Your diamond will catch the light on countless ordinary mornings. It will be the first thing you notice on your hand and, in time, the thing you stop noticing because it has become part of you. Choosing it is one of the most considered decisions of your life, and the number most people reach for first is the carat.
Carat weight is the simplest number on a diamond certificate and the least complete. It tells you what a diamond weighs. It does not tell you how large the diamond looks on your finger, how much of it faces up towards the world, or whether its weight sits where you can see it. Two diamonds of identical carat weight can look noticeably different in size, and the difference comes down to shape, proportions and millimetres.
This guide explains what carat weight measures, why millimetre dimensions and depth tell you more, how each diamond shape spends its carats, and how to read the three numbers on a certificate that reveal the real size of a stone.
In this guide
- What carat weight actually measures
- Why millimetres tell you what carat cannot
- Depth, spread and where the weight hides
- Carat to millimetre size chart by shape
- How shape changes the way carat looks on the hand
- Choosing a size with intention
- Frequently Asked Questions
What Carat Weight Actually Measures
A carat is a unit of weight. One carat equals 0.2 grams, and each carat divides into 100 points, so a 0.75 carat diamond is a 75 point diamond. The word comes from the carob seed, which early gem traders used as a counterweight on their scales because the seeds were reasonably consistent in size. The measurement has moved on. The idea has not.
Because carat weight is the easiest of the 4Cs to understand, it tends to become the headline. Price scales with it, since larger stones are rarer in nature and take more rough to grow and cut in a laboratory. But weight and visual size are not the same thing. A diamond is a three dimensional object, and only the top of it faces you. Everything beneath the girdle, the thin edge that runs around the widest point of the stone, is weight you pay for and do not see.
A practical way to hold this in mind: carat tells you how much diamond you are getting. Millimetres tell you how much diamond you will see.
Why Millimetres Tell You What Carat Cannot
Every GIA or IGI certificate carries a measurements line, written as length by width by depth in millimetres. For a round brilliant, the first two numbers are the diameter measured at two points. For an oval, cushion, emerald or pear, they are the length and the width. The third number is always the depth, from the table at the top of the stone to the culet at the bottom.
Those three numbers are the honest description of a diamond's size. The first two tell you the footprint the stone will occupy on your hand. The third tells you how much of the carat weight sits below the surface, where it contributes to the number on the certificate but not to the view from above.
Consider a 1.00 carat oval with a certificate reading 7.9 by 5.6 by 3.6 millimetres. Dividing length by width gives a ratio of 1.41, a balanced oval. Dividing depth by width gives a depth of roughly 64 percent, which is comfortable territory for the shape. Now consider another 1.00 carat oval at 7.4 by 5.3 by 3.9 millimetres. It weighs exactly the same, costs about the same, and sits visibly smaller on the finger because more of its weight is hiding in the pavilion. The carat number cannot tell these two stones apart. The millimetres can.
This is why the first thing we look at when assessing any diamond is the measurements line, and why we encourage you to do the same. For more on what each laboratory reports and how to read a certificate, the IGI vs GIA guide walks through both.
Depth, Spread and Where the Weight Hides
In the trade, the word for how large a diamond looks for its weight is spread. A stone with good spread carries its carats across its face. A stone with poor spread carries them in its depth. Both are real diamonds of real weight. Only one of them gives you the full visual return on what you have invested.
Depth is the main reason a diamond loses spread. When a cutter works with a piece of rough, there is always a tension between keeping weight and keeping proportions. Cutting a stone slightly deeper, or leaving the girdle slightly thicker, preserves weight that would otherwise be lost, and that extra weight carries the stone past a price threshold such as 1.00 or 1.50 carats. The result is a diamond that weighs more than it looks, priced on the weight rather than the look.
Depth percentage, printed on most certificates, is the quickest way to spot this. It expresses the depth of the stone as a percentage of its width. For a round brilliant, a depth in the high fifties to low sixties is the familiar range where light performance and spread both hold up. For fancy shapes such as ovals and cushions, the comfortable range sits a little higher, since the shapes carry their weight differently. Treat these as a guide rather than a rule. A stone a point or two outside the range can still be beautiful. A stone well above it is telling you where its weight went.
The reverse also exists. A diamond cut very shallow will measure large for its weight, but it leaks light through the bottom and loses the brilliance that makes a diamond worth wearing. Spread is something to seek within the bounds of good cutting, never at its expense. The goal is a stone whose weight is honest: what you see on the hand is what you paid for.
