Carat is weight, not size
A metric carat is 200 milligrams (0.2 gram), and each carat is divided into 100 “points,” so a 0.75 ct diamond is “75 points.”[1] Because carat measures weight, how big a diamond looks face-up depends on its shape and how it’s cut. A diamond cut with extra depth carries weight where you can’t see it, so it can look smaller than a lighter, well-cut stone.[2]
The millimeter figures below are estimates based on typical proportions. A grading report lists a stone’s actual measurements, and those are what Terra uses when designing a setting.
Round diamond size chart
| Carat weight | Approx. diameter | Points |
|---|---|---|
| 0.25 ct | 4.1 mm | 25 |
| 0.50 ct | 5.2 mm | 50 |
| 0.75 ct | 5.9 mm | 75 |
| 1.00 ct | 6.5 mm | 100 |
| 1.50 ct | 7.4 mm | 150 |
| 2.00 ct | 8.2 mm | 200 |
| 3.00 ct | 9.4 mm | 300 |
Notice that doubling the weight from 1 to 2 carats adds only about 1.7 mm of diameter: weight grows with volume, while the face-up width grows much more slowly.
Compare shapes at one weight
A 1.00 ct round measures about 6.5 mm across face-up.
Sizes are estimates; a stone’s report gives its exact measurements.
Shapes Terra designs with
These are the center-stone shapes available in the Engagement Ring Designer. Each shape’s personality and sparkle are covered in Diamond Education; here’s how they compare in outline at one carat.
| Shape | Approx. size at 1 ct | Typical length ÷ width |
|---|---|---|
| Round | 6.5 mm | 1.00 |
| Oval | 7.7 × 5.7 mm | 1.35 |
| Cushion | 6.0 × 5.5 mm | 1.09 |
| Emerald | 7.0 × 5.0 mm | 1.40 |
| Pear | 8.5 × 5.5 mm | 1.55 |
| Radiant | 6.5 × 5.0 mm | 1.30 |
| Princess | 5.5 mm | 1.00 |
| Asscher | 5.5 mm | 1.00 |
For Terra’s review: Confirm these typical one-carat outlines match the stones Terra usually sources, especially oval, pear and emerald ratios, which vary widely between cutters.
Getting the most size
- Elongated shapes such as oval, pear and emerald spread their weight over a longer outline and often look larger per carat.
- Prices often step up at “magic” weights like 1.00 ct; a 0.90–0.99 ct stone can look nearly identical. Ask Terra to compare both.
- A thin halo or slim band can make the center look larger; a wide band can make it look smaller.
- Look at a stone’s millimeter measurements on its report, not just the carat weight.
For Terra’s review: Confirm the advice about weights just under whole-carat marks, which is a market observation rather than a lab fact.
Put it into practice
Try your choices in Terra’s design tools, then send your saved design for a personal review. Nothing is ordered or priced online; Terra confirms every detail with you first.
Also helpful: Diamond Education (the 4Cs) · Ring Size Guide