Diamond Carat Guide

Carat is a measure of weight, not size. It moves the price more directly than any other grade, and it says less about how a stone will look than most buyers expect.

What is carat weight?

A carat is a unit of mass: 200 milligrams, or 0.2 grams. It describes how much a diamond weighs on a scale, not how large it looks in a ring. Two stones of the same weight can face up at noticeably different sizes, depending on shape and how they were cut.

It is not the same word as karat, which measures the purity of gold. An 18-karat ring and a one-carat diamond share a pronunciation and nothing else.

Carats and points

One carat divides into 100 points. The trade often quotes smaller stones in points rather than decimals.

0.25 ct: 25 points
0.50 ct: 50 points
0.75 ct: 75 points
1.00 ct: 100 points

1 carat = 200 mg = 0.2 g = 100 points

Carat weight vs. diamond size

Weight rises faster than size. Doubling the weight of a round adds only about a quarter to its diameter, because the extra carbon goes into depth as well as width. Shape changes the picture again: elongated cuts spread more of their weight face up than a round does.

The figures are for well-proportioned stones. A stone cut deep carries weight below the girdle where it cannot be seen, so always read the measurements on the report alongside the carat figure.

Round brilliant: carat weight and approximate diameter
Carat	Approx. diameter
0.25 ct	4.1 mm
0.50 ct	5.1 mm
0.75 ct	5.8 mm
1.00 ct	6.4 mm
1.50 ct	7.3 mm
2.00 ct	8.1 mm
3.00 ct	9.3 mm

5 mm. Every silhouette is drawn to this scale, at typical proportions.

Six more shapes

Every shape as a table

Approximate face-up dimensions (length × width) by carat weight, for twelve diamond shapes
Shape	0.25 ct	0.50 ct	0.75 ct	1.00 ct	1.50 ct	2.00 ct	3.00 ct
Radiant	4.1 × 3.4 mm	5.1 × 4.3 mm	5.9 × 4.9 mm	6.5 × 5.4 mm	7.4 × 6.2 mm	8.1 × 6.8 mm	9.3 × 7.8 mm
Oval	5.1 × 3.6 mm	6.4 × 4.6 mm	7.3 × 5.2 mm	8.0 × 5.7 mm	9.2 × 6.6 mm	10.1 × 7.2 mm	11.6 × 8.3 mm
Emerald	4.4 × 3.1 mm	5.6 × 3.8 mm	6.4 × 4.4 mm	7.0 × 4.8 mm	8.0 × 5.5 mm	8.8 × 6.1 mm	10.1 × 7.0 mm
Round	4.1 mm	5.1 mm	5.8 mm	6.4 mm	7.3 mm	8.1 mm	9.3 mm
Pear	5.7 × 3.5 mm	7.1 × 4.5 mm	8.2 × 5.1 mm	9.0 × 5.6 mm	10.3 × 6.4 mm	11.3 × 7.1 mm	12.9 × 8.1 mm
Asscher	3.6 mm	4.6 mm	5.2 mm	5.7 mm	6.6 mm	7.2 mm	8.3 mm
Princess	3.5 mm	4.4 mm	5.1 mm	5.6 mm	6.4 mm	7.0 mm	8.0 mm
Cushion	3.7 × 3.6 mm	4.7 × 4.5 mm	5.4 × 5.1 mm	5.9 × 5.7 mm	6.8 × 6.5 mm	7.5 × 7.1 mm	8.6 × 8.2 mm
Marquise	6.7 × 3.3 mm	8.4 × 4.2 mm	9.6 × 4.8 mm	10.6 × 5.3 mm	12.1 × 6.1 mm	13.3 × 6.7 mm	15.3 × 7.6 mm
Heart	4.1 mm	5.2 mm	6.0 mm	6.6 mm	7.5 mm	8.3 mm	9.5 mm
Trillion	4.6 mm	5.8 mm	6.6 mm	7.3 mm	8.4 mm	9.2 mm	10.5 mm
Hexagon	4.1 × 3.6 mm	5.1 × 4.6 mm	5.9 × 5.2 mm	6.5 × 5.8 mm	7.4 × 6.6 mm	8.1 × 7.3 mm	9.3 × 8.3 mm

Round figures are the standard reference diameters. Other shapes are estimated from the trade's weight formula at a typical ratio and depth for each cut; hexagon and trillion have the least standardised proportions and vary most. Select any weight to see natural stock in that shape and weight band.

