Siamrington · Diamond Guide
Diamond Carat Size
Explore how carat weight, shape, and real-world proportions come together. View each Siamrington diamond silhouette in relative scale – from delicate accent stones to bold centre stones – and compare two side by side.
At a glance
A few gentle reference points before you begin comparing – carat weight and diameter for well-cut round brilliants.
0.05 ct
≈ 2.39 mm
0.10 ct
≈ 3.02 mm
0.20 ct
≈ 3.80 mm
0.30 ct
≈ 4.35 mm
0.40 ct
≈ 4.79 mm
0.50 ct
≈ 5.16 mm
1.00 ct
≈ 6.50 mm
1.50 ct
≈ 7.44 mm
2.00 ct
≈ 8.19 mm
3.00 ct
≈ 9.37 mm
Approximations for round brilliants. Other shapes vary by proportion – see the comparison tool below.
Diamond A
Approx. diameter
6.50 mm diameter
Diamond B
Approx. diameter
8.19 mm diameter
Silhouettes in relative scale
Each outline below is drawn from its approximate millimetre measurements, shown to scale against one another. You may gently bring your ring or loose diamond close to the screen to sense how it compares.
1.00 ct · Round Brilliant
6.50 mm diameter
2.00 ct · Round Brilliant
8.19 mm diameter
Face-up presence
Diamond B presents approximately 58.7% more face-up area than Diamond A.
Diamond A
Covers approximately 41% of a 16 mm finger.
Diamond B
Covers approximately 51% of a 16 mm finger.
Coverage is based on a graceful, average ring finger width of 16 mm (approximately a US size 6–7). Hands are wonderfully unique, so treat this as guidance rather than a strict rule.
Round brilliant reference
Below are gentle approximations for well-cut round diamonds. Other shapes in this tool are scaled gracefully from these measurements using typical Siamrington proportions.
| Carat weight | Approx. diameter (mm) |
|---|---|
| 0.01 ct | 1.40 mm |
| 0.02 ct | 1.76 mm |
| 0.03 ct | 2.02 mm |
| 0.04 ct | 2.22 mm |
| 0.05 ct | 2.39 mm |
| 0.06 ct | 2.54 mm |
| 0.07 ct | 2.68 mm |
| 0.08 ct | 2.80 mm |
| 0.09 ct | 2.91 mm |
| 0.10 ct | 3.02 mm |
| 0.15 ct | 3.45 mm |
| 0.20 ct | 3.80 mm |
| 0.25 ct | 4.09 mm |
| 0.30 ct | 4.35 mm |
| 0.35 ct | 4.58 mm |
| 0.40 ct | 4.79 mm |
| 0.45 ct | 4.98 mm |
| 0.50 ct | 5.16 mm |
| 0.60 ct | 5.48 mm |
| 0.70 ct | 5.77 mm |
| 0.75 ct | 5.91 mm |
| 0.80 ct | 6.03 mm |
| 0.90 ct | 6.28 mm |
| 1.00 ct | 6.50 mm |
| 1.10 ct | 6.71 mm |
| 1.20 ct | 6.91 mm |
| 1.25 ct | 7.00 mm |
| 1.30 ct | 7.09 mm |
| 1.40 ct | 7.27 mm |
| 1.50 ct | 7.44 mm |
| 1.75 ct | 7.83 mm |
| 2.00 ct | 8.19 mm |
| 2.50 ct | 8.82 mm |
| 3.00 ct | 9.37 mm |
| 3.50 ct | 9.87 mm |
| 4.00 ct | 10.32 mm |
| 5.00 ct | 11.11 mm |
Siamrington creates tools to assist your eye and intuition. For formal grading and certification, always refer to the original laboratory report (such as GIA or IGI).
Questions about diamond size
How many millimetres is a 1 carat diamond?+
A well-cut round brilliant of 1 carat measures approximately 6.5 mm across. This is a gentle approximation – every Siamrington diamond is individually graded, so the exact figure on its certificate should always be your reference.
Does a 2 carat diamond look twice the size of a 1 carat diamond?+
Not quite. Carat measures weight, not width, so diameter grows more gently than weight does – a 2 carat round brilliant is around 26% wider than a 1 carat stone, though its face-up area appears close to double.
Why do some diamond shapes appear larger at the same carat weight?+
Elongated shapes such as oval, pear, and marquise spread their weight across a larger surface, so they can appear more generous on the hand than a round or princess cut diamond of the same carat weight.
Does carat weight or cut affect how large a diamond looks?+
Both play a part. Carat weight sets the overall size, while cut proportions determine how gracefully that weight is distributed across the face-up surface – a well-cut stone can appear more generous than its carat weight alone would suggest.
Considering a bespoke centre stone?
Our consultants can help you select the carat, shape, and cut that feel most like you – and source a GIA-certified diamond to match.
Begin a bespoke consultation