Length to width ratio is the length of a diamond divided by its width. The number tells you how elongated or how compact the outline will look when the stone is viewed face up, and it is one of the first things we read on a grading report when we shortlist ovals, pears, marquises and other fancy shapes.
How the ratio is calculated
A diamond report lists the measurements in millimetres. For a fancy shape this is usually written as length, then width, then depth. Divide the length by the width and you have the ratio. A stone that measures 9.00 mm by 6.00 mm has a ratio of 1.50. A stone that measures 8.00 mm by 6.40 mm has a ratio of 1.25 and will look noticeably fuller.
We always check the actual outline as well as the number. Two ovals with the same ratio can still look different if the shoulders are flat, if one end is fuller than the other, or if the width is not even along the length. That is why we share photographs and, where we can, a video of the stone before you approve it in writing.
Ranges GIA mentions, and why taste still matters
There is no single correct ratio. In its holiday buying guide, the Gemological Institute of America gives an optimal length to width range for ovals of 1.33 to 1.66 to 1, a preferred range for pears of 1.50 to 1.75 to 1, and a range to look for in a marquise of 1.75 to 2.25 to 1. In a separate oval cut guide, GIA notes that many people prefer ovals between 1.30 to 1 and 1.50 to 1, with a more rounded look around 1.30 to 1.40 and a more traditional oval around 1.40 to 1.50.
We treat those figures as a starting point, not a rule. A slightly longer oval can look graceful on a narrow finger. A slightly fuller oval can feel generous on a wider hand. What matters is that you like the outline, that the shoulders and ends are even, and that any bow tie across the centre is a level you are comfortable with. Our guide to the bow tie effect in oval, pear and marquise diamonds explains that last point in more detail.
| Shape | GIA length to width guidance |
|---|---|
| Oval | Optimal 1.33 to 1.66 to 1 in GIA's holiday buying guide; many people prefer 1.30 to 1.50 to 1 in GIA's oval cut guide |
| Pear | Preferred 1.50 to 1.75 to 1 in GIA's holiday buying guide |
| Marquise | Look for 1.75 to 2.25 to 1 in GIA's holiday buying guide |
What the ratio does not tell you
The ratio describes the outline, not the quality of the cut. GIA does not assign a cut grade to most fancy shapes in the same way it does for a standard round brilliant. The report will still record polish, symmetry and the millimetre measurements, which we review with you. A well proportioned stone can still show a strong bow tie, and a stone just outside a published range can still look beautiful in the hand.
The setting changes the impression too. A halo adds a frame around the outline. Side stones can make a centre look longer or more compact. Claws at the tips of a marquise or a pear also protect those points, which is a durability point GIA highlights as well as an aesthetic one. We design the mount around the actual stone, and nothing is made until you approve the design and the stone in writing. Making time is then 5 to 6 weeks, and the finished ring is UK hallmarked before insured, tracked FedEx delivery.
If you would like us to shortlist stones by outline as well as by the report, start with a video consultation, browse the diamonds we can source, or read our guides to oval cut diamonds and marquise engagement rings.
Frequently asked questions
What is length to width ratio on a diamond?
It is the length of the stone divided by its width, both taken from the measurements on the grading report. The number describes how elongated or how compact the outline looks when the diamond is viewed face up.
What length to width range does GIA mention for ovals, pears and marquises?
In its holiday buying guide, GIA gives an optimal range for ovals of 1.33 to 1.66 to 1, a preferred range for pears of 1.50 to 1.75 to 1, and a range to look for in a marquise of 1.75 to 2.25 to 1. Those figures are guidance, not a grade, and personal preference still matters.
Does a higher ratio make a diamond look larger?
A more elongated outline can look longer on the finger than a compact stone of the same carat weight, but the report measurements, the cut and the setting all play a part. We look at the stone with you rather than choosing by the ratio alone.
Do fancy shapes receive a GIA cut grade?
GIA does not assign a cut grade to most fancy shapes in the same way it does for a standard round brilliant. The report still records the measurements, plus polish and symmetry, which we review with you on a video consultation.