Carat to Millimetre Size Chart by Shape
The chart below gives approximate face up measurements for well proportioned lab-grown diamonds at the most commonly chosen carat weights. The chart lists round and cushion stones by diameter or width, and elongated shapes as length by width. Use it to picture a size, then confirm the specific stone against its certificate.
| Shape | 0.50 ct | 1.00 ct | 1.50 ct | 2.00 ct | 3.00 ct |
|---|---|---|---|---|---|
| Round | 5.1 mm | 6.4 mm | 7.3 mm | 8.1 mm | 9.3 mm |
| Oval | 6.5 × 4.5 mm | 8.0 × 5.5 mm | 9.0 × 6.5 mm | 10.0 × 7.0 mm | 11.5 × 8.0 mm |
| Cushion (square) | 4.5 mm | 5.7 mm | 6.5 mm | 7.3 mm | 8.3 mm |
| Emerald | 5.5 × 4.0 mm | 7.0 × 4.5 mm | 8.0 × 5.5 mm | 8.5 × 6.0 mm | 9.7 × 7.0 mm |
| Pear | 7.0 × 4.5 mm | 9.0 × 5.5 mm | 10.0 × 6.5 mm | 11.0 × 7.0 mm | 13.0 × 8.0 mm |
| Radiant | 4.3 mm | 5.5 mm | 6.5 mm | 7.0 mm | 8.0 mm |
Figures are industry averages for well cut stones and are illustrative only. Individual diamonds vary with ratio, depth and girdle thickness. Always confirm the measurements on the stone's own GIA or IGI certificate.
Read across any row and the pattern is clear. Elongated shapes carry more of their weight across the face, which is why a 1.00 carat oval or pear measures longer than a 1.00 carat round, and why a 1.00 carat square cushion measures smaller than both. Read down any column and you will notice the steps get smaller as the carats get larger. Moving from 1.00 to 1.50 carats adds close to a millimetre to a round; moving from 2.00 to 3.00 adds a little more than that for twice the weight. Visual size grows more slowly than carat weight, and price grows faster.
How Shape Changes the Way Carat Looks on the Hand
Each diamond shape spends its carats in its own way. Knowing how yours does is the difference between choosing a weight and choosing a look. The diamond shapes guide covers the character of each cut; here the focus is purely on size.
Oval
The oval is the most generous shape per carat among the brilliant cuts, and the reason is its length. A well proportioned oval diamond at 1.00 carat covers more of the finger from knuckle to knuckle than a round of the same weight, and its length follows the line of the hand, which is why it reads as both larger and more flattering. The length to width ratio decides how far this goes. An oval around 1.35 is soft and full; an oval around 1.50 is longer and more slender and covers more finger for the same carat. Push the ratio much past 1.55 and the trade off arrives in the form of a darker bow tie across the centre. Our oval engagement ring guide goes deeper on ratio and bow tie. The short version for size: a 1.00 carat oval at 1.45 will look closer to a 1.20 carat round on the hand.
Cushion
The cushion is the shape where carat weight and visual size part ways most often. A square cushion cut diamond is cut deep by design, with a high crown and a full pavilion that give it its pillowy sparkle, and that depth means a 1.00 carat square cushion measures noticeably smaller across the top than a 1.00 carat round. It is not a flaw. It is simply where the weight lives. If you love the cushion's softness and want more presence on the hand, the elongated cushion answers both: the same rounded corners and crushed ice or chunky facet pattern, stretched to a ratio around 1.20 to 1.40 so that more of the carat sits in the length. Between two 1.50 carat cushions, the elongated one will almost always look larger.
Emerald and radiant
The emerald cut is a step cut with a large open table, so although its millimetres sit close to an oval's, the broad flat facets make the stone read as expansive and clean rather than bright. Its depth matters more than most: a deep emerald cut shows a visibly smaller window and a darker centre. The radiant shares the emerald's outline with brilliant faceting beneath, and like the cushion it tends to carry weight in its depth, so a radiant will usually measure a touch smaller than a round of the same carat while sparkling at least as hard.
Pear and marquise
The pear and the marquise are the longest shapes per carat of all. A 1.00 carat pear stretches to around nine millimetres from point to curve, and a marquise longer still, which is why both feel so substantial on the hand and why both are so often worn east-west, where their length runs across the finger rather than along it. The same length that gives them presence also narrows them, so a pear or marquise with good width across its belly will always look larger than a thin one of the same weight.
Round
The round brilliant is the benchmark for every other shape, and it is the only shape with a formal cut grade on its certificate. Its proportions are so well understood that a 1.00 carat round of excellent cut will measure within a tenth of a millimetre or so of 6.4 millimetres almost every time. If a 1.00 carat round measures well under that, its weight is in its depth. The round gives up a little face up size compared with the elongated shapes and returns it as the most consistent light performance of any cut.
