Diamond Carat to MM Calculator
Carat weight against actual spread.
Convert diamond carat weight to millimetre diameter and back for round brilliants, with the effect of cut depth on spread.
Diameter
6.44 mm
1.00 ct round brilliant
Weight goes as the cube of the diameter, so doubling the carat adds only about 26% to the spread. A well-cut stone also spreads wider than a deep one of the same weight, which is why two stones of identical carat can look different sizes.
How the Diamond Carat to MM Calculator works
Carat is a weight, not a size, and the difference matters more than almost anything else in buying a stone. Weight goes as the cube of the diameter, so a two carat diamond is only about 26% wider than a one carat — not twice as wide. A well-cut stone also spreads wider than a deep one of identical weight, which is why two stones of the same carat can look noticeably different.
Also known as: diamond size calculator · carat to millimetre converter
Why weight and size are not the same thing
A carat is a weight — exactly 0.2 grams — and a diamond's visible size is its face-up diameter. Weight scales with volume, which goes as the cube of the linear dimension, so size grows far more slowly than weight.
The consequence is counterintuitive and expensive. A two carat round brilliant is not twice as wide as a one carat; it is about 26% wider, 8.2 mm against 6.5. A four carat stone is only twice the width of a one carat.
The estimating formula for a round brilliant is diameter squared times depth times 0.0061, where the constant folds in diamond's specific gravity of 3.52. A well-cut stone has a depth around 61.5% of its diameter, which is what makes 6.5 mm come out at almost exactly one carat.
A worked example
A 6.5 mm round brilliant at a typical depth of 4.0 mm: 6.5 × 6.5 × 4.0 × 0.0061 = 1.03 carats. Increase the depth to 4.4 mm without changing the diameter and the same visible size weighs 1.13 carats — ten percent more weight for no more spread.
That is exactly what a deeply cut stone does: it carries weight below the girdle where nobody sees it. The buyer pays for the carat and looks at the millimetres.
Where the answer misleads
Spread varies with cut quality, and the formula assumes reasonable proportions. A stone with a very thick girdle or a deep pavilion can weigh several percent more than its diameter suggests, and one cut for spread can weigh less. Certificates list both the measurements and the weight, and comparing them is the check.
Cutting for maximum spread also has a cost. The proportions that return the most light are fairly specific, and a stone pushed shallow to look bigger usually looks duller. A well-cut 0.9 carat frequently outperforms a poorly cut 1.1 in both brilliance and apparent size.
This formula is for round brilliants only. Fancy shapes vary far more between individual stones, and their spread depends on ratios the weight alone cannot capture.
Frequently asked questions
How many mm is a 1 carat diamond?
About 6.5 mm across for a well-cut round brilliant. Half a carat is about 5.2 mm and two carats about 8.2 mm — the diameter grows with the cube root of the weight, so it rises far more slowly than the carat number suggests.
Why do two stones of the same carat look different sizes?
Because weight can be hidden in depth. A stone cut with a deep pavilion or a thick girdle carries weight below the girdle where it does not show, so it spreads narrower than a shallower stone of the same carat. Face-up spread is what you see; carat is what you pay for.
Is a bigger millimetre size always better?
No. A stone cut for maximum spread at a given weight often loses brilliance, because the proportions that return light are fairly specific. A well-cut 0.9 carat frequently outperforms a poorly cut 1.1.
Does this work for other shapes?
The formula here is for round brilliants. Fancy shapes have their own factors and vary far more between individual stones, so use the coloured gemstone calculator with a shape factor, and treat the result as an estimate.