Worked examples from current stock

The chart gives the typical figure. Real stones land close to it, and the gap is proportion: a slightly shallower stone spreads wider, a deeper one faces up smaller.

Carat weight and price

Price per carat rises with weight, because large rough is rarer than small rough. A two-carat stone costs more than two one-carat stones of the same grades.

The rise is not smooth. It steps up at the weights buyers ask for by name, so a stone that reaches a milestone costs noticeably more than one a few points short of it, though the two look the same.

0.50ctct milestone
0.75ctct milestone
1.00ctct milestone
1.50ctct milestone
2.00ctct milestone

Same weight, different price

Two stones of identical weight can sit far apart on price. Cut, colour and clarity set the rest, and a difference of a grade or two in colour or clarity moves the figure more than most buyers expect.

Natural and lab-grown

At the same weight and grades, a lab-grown diamond costs considerably less than a natural one. Weight, size and material are the same; origin, not material, is the difference, and the market prices natural stones on their finite supply.

Natural diamonds Lab-grown diamonds

How carat interacts with the other Cs

Weight decides how much diamond there is. Cut decides how much of it you see. A well-proportioned stone spreads its weight face up and returns light to the eye; a stone cut deep hides weight below the girdle and looks smaller and darker for it.

Every stone we list carries an independent GIA or IGI report with the grades that matter here:

Cut: How the proportions return light. Graded for round brilliants only.
Polish: The finish of the facet surfaces.
Symmetry: How precisely the facets align with each other and the outline.
Colour: D to J on the colourless scale, or a fancy-colour description.
Clarity: Inclusions and blemishes, from FL down to I2.
Measurements: Length, width and depth in millimetres. Read these with the carat figure.

How to read a grading report Diamond colour guide

Choosing the right carat weight

Hand and finger size
The same stone reads larger on a slender finger than on a broad one. Elongated shapes such as oval, pear and marquise lengthen the hand and cover more of it for their weight.
Just under a milestone
A 0.90 ct stone looks almost identical to a 1.00 ct stone and costs noticeably less. The same holds at 1.40 against 1.50, and 1.90 against 2.00.
Setting style
A halo of small stones around the centre makes it read larger. Thin prongs show more of the stone than heavy ones, and a bezel frames it but covers the edge.
Origin
At the same weight and budget, lab-grown often allows a higher colour and clarity than natural. Natural offers finite supply. Neither is the wrong choice.

Frequently Asked Questions

What buyers ask most often about weight before choosing a stone.

See how weight trades off against the other Cs in the 4Cs calculator.

Is a bigger carat always better?

No. Weight is one grade among several. A smaller stone that is well cut, with good colour and clarity, often looks better than a heavier one cut deep or heavily included. A bigger stone always costs more; whether it looks better depends on the other grades.

What's the most popular carat weight for engagement rings?

One carat is the reference point most buyers start from, partly because it is a round number and partly because it is where prices step up. Many engagement stones sit either side of it. The right weight is the one the budget allows without giving up cut quality.

Does carat weight affect durability?

No. A diamond is equally hard at any weight. Durability depends on shape and setting: points, thin corners and a thin girdle are the vulnerable areas whatever the stone weighs. A larger stone does sit higher off the hand, so a protective setting matters more.

How is carat weight measured for fancy-shaped diamonds?

Exactly as for a round: the stone is weighed on a calibrated scale and the report states the weight to two decimal places. What changes is how that weight spreads face up. An oval or marquise covers more area than a round of the same weight; a deep emerald or radiant can cover less. That is why reports give measurements in millimetres.

Does carat weight mean the same thing for natural and lab-grown diamonds?

Yes. A carat is 200 milligrams whatever the origin, and natural and lab-grown diamonds have the same density, so two stones of equal weight and equal proportions measure the same. The difference between them is price per carat, not size per carat.

Shop Diamonds by Carat

Choose a shape and a weight band to open the catalogue already filtered to it. If nothing in stock fits, enquire and we will source to the specification.

Shape

Carat weight

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