger the diamond, the more it will cost. In fact, diamond cost increases exponentially as the carats increase. Large diamonds are rare and must be cut with extreme care to preserve their weight, which drives up the price. The price of a 2-carat round diamond is almost 3.5 times the price of a 1-carat diamond with identical properties! 

The actual number of carats impacts diamond price too. One carat diamond rings are incredibly popular, so diamonds that are exactly 1-carat are priced at a premium. Look for a diamond slightly above or slightly below the 1-carat mark to save on cost. The same applies to diamonds that are at a quarter and half-carat sizes, like 1.25 or 1.50 carats. You'll likely end up paying much more for a diamond that is 1.25 carats compared to a similar diamond that is 1.21 carats, but the size difference is negligible. 

Evaluating Diamond Measurements Online

A 0.92-carat ideal cut round diamond from White Flash. White Flash provides full specifications for each diamond. 

With all of the numbers, ratios and percentages to keep track of, evaluating a diamond's measurement may seem like a daunting task. Fortunately, diamond graders have done some of the hard work for you. Diamonds with high cut grades-very good, excellent, or ideal-will have the best proportions. The cut is the most important of the 4C's when it comes to a diamond's appearance and well-cut diamonds will give you the traditional sparkly diamond.

When you do find that perfect diamond, look at it in 3D and read all of the information provided online. Retailers like James Allen, Blue Nile, Brian Gavin, White Flash, Ritani, and With Clarity include the cut rating, length to width ratio, depth percentage, table percentage, and girdle thickness for each diamond. Most of these retailers have an option to talk with an expert if you need some buying advice.  

Courtney Beck Antolik

Courtney Beck Antolik is a geologist and science writer based in Denver, CO. She received a B.S. (2010) in Geology from Texas A&M University and an M.S. (2013) in Geological Sciences from the University of North Carolina at Chapel Hill. Courtney’s experience with minerals includes a zircon geochronology project and a semester teaching mineralogy labs. Her favorite mineral is beryl, but diamond is a close second.

Never Stop Learning

When you join the IGS community, you get trusted diamond & gemstone information when you need it.

Become a Member

Get Gemology Insights

Get started with the International Gem Society’s free guide to gemstone identification. Join our weekly newsletter & get a free copy of the Gem ID Checklist!