Round Cut Diamond Size Chart
How big isa round, really?Carat to millimetres.
A round diamond's true size is its diameter in millimetres. This chart converts every popular carat weight to a real measurement, using standard well-cut round-brilliant proportions.
Why carat isn't the same as size.
Carat measures weight, not dimensions. The size you actually see is the stone's measurement in millimetres — and two diamonds of the same carat can look different sizes depending on how they are cut.
A shallow, well-spread stone shows more face-up area; a deep-cut one hides weight underneath. So when comparing diamonds, compare the millimetre diameter, not just the carat weight.
"The number that decides how big a round looks isn't the carat — it's the diameter in millimetres."
The chart below gives you both, so you can picture the real size before you choose.
What makes the round special.
Its 57–58 facets are arranged to return the maximum light — the round is the benchmark every other shape is measured against for sparkle.
Length equals width, so a round is described by a single diameter — no length-to-width ratio to weigh up.
More than any other shape, a round lives or dies by its cut. An excellent cut is what unlocks all that brilliance.
Busy brilliant faceting masks small inclusions and slight tint, so an eye-clean, near-colourless round looks superb.
What each size actually looks like.
These round shapes are drawn to relative scale from real diameters. Notice how gently size grows — a 2 carat is larger than a 1 carat, but not twice the size.
Drawn to relative scale for a well-cut round. On-screen sizes are proportional, not literal millimetres — see the chart below for exact diameters.
Round Cut carat to millimetre chart.
Approximate diameter for a well-cut round brilliant. These are standard, well-established proportions; individual stones vary slightly with cut and depth. The exact diameter is listed on every IGI report.
| Carat Weight | Diameter (mm) | L/W Ratio | Source |
|---|---|---|---|
| 0.25 ct | 4.1 mm | 1.00 | Standard |
| 0.50 ct | 5.2 mm | 1.00 | Standard |
| 0.75 ct | 5.9 mm | 1.00 | Standard |
| 1.00 ct | 6.5 mm | 1.00 | Standard |
| 1.25 ct | 6.9 mm | 1.00 | Standard |
| 1.50 ct | 7.4 mm | 1.00 | Standard |
| 1.75 ct | 7.8 mm | 1.00 | Approx. |
| 2.00 ct | 8.2 mm | 1.00 | Standard |
| 2.50 ct | 8.8 mm | 1.00 | Approx. |
| 3.00 ct | 9.4 mm | 1.00 | Standard |
| 4.00 ct | 10.4 mm | 1.00 | Approx. |
| 5.00 ct | 11.2 mm | 1.00 | Approx. |
For a round, cut is everything.
A round has no length-to-width ratio to choose — it is always a perfect circle. What decides how it looks is the cut. Here's what to aim for.
Length equals width, so size is a single diameter — no proportions to balance, just how big across it measures.
Prioritise an Excellent or Ideal cut grade. A top-cut round outshines a larger, poorly cut stone every time.
The best-cut rounds show a crisp hearts-and-arrows pattern — a sign of top optical symmetry and light return.
Four things worth knowing first.
Beyond size, a few round-specific pointers help you choose a stone that looks its best.
For a round, cut outranks every other factor — it's what creates the sparkle. Choose Excellent or Ideal before spending on higher colour or clarity.
A round's busy faceting hides tint well, so a near-colourless stone looks perfectly white — especially set in white gold or platinum.
Brilliant faceting masks small inclusions, so an eye-clean VS round looks flawless to the naked eye at far better value.
A 1 carat round (about 6.5 mm) is the instantly recognisable engagement size and sits beautifully on almost any hand.
How the setting changes the size you see.
The same stone can look bigger or smaller depending on how it is set. If maximising visual size matters, the setting is as important as the carat weight.
A solitaire shows the true size; a halo frames it to read larger; a slim band makes the centre look bigger by comparison. Choosing well means you can often get the presence you want without moving up in carat.
Browse round diamonds by size.
Found your size? Go straight to the collection — every stone IGI certified and lab grown.
Round size questions, answered.
The questions we hear most about round diamond sizing. If yours isn't here, just ask.
Chat on WhatsAppEverything you need to choose with confidence.
Find your perfect round.
Every round is IGI certified and made to order, in the exact carat and setting you choose. Not sure which size suits the hand? Tell us and we'll guide you.