Diamond Size Calculator: How Big Is a Diamond by Carat?
Last updated: May 2026 | Independently researched by Diamond Guide SA
A 1 carat round brilliant diamond measures about 6.5 mm across. Carat is a measure of weight, not size, so the millimetre dimensions depend on the shape and the cut. A 0.5 carat round is roughly 5.2 mm, a 1.5 carat is about 7.4 mm, and a 2 carat is around 8.2 mm. Use the free tool below to see the true, to-scale size of any diamond by carat and shape, then check the full carat-to-millimetre chart.
6.5 mm×6.5 mm
1 ct round · approximate actual size on screen
Want to see stones this size in person?
Our Editor's Choice, ProDiam in Bedfordview, lets you view loose diamonds to scale before buying.
Sizes are based on well-cut proportions and scale with the cube root of carat weight from standard 1 carat reference dimensions. Actual stones vary with cut depth and spread. Use the calibration option for a true-to-life on-screen size.
Round Diamond Size Chart (Carat to mm)
| Carat weight | Diameter (mm) | How it reads on the hand |
|---|---|---|
| 0.25 ct | 4.1 mm | Delicate, understated |
| 0.50 ct | 5.2 mm | Classic, modest presence |
| 0.75 ct | 5.9 mm | Classic, modest presence |
| 1.00 ct | 6.5 mm | Strong, classic engagement size |
| 1.25 ct | 7.0 mm | Strong, classic engagement size |
| 1.50 ct | 7.4 mm | Strong, classic engagement size |
| 2.00 ct | 8.2 mm | Large, high-impact |
| 2.50 ct | 8.8 mm | Large, high-impact |
| 3.00 ct | 9.4 mm | Large, high-impact |
Diamond Size by Shape (at 1 Carat)
All ten diamonds below weigh exactly 1 carat, yet their face-up size differs a lot. Elongated shapes spread wider and look bigger; deeper cuts hold weight below the surface.
| Shape | Size at 1 ct (L × W, mm) | Notes |
|---|---|---|
| Round brilliant | 6.5 × 6.5 mm | The size benchmark |
| Oval | 7.7 × 5.7 mm | Looks larger than round |
| Marquise | 10.0 × 5.0 mm | Largest visual spread |
| Pear | 8.7 × 5.7 mm | Elongated, looks big |
| Emerald | 7.0 × 5.0 mm | Long, elegant |
| Radiant | 6.2 × 5.6 mm | Slightly rectangular |
| Cushion | 5.75 × 5.75 mm | Compact, soft corners |
| Asscher | 5.6 × 5.6 mm | Square step cut |
| Princess | 5.5 × 5.5 mm | Square, weight held deep |
| Heart | 6.3 × 6.3 mm | Romantic, brilliant cut |
Carat Is Weight, Not Size
One carat equals 0.2 grams. Because it measures weight, two diamonds of the same carat can look completely different sizes. A stone cut to ideal proportions shows its full diameter and maximum sparkle. A stone cut too deep to save weight looks smaller from the top and often sparkles less. This is why cut quality matters more than carat when you want a diamond to look big and brilliant.
Size also grows slowly with weight. Doubling the carat does not double the width: a 2 carat round is only about 26% wider than a 1 carat, because a diamond is three-dimensional and weight scales with volume. That is why jumping from 1 ct to 2 ct adds a lot of cost but a modest amount of visible width.
How to picture the size in real life
- A 1 carat round (6.5 mm) is roughly the width of a standard pencil eraser.
- A 0.5 carat round (5.2 mm) is close to the size of a small dress-shirt button hole.
- Use the "Compare carats" tab above to see the jump between sizes side by side.
- Use the "On a finger" tab to picture the stone on the hand to scale.
Diamond Size FAQs
How big is a 1 carat diamond?
A 1 carat round brilliant diamond measures about 6.5 mm in diameter when cut to good proportions. Fancy shapes of the same weight look different: a 1 carat oval is roughly 7.7 × 5.7 mm and a 1 carat princess about 5.5 × 5.5 mm. Carat is a measure of weight, not size, so the millimetre dimensions depend on the cut.
What is the mm size of a 2 carat diamond?
A 2 carat round brilliant diamond is approximately 8.2 mm in diameter. It is heavier and wider than a 1 carat stone, but because size scales with the cube root of weight, a 2 carat diamond is only about 26% wider than a 1 carat, not twice as wide.
Is a 1 carat diamond big?
A 1 carat diamond is considered a substantial, classic engagement-ring size in South Africa and worldwide. At about 6.5 mm across for a round, it has clear presence on the hand without being oversized. Elongated shapes like oval, marquise and pear make the same carat weight look noticeably larger.
Which diamond shape looks the biggest for its carat weight?
Elongated shapes look the largest face-up for the same carat weight, because more of the diamond's weight is spread across the top. Marquise, oval and pear cuts appear biggest, while princess, asscher and cushion cuts hold more weight in their depth and can look smaller per carat.
Does carat mean size or weight?
Carat measures weight: 1 carat equals 0.2 grams. Two diamonds of the same carat weight can look different sizes depending on shape and how well they are cut. A shallow-cut stone spreads wider (looks bigger) but can lose sparkle, while a deep-cut stone looks smaller face-up but hides more weight underneath.
Why do two 1 carat diamonds look different sizes?
Cut proportions and shape. A diamond cut to ideal proportions shows its full face-up size and maximum sparkle. A diamond cut too deep hides weight in the pavilion and looks smaller from the top. This is why cut quality, not just carat, decides how big a diamond actually looks.
See diamonds this size in person
A size chart only goes so far. To judge real presence, view loose stones to scale before you buy. Our Editor's Choice dealer, ProDiam in Bedfordview, manufactures GIA ideal-cut natural diamonds and lets you compare sizes side by side at factory-direct pricing.