A fuller oval and a slender oval can carry the same carat weight and create different silhouettes. Length-to-width ratio names that proportion: divide the face-up length by the face-up width, keeping both measurements in millimeters.
Calculate the ratio
Use the two face-up dimensions from the report, not the depth. If an illustrative oval measures 10.0 × 6.5 mm, its ratio is 10.0 ÷ 6.5, or approximately 1.54.
| Illustrative dimensions | Calculation | Approximate ratio |
|---|---|---|
| 8.0 × 8.0 mm | 8.0 ÷ 8.0 | 1.00 |
| 9.0 × 7.0 mm | 9.0 ÷ 7.0 | 1.29 |
| 10.0 × 6.5 mm | 10.0 ÷ 6.5 | 1.54 |
| 11.0 × 6.0 mm | 11.0 ÷ 6.0 | 1.83 |
These are mathematical examples, not measurements of Just Brilliant inventory or recommended proportions for every shape.
Compare outlines within a shape
For ovals, the ratio helps distinguish a fuller outline from a more elongated one. For cushions, the rounded corners and the curve of the sides also affect the impression. Radiants have cropped corners, so an identical ratio does not make a radiant and a cushion look identical.
A pear's shoulders and point, or a marquise's curved sides, introduce additional variables. The same ratio across different shapes is not an exact visual match.
There is no universal ideal ratio for personal taste. The outline should suit the intended design, while the stone's optical appearance and physical condition require separate assessment.
Keep millimeters beside the ratio
This is why a shortlist should record shape, length, width and ratio together. Carat weight belongs on that list too, but it measures mass rather than the outline on the hand.
Do not calculate a precise face-up area by treating every fancy diamond as a rectangle. Rounded or pointed outlines occupy less area than the rectangle drawn around their maximum measurements.
Use a fair visual comparison
View images at a common scale, with the same orientation. A cropped product photograph that fills the frame can make two differently sized diamonds appear equally large.
Try a paper outline or a correctly scaled screen comparison to discuss proportions, then see the actual jewel. State clearly if a visual is schematic. A drawing is useful for exploring silhouette; it cannot establish the stone's color, clarity or light performance.
Turn the shortlist into a design comparison
The equal-scale outlines below use a common 6.0 mm width. Lengths of 7.2, 9.0 and 10.8 mm produce ratios of 1.20, 1.50 and 1.80. Because the width stays fixed, you can see what increasing length changes. This is a proportion study, not a carat comparison.
For your own shortlist, record the actual outline as well as the numbers. Trace or print the outline with a ruler reference, then mark the proposed prong positions and side stones. Check the printed reference against a ruler: a page or screen resized automatically will not preserve physical size.
If two candidates share a ratio, compare their absolute measurements before deciding that they will occupy the same space. A 9 × 6 mm stone and a 12 × 8 mm stone both have a 1.50 ratio, but the second extends 3 mm farther in length and 2 mm farther in width. The mounting adds another footprint beyond the stone itself.
Keep the ratio beside the dimensions and the silhouette. Together they describe a design choice much more clearly than “large” or “elongated” alone.
Sources: GIA oval-cut diamond guide; GIA guide to diamond shapes.
