1.5-Carat Diamond Size: How Big Is It in Millimeters?
A 1.5-carat round diamond is about 7.4 mm across. Compare shape benchmarks, the step up from 1 carat, and actual report measurements.
A 1.5-carat round brilliant diamond is approximately 7.4 mm across, or about 0.29 inches. It is a useful shopping benchmark, not an exact specification: Brilliance’s size chart gives 7.3 mm, while VRAI’s tabulated reference gives 7.33–7.53 mm. The particular diamond’s measurements matter more than the chart. (Brilliance, VRAI)
The “1.5 carat” label describes weight, not diameter. One carat equals 200 milligrams, so a 1.5-carat diamond weighs 0.30 grams. Its shape and proportions determine how that weight translates into visible size. (GIA carat-weight guide)
What size is a 1.5-carat diamond by shape?
For a round diamond, compare diameter. For other shapes, compare length and width—the outline you see when looking down at the stone.
The following are rounded midpoints from VRAI’s tabulated size references for its lab-grown diamonds. They illustrate possible dimensions at 1.5 carats, not universal averages or required measurements for every diamond of that shape.
| Diamond shape | Approximate face-up reference at 1.5 ct |
|---|---|
| Round brilliant | 7.4 mm diameter |
| Oval | 9.6 × 6.5 mm |
| Princess | 6.4 × 6.4 mm |
| Cushion | 7.2 × 6.8 mm |
| Emerald | 8.3 × 5.8 mm |
| Pear | 10.6 × 6.4 mm |
| Marquise | 12.0 × 5.9 mm |
An elongated outline can give you more length along the finger without giving you more width across it. For example, the oval reference is longer than the round’s diameter but narrower across the finger. Likewise, a princess diamond’s side length is not directly equivalent to a round’s diameter; their outlines differ.
If you prefer an oval, use our oval carat-size chart to explore proportions in more detail rather than treating one length-to-width combination as ideal.
How much bigger is 1.5 carats than 1 carat?
For round brilliants, useful rounded references are 6.5 mm at 1 carat, 7.4 mm at 1.5 carats, and 8.2 mm at 2 carats, based on VRAI’s size table.
Using those diameters, a 1.5-carat round is approximately:
- 0.9 mm wider than a 1-carat round—about a 14% increase in diameter.
- 30% larger in circular face-up area than a 1-carat round, calculated from those benchmark diameters.
- 0.8 mm narrower than a 2-carat round.
That explains why 50% more weight does not mean a diamond looks 50% wider. The area calculation is a geometric comparison, not a prediction of perceived size or sparkle. For the next step up, see our 2-carat diamond size guide.
Check the actual stone before deciding
Read the measurements line. On a GIA report, round diamonds are listed as minimum diameter–maximum diameter × depth; fancy shapes use length × width × depth. A hypothetical round measurement of 7.35–7.42 × 4.55 mm therefore describes a stone about 7.4 mm across—not 4.55 mm across. (GIA report-reading guide)
Do not chase millimeters alone. Weight can sit in an extremely thick girdle, steep crown or deep pavilion rather than contribute efficiently to the face-up outline. GIA’s round-brilliant cut system penalizes excess weight in these features and also evaluates appearance. A larger measurement is not, by itself, evidence of a better cut. (GIA cut overview)
Preview it at actual scale. For a first impression, draw a 7.4 mm circle on paper and place it over your finger. Then try a ring with a similar-size stone in the setting you intend to buy. Compare it with a 6.5 mm and an 8.2 mm example under the same conditions. This makes the 1.5-carat choice concrete without relying on enlarged listing photos or someone else’s idea of what looks “big.”