Choosing a Size with Intention
With the mechanics in hand, the choice becomes a considered one rather than a numerical one. A few principles help.
Start with the hand that will wear it. Finger length and width change how any carat weight reads: a 1.50 carat oval that feels balanced on a longer finger can feel substantial on a shorter one, and a slender finger carries a 1.00 carat stone with real presence. Our guide to which diamond shape suits your hand works through this by finger type.
Choose the shape before the carat. Once you know you love an oval or an elongated cushion, you are choosing within a shape that already gives you more face for the weight, and a 1.20 carat stone in that shape may give you exactly the look you had pictured for 1.50.
Look just beneath the thresholds. Prices step up at the round numbers, 1.00, 1.50, 2.00, while the millimetres move smoothly. A 0.92 carat round of excellent cut can measure within a fraction of a millimetre of a 1.00 carat stone with a thick girdle, and a 1.90 carat oval with good spread can outsize a deep 2.05. The stone that looks larger is the better choice, whatever the certificate weighs.
Let the setting do some of the work. A delicate band makes any centre stone read larger. A halo adds visible diameter. A bezel wraps the stone in metal and will always read a touch smaller than the same diamond in fine claws, which is worth knowing before you fall for a bezel at a given weight.
And let lab-grown change the arithmetic. Because lab-grown diamonds cost substantially less per carat than mined stones of the same quality, the question is no longer whether you can reach a size but which size feels right. That freedom is best spent on a well cut, well spread stone in the shape you love, not simply on the heaviest number the budget allows. Our guide to engagement ring costs in Australia sets out what different budgets reach.
Frequently Asked Questions
Is carat weight the same as diamond size?
No. Carat measures weight, with one carat equal to 0.2 grams, while size describes the millimetre dimensions you see from above. Two diamonds of the same carat weight can look different in size depending on shape, depth and girdle thickness, so the measurements line on the certificate is the better guide to how large a stone will look on your hand.
How many millimetres is a 1 carat diamond?
It depends on the shape. A well cut 1.00 carat round brilliant measures about 6.4 millimetres across. A 1.00 carat oval measures around 8.0 by 5.5 millimetres, a square cushion around 5.7 millimetres, an emerald cut around 7.0 by 4.5 millimetres and a pear around 9.0 by 5.5 millimetres. Individual stones vary, so always check the certificate.
What is diamond spread?
Spread is how large a diamond looks for its carat weight. A diamond with good spread carries its weight across the face where you can see it, while a diamond with poor spread carries weight in its depth or girdle. Comparing the millimetre measurements of two stones of the same weight is the simplest way to judge spread.
Does a deeper diamond look smaller?
Yes. Depth adds weight beneath the girdle without adding to the face up size, so a deeply cut diamond measures smaller across the top than a well proportioned diamond of the same carat. The depth percentage on a certificate shows this at a glance: for a round brilliant, a figure in the high fifties to low sixties is the familiar range, and a figure well above it means the stone is carrying hidden weight.
Which diamond shape looks biggest per carat?
Elongated shapes look largest for their weight. Marquise, pear and oval diamonds spread more of their carat weight across their length, so they cover more of the finger than a round of the same weight. Square cushions, radiants and princess cuts carry more weight in their depth and tend to look smallest per carat.
Is a 0.90 carat diamond noticeably smaller than a 1.00 carat?
On the hand, rarely. A 0.90 carat round of excellent cut measures only a fraction of a millimetre smaller than a 1.00 carat stone, a difference that is very hard to see once the ring is on. The price difference between the two is usually far more visible than the size difference, which is why stones just under the round-number thresholds offer such good value.
Do lab-grown diamonds weigh the same as mined diamonds?
Yes. Lab-grown diamonds are chemically, physically and optically identical to mined diamonds, so a 1.00 carat lab-grown diamond weighs exactly 0.2 grams and measures the same as a mined diamond of the same shape and proportions. Carat to millimetre charts apply equally to both.
See the Size, Not Just the Number
A diamond certificate tells you its weight in a single number and its size in three. The three are the ones that describe the stone you will actually see, on every ordinary morning and every extraordinary day, for the rest of your life. Reading them well is how you arrive at a diamond whose weight is honest and whose presence on your hand feels exactly as you pictured it.
We assess every diamond we offer for spread as well as for cut, colour, clarity and carat, and we are glad to put two stones side by side with their measurements so you can see the difference for yourself. Explore the lab-grown diamond engagement ring collection, or book a virtual consultation and we will help you choose the size that feels undeniably yours, with clarity and quiet confidence.
