A full carat of diamond here, spread across a whole bouquet rather than gathered into one stone. Same weight, entirely different object. Antique Bouquet Diamond Pendant, 1.0 carats, 18K white gold
A carat is a unit of weight, not size: exactly 0.2 grams, or 200 milligrams, so five carats weigh a gram. That definition causes most of the confusion in diamond buying, because people hear a weight and picture a width. A one carat diamond in a standard modern round brilliant measures roughly 6.4 to 6.5 mm across. That is about the width of a pencil eraser, and it is almost always smaller than people expect the first time they see one.
Weight against width, for round brilliants
| Carat weight | Approximate diameter |
|---|---|
| 0.25 ct | 4.1 mm |
| 0.50 ct | 5.2 mm |
| 0.75 ct | 5.8 mm |
| 1.00 ct | 6.5 mm |
| 1.50 ct | 7.4 mm |
| 2.00 ct | 8.2 mm |
Notice how slowly the width grows. Doubling the weight from one carat to two adds only about a quarter to the diameter, because weight increases with the cube of the dimensions. That is why a two carat stone costs far more than twice a one carat but does not look twice as big, and it is worth knowing before spending the difference.
Why identical weights look different
Depth. A stone cut deep hