When it comes to lab grown diamonds, there’s a lot of noise out there. Everyone’s throwing around carat numbers like they mean size, value, status, all of it. But here’s what no one tells you: carats don’t measure size. They measure weight. And if you’ve ever worn a diamond that looks smaller than it sounds, you know exactly what I mean.
If you’re buying lab diamonds, especially in 2025 when you’ve got more freedom than ever to go big without breaking your budget, you need to know what really counts. Millimeter sizing? That’s the one to pay attention to. Because that’s what shows up on your hand, that’s what people see.
This isn’t just a sizing guide; it’s the part no one explains. And if you’re smart enough to choose lab created diamonds, you’re smart enough to want the whole story.
Carat: What You Think It Means Vs. What It Actually Means
A carat is 200 milligrams. That’s it. It’s a unit of weight, not a measurement of how big or bold the diamond looks on your hand.
Two diamonds can both weigh 1 carat. But if one is deep and shallow, they’ll look completely different. One might have a wider surface and sparkle like crazy. The other might look smaller, even though it's heavier.
This matters more with lab grown diamonds, where buyers often want maximum sparkle for the price. The truth is, not every 1-carat diamond delivers that just because it says “1.00” on paper.
Millimeter: The Real Size
Millimeters are how we measure what your eye actually sees—the width and height of the diamond on the surface. Think of it like this:
- Carat tells you the mass.
- Millimeters tell you the visual footprint.
- Both matter. But only one tells you what people actually see when you wear it.
For example, a round brilliant 1-carat lab-created diamond is usually around 6.4 to 6.5 mm in diameter. If you see a 1-carat diamond that’s noticeably smaller than that, the proportions are off. Maybe it’s too deep, maybe it’s poorly cut. Either way, you’re not getting what you think you are.
Carat vs. Millimeter: Why It Matters for Lab-Grown Diamonds
Let’s say you’ve got two 1-carat lab diamonds. One looks slightly bigger. Why?
Because it’s shallower. Less depth, more width across the top. And that top surface, the table, is what most people see when you wear it.
The trade-off? Shallow stones can sometimes lose brilliance. But it’s not always a dealbreaker. Some cutters deliberately widen the table just a touch to give more face-up size without killing sparkle.
If you’re comparing lab grown diamonds online, this is where people get tripped up. The photo looks huge. The carat weight sounds solid. But the millimeter size tells the truth. A well-cut 0.90 carat lab grown diamond can often look better than a badly cut 1.00 carat. And it can cost a lot less.
The Shape Factor
Size perception isn’t just about numbers. Shape plays a role, too. Different diamond shapes carry weight differently.
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Round: Standard reference point for size. A 1-carat round is about 6.4 mm.
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Oval: Faces larger than round, usually around 7.7 x 5.7 mm for 1 carat.
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Cushion: Deep cut. It may look smaller than a round of the same weight.
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Emerald: Long and lean. It can look bigger due to its spread, even if the carat is lower.
This is why understanding how your diamond is shaped and how that affects face-up size is key when buying lab created diamonds. Especially if you're aiming for that “wow” look without jumping up a price bracket.
Proportions and Cut: The Silent Difference-Maker
You can have the perfect carat and even the correct mm dimensions, and it still won’t look great if the cut is off.
The cut is what makes a diamond bounce light back. A deep or shallow cut leaks light, making the diamond look dull or, worse, smaller.
This applies to both natural and lab grown diamonds, but here’s where lab diamonds get the edge: they’re cut with better precision, more often. Most lab-created diamonds are made with ideal proportions from the start because the raw material isn’t as rare or expensive. That gives cutters more freedom to focus on visual beauty.
So yes, a 1-carat lab grown diamond often looks better than a natural 1-carat, simply because it’s cut smarter.
Don’t Rely on Carat Alone: Ask for Millimeter Specs
If you're shopping for lab diamonds online or in-store, don’t just look at the carat number. Ask for the mm dimensions.
For rounds, anything under 6.3 mm for a full carat should raise questions. The same goes for ovals under 7.5 mm or cushions under 5.7 mm. These aren’t hard rules, but they’re good markers for whether you’re getting what you think you’re getting.
Also, check the cut grade. For rounds, stick with “Excellent” or “Ideal.” For other shapes, look at symmetry, polish, and length-to-width ratio.
Want to Maximize Look for Less?
Here’s a smart move: shop for just under the magic numbers. Go for 0.90 carats instead of 1.00. Go for 1.70 instead of 2.00. The difference in look is barely noticeable, but the price difference is real.
And if the cut is on point, a 0.90 carat lab created diamond can sparkle harder than an average 1.00 carat diamond any day of the week.
Final Words
When it comes to sizing lab grown diamonds, carat gets all the attention, but it’s only half the story. Millimeters, shape, and cut are what actually determine how that diamond looks on your hand.
And with lab diamonds, that strategy pays off more than ever. You get more control, better visuals, and often a better price. Just don’t fall for the weight trap. You’re buying what people see, not what it weighs in a lab.
Need help figuring it out in real time? Stop by Chawi Jewelers. We’ll break it down for you—no pressure, no buzzwords, just real talk about real diamonds that shine like they mean it.