Carat weight vs size: why a 2 ct diamond isn't twice as big

A quick guess

Picture a 1-carat round diamond. Now picture a 2-carat one beside it. How much wider is the 2-carat?

Most people say "twice as wide", or at least "a lot". The answer is 26%. A 1.00 ct T73 is 6.48 mm across; a 2.00 ct T73 is 8.16 mm. Double the weight buys you less than two millimeters.

Here is why.

A carat is a weight

Carat measures weight, not size. One carat is 200 milligrams, a fifth of a gram. For smaller stones, the carat has 100 points. So a 0.75 ct diamond is a "75-pointer", and a half carat is 50 points.

A lab weighs every stone on an electronic microbalance. GIA's microbalance reads to the fifth decimal place, a hundred-thousandth of a carat. The report gives the weight rounded to two decimals. Carat is the only one of the four Cs that the lab measures directly. Nobody's eye is involved.

And diamond is rare by nature. GIA says that miners dig about a ton of rock to get less than half a carat of rough diamond.

Weight grows with the cube

Weight is volume times density. Our 3D models give the volume of every cut, and the density of diamond does the rest. For a cut of a given shape, volume grows with the cube of its size. Make a stone twice as wide, twice as long and twice as deep, and it holds eight times as much diamond.

Now turn that around. When you double the weight, you multiply every dimension by the cube root of 2, which is 1.26. That's the 26%.

The face-up area grows with the square of the width. So the 2 ct shows about 59% more surface than the 1 ct. That's a real step up, and you can see it. It just isn't double.

Try it yourself

1. Open 3D Gemstone Simulator with the T73 Round Brilliant in diamond.
2. In the report, find Carat weight. Click through every step from 0.25 to 5.
3. Watch the stone on screen. Then watch the scale bar under it, and read Measurements in the report at each step.
4. Before you click 5, write down your guess for its width.
5. The challenge: which carat step shows about twice the face-up area of the 0.50 ct? Use the measurements to find it, not your eye.

What the simulator shows

The stone didn't grow on screen. The simulator always makes the stone fill the view, so you can study its pattern at any weight. The scale bar and the measurements show the size. As the carat goes up, each millimeter takes up less of the picture, so the bar gets shorter. From time to time it jumps to a rounder length (0.5, 1 or 2 mm). Read the number on it.

The measurements follow the cube root. Here's the T73 in diamond:

The 5 ct has five times the weight of the 1 ct, but it is only 71% wider. To be twice as wide as the 1 ct, a T73 must weigh 8 carats.

The challenge. The area grows with the weight to the power two-thirds. So twice the area of the 0.50 ct needs 2.83 times its weight: about 1.41 ct. The nearest step is 1.50 ct: 7.42 mm against 5.14 mm, so 2.08 times the area. Three times the weight, twice the face.

Same carat, different size. Carat sets only the weight. The cut and the material decide how that weight spreads out. In moissanite, the same 1.00 ct T73 is 6.67 mm across, because moissanite is less dense than diamond. And a badly cut diamond hides weight in a deep pavilion and a thick girdle. Diamond cut grade: Excellent vs Poor shows this side by side.

Magic sizes

Some weights sell better than others: half a carat, three-quarters, one carat. Demand makes these round numbers special, not looks. A 0.99 ct T73 is 0.02 mm narrower than a 1.00 ct, and nobody can see that difference. But it is below the 1-carat line, so its price is usually much lower.

Price doesn't go up in a straight line either. Larger diamonds are rarer and people want them more. So a 2-carat stone costs more than two 1-carats when the other three Cs are equal.

Key points

• One carat is 200 mg, and it has 100 points. Labs weigh to five decimals and report two.
• Weight grows with the cube of size. So double the weight is only 26% wider, with 59% more face-up area.
• The simulator keeps the stone the same size on screen. Read the scale bar and the measurements for its true size.
• The same carat can face up larger or smaller. The material and the cut decide.
• Round "magic" weights have a price step that the eye cannot see: 0.99 ct is 0.02 mm narrower than 1.00 ct.

Go further: the T73 Round Brilliant cut page lists its weight and measurements from the 3D model.