“I want a 1 carat diamond.” Great! But did you know that “one carat diamond” could be 6.2mm, 6.5mm, or 6.6mm across? The sad truth is that, unfortunately, unless you pay close attention to the measurements of your stone, you can get caught paying for diamond weight you don’t see. Ain’t nobody got time for that! So, let’s talk about carat weight vs diamond size and make sure you get what you pay for.

Different diamond sizes on fingers

Carat Size vs Diamond Size

Is Carat a weight or a size?

A “carat” is a weight measurement, meaning one fifth (1/5) of one gram. So, if a diamond weighs one carat, it weighs .20 grams. However, that doesn’t really help you in terms of seeing how big it looks. That’s because a diamond can carry weight in its height or depth, instead of spread across its diameter.

Does carat affect diamond size?

Generally speaking, yes, but it’s not that simple. While a 3 carat stone will, obviously, look larger than a .50 ct stone – the same is not necessarily true for a .90 ct and a 1.10 ct. So, it is incredibly important to look at a diamond’s measurements as part of your diamond search! You may find a well-proportioned .97 ct diamond that is not only bigger than a 1.00 ct, but is also less expensive! This is particularly common in fancy cut (non-Round) diamonds, because their facet patterns are less specific.

What size is 1 carat diamond?

A 1 carat Round diamond should measure around 6.5 mm. However, the millimeter diameter of a one-carat diamond will depend upon its shape and how well it is cut. For example, check out the HUGE difference in millimeter sizes of these one-carat diamonds! From 6.11 mm all the way up to 6.53 mm! This wide variety of size occurs because some diamonds are cut more deeply than others, causing carat weight to hide in the diamond’s bottom. When this happens, a 1 carat Round can look closer to a .90 ct! No one wants that. This is one reason an “ideally” cut Round diamond should have a depth percentage of less than 62%. That way, you make sure your diamond faces up the appropriate size!

Search field showing different diamond millimeters in same carat weight

Whoa! 6.1mm vs. 6.5mm?!

0.5mm makes a big difference in diamonds.

Check it out – which would you prefer?!

Side by side comparison of 6mm and 6.5mm diamonds

6mm vs 6.5mm diamond

How does diamond shape affect carat size?

Some diamond shapes look bigger than other shapes, even though they’re the same carat weight! This is because slender shapes, like Oval and Marquise, not only have the same depth percentages as Rounds, but also have all that extra length! So, choosing an Oval or Marquise is a quick way to get a bigger look for less money.

Shape Carat Weight Millimeter Size
Round 1.00ct 6.5mm
Oval or Pear 1.00ct 7.7mm x 5.7mm
Marquise 1.00ct 10mm x 5mm


1ct Round vs Oval vs Pear vs Marquise Side by Side Comparison on Fingers

Round 1 carat; Oval 1 carat; Marquise 1 carat; Pear 1 carat

Likewise, some diamond shapes look smaller than others. For example, many square shapes, like Cushion and Princess, are cut with much deeper depth percentages. So, these diamond shapes appear smaller than comparable Rounds.

Shape Carat Weight Millimeter Size
Round 1.00ct 6.5mm
Square Cushion 1.00ct 5.9mm
Princess 1.00ct 5.5mm
Asscher 1.00ct 5.5mm
1ct Round vs Princess vs Cushion Side by Side Comparison on finger

Round 1 carat diamond; Princess 1 carat diamond; Cushion 1 carat diamond

How do you determine carat size?

Put it on a jeweler’s scale. Loose diamonds are best measured by a special jeweler’s scale. These special scales work in carats for gemstones and “pennyweight” (abbreviated, confusingly, as “dwt”) for metals. For diamonds and gemstones that are already set into jewelry, jewelers and appraisers estimate carat weight by taking diameter measurements.

Do Lab Diamonds Weigh the Same as Natural Diamonds?

Yes. Lab Diamonds are the exact same mineral as Natural Diamonds. So , a 1 carat Lab Diamond will look the same size as a 1 carat Natural Diamond. Interestingly, this is not the same for diamond simulants like Moissanite or Cubic Zirconia. Moissanite weighs a little less than Diamond. This is why we list our Moissanite by “diamond equivalent” size – so you get the size you really want! Cubic Zirconia, on the other hand, weighs significantly more than Diamond. This super-heavy weight is one way you can tell Cubic Zirconia apart from Diamond without testing.

.95ct Lab Diamond vs .95ct Natural Diamond showing same carat weight

Both this Lab Diamond and Natural Diamond are around .95ct!

How to Get What You Pay For

1. Look at a Diamond’s Measurements

Don’t just look for a “one carat” – look for a one carat that measures up to its expected measurements (or over!) The easiest way to do this to take the following steps:

  1. Set a broad carat weight range (think .95-1.05 carat for a “one carat”).
  2. Set cut grade to “Excellent” and include ideal parameters (you can find these on our Diamond Resource pages! For example, an Ideal Round has a depth percentage lower than 62%).
  3. Compare the measurements of diamonds that come up and see what an average size is.
  4. Choose a diamond at or above average!

You can also check out Stone Algo’s size chart, here.

Step by step directions to get a larger diamond

2. Use a Ruler!

One of my favorite ways to make sure your expectations meet reality is to use a ruler to really see what a millimeter is. No, seriously! Find the diamond you think you want, and whip out a ruler and see what that millimeter size really looks like. When my husband and I were looking for my stone, we even cut out a little slip of paper in its size to put on my finger (super scientific approach, I know)! Visualizing actual size, instead of getting stuck on carat weight, is a much better way to get what you really want.


3. Don’t sacrifice quality

BEWARE of falling into the trap of just selecting the biggest stone. Cut quality (NOT carat weight) is the number one factor in choosing your diamond, because it is directly responsible for beauty and sparkle. Often, the largest measuring stones are cut too shallowly, allowing light to leak through the bottom (resulting in lost sparkle) or a fisheye (see below!). So, always make sure to set your ideal parameters, and then double check for even contrast/brilliance. Evenly spaced contrast (dark areas) and brilliance (bright areas) throughout your stone, without any weird patterns, confirms that your diamond is reflecting and refracting light properly – max sparkle! So, your goal is to choose a stone that measures up big AND has a great cut!

Poor cut vs Ideal cut cushion diamond side by side

Can You Help Me Get a Good Carat Size vs Diamond Weight?

For sure! Drop us a note here, or email us are [email protected]! We love helping customers get the most beautiful diamonds, with the biggest bang for their buck!