Diamond shapes — round, oval, marquise, pear, cushion, emerald and asscher — differ in far more than outline: each has an ideal length-to-width ratio where it looks its best, and each carries its carat weight differently, so a one-carat oval and a one-carat asscher are dramatically different sizes on the finger. Below is the working chart I use at the bench: millimetre sizes for every major shape from 0.10 to 3.00 carats, the ratio sweet spots, and the honest trade-offs between them.
How to read this chart (the fine print that matters)
Carat is weight, not size. Two stones of identical weight can face up very differently depending on how deep they're cut — a deep stone hides weight underneath where you can't see it; a shallow one spreads it wide but can leak light. The measurements below assume well-proportioned cutting and are accurate to roughly ±0.2 mm; treat them as the centre of the range, not gospel. When you're comparing actual stones, always compare the millimetre measurements on the certificate, never the carat weight alone — it's the single most useful habit a diamond buyer can learn.
Diamond size chart: mm by carat weight for every shape
| Carat | Round Ø | Oval | Marquise | Pear | Cushion | Emerald | Asscher |
|---|---|---|---|---|---|---|---|
| 0.10 | 3.0 | 3.7 × 2.7 | 4.9 × 2.5 | 4.0 × 2.6 | 2.7 | 3.4 × 2.4 | 2.7 |
| 0.25 | 4.1 | 5.0 × 3.6 | 6.7 × 3.3 | 5.5 × 3.5 | 3.6 | 4.7 × 3.2 | 3.6 |
| 0.50 | 5.1 | 6.3 × 4.5 | 8.4 × 4.2 | 6.9 × 4.5 | 4.6 | 5.9 × 4.1 | 4.5 |
| 0.70 | 5.7 | 7.1 × 5.1 | 9.4 × 4.7 | 7.7 × 5.0 | 5.1 | 6.6 × 4.5 | 5.1 |
| 1.00 | 6.4 | 7.9 × 5.7 | 10.6 × 5.3 | 8.7 × 5.6 | 5.7 | 7.4 × 5.1 | 5.6 |
| 1.50 | 7.4 | 9.1 × 6.5 | 12.1 × 6.1 | 9.9 × 6.4 | 6.5 | 8.5 × 5.8 | 6.4 |
| 2.00 | 8.1 | 10.0 × 7.1 | 13.3 × 6.7 | 10.9 × 7.0 | 7.2 | 9.3 × 6.4 | 7.1 |
| 2.50 | 8.7 | 10.8 × 7.7 | 14.3 × 7.2 | 11.8 × 7.6 | 7.7 | 10.0 × 6.9 | 7.6 |
| 3.00 | 9.3 | 11.4 × 8.2 | 15.2 × 7.6 | 12.5 × 8.1 | 8.2 | 10.7 × 7.4 | 8.1 |
All measurements in millimetres, length × width; round, cushion and asscher shown as a single dimension (cushion and asscher assume a square outline). Oval calculated at a 1.40 ratio, marquise at 2.00, pear at 1.55, emerald at 1.45.
Round brilliant — ratio 1.00–1.02
The benchmark, and the shape every other is measured against. Fifty-seven facets engineered over a century for maximum light return — a story I've told in full in my round brilliant history. It's also the most expensive shape per carat: demand is highest and cutting one wastes the most rough. A round hides clarity issues well and forgives lower colour grades. If sparkle is the brief, this is the answer. Browse them in our solitaire settings.
Oval — best ratio 1.30–1.50
The oval faces up around 10% larger than a round of the same weight — that 7.9 × 5.7 mm one-carat oval covers noticeably more finger than a 6.4 mm round. The sweet spot is 1.35–1.45: below 1.30 it reads as a squashed round, beyond 1.50 it starts to flirt with bow-tie territory. And about that bow-tie — the dark bands across the centre that every elongated brilliant cut shows to some degree. A faint one is normal; a black one is a cutting fault. Ask to see the stone (or a video of it) face-up in normal light before you commit. See our oval engagement rings.
Marquise — best ratio 1.85–2.10
The boat-shaped cut with royal French history and the largest face-up spread of any shape, full stop: a one-carat marquise stretches over 10 mm long. It makes fingers look longer and carats look bigger, and after decades out of fashion it's rising fast again. The classic ratio is 2.00; shorter reads vintage, longer gets fragile and floppy-looking. Two non-negotiables from the bench: the two points must be protected by claws or V-tips (points are where chips happen), and symmetry is everything — the two halves must mirror perfectly, and the bow-tie rules from the oval apply here too.
Pear — best ratio 1.45–1.75
Half round, half marquise: a teardrop with one protected point. The sweet spot for a balanced, classic teardrop is 1.50–1.60 — shorter looks stubby, longer goes dramatic (which can be exactly the point in a pendant or toi et moi pairing). The rounded shoulders should be full and even, the point crisp and centred on the axis. Worn point-toward-the-nail it lengthens the finger beautifully. Like all elongated brilliants: check the bow-tie, and insist on a V-claw over the tip.
Cushion — best ratio 1.00–1.05 (square) or 1.15–1.30 (elongated)
The pillow-shaped romantic — soft corners, big crushed-ice or chunky-facet sparkle, and the shape of choice for antique-leaning designs. Cushions are cut deep, which is why the chart shows a one-carat square cushion at just 5.7 mm: it faces up the smallest of the brilliant cuts per carat. Compensate by prioritising spread (millimetres, not carats) when comparing stones. Choose square (1.00–1.05) for classic proportions or elongated (1.15–1.30) for finger coverage; avoid the awkward in-between around 1.10, which reads as neither.
Emerald cut — best ratio 1.30–1.50
The step cut: long parallel facets, a hall-of-mirrors flash instead of sparkle, and unapologetic Art Deco elegance. The classic ratio is around 1.40–1.45. One thing the emerald cut demands is honesty in the stone itself — those big open facets are a window, not a disguise, so buy a grade higher on colour and clarity than you would for a brilliant cut (VS2+ is my floor here, as I explain in the clarity guide). Its corners are cropped, which makes it surprisingly durable. Browse emerald cut engagement rings.
Asscher — best ratio 1.00–1.05
The square step cut, patented by the Asscher brothers of Amsterdam in 1902 — the family firm that famously cleaved the Cullinan diamond. Its high crown and deep pavilion create the mesmerising "windmill" pattern of concentric squares. That depth costs spread: at 5.6 mm, a one-carat asscher is the smallest face-up of the seven shapes, so buy for the hypnotic geometry, not for size per dollar. The same step-cut honesty rules as the emerald apply: colour and clarity have nowhere to hide.
So which shape looks biggest — and which is best value?
Per carat, the face-up ranking runs roughly: marquise, pear and oval largest; round in the middle; cushion, emerald and asscher smallest. On price, fancy shapes typically cost 10–25% less per carat than rounds, so an oval or pear delivers a double win: cheaper per carat and bigger per carat. The rounds and asschers of the world buy something else — perfection of light and geometry. Neither choice is wrong; they're different briefs. For how shape fits into the wider decision, start with the 4Cs and my Australian diamond buying guide — and every measurement in this article applies identically to lab-grown diamonds, which are cut to the same proportions.
If you're weighing two shapes against each other for the same hand, this is exactly the conversation of a bespoke consultation — I'll put physical stones of both shapes on the finger and the decision usually makes itself in minutes.
Diamond shapes FAQ
How big is a 1 carat diamond in mm?
A well-cut round is about 6.4 mm across. The same weight is roughly 7.9 × 5.7 mm as an oval, 10.6 × 5.3 mm as a marquise, 8.7 × 5.6 mm as a pear, 5.7 mm as a square cushion, 7.4 × 5.1 mm as an emerald cut and 5.6 mm as an asscher.
What is the best length-to-width ratio for an oval diamond?
1.35–1.45 for a classic, balanced oval. Under 1.30 looks like an off-round; over 1.50 increases bow-tie risk.
Which diamond shape looks the biggest?
Marquise, followed by pear and oval — elongated shapes spread their weight across the finger. Deep cuts like the asscher and cushion look the smallest per carat.
Which diamond shape is the cheapest per carat?
Fancy shapes generally run 10–25% below rounds, with elongated brilliants (oval, pear, marquise) often the strongest value once you factor in their larger face-up size.
Do these sizes apply to lab-grown diamonds?
Yes — lab-grown diamonds are cut to identical proportions, so every millimetre figure in the chart holds for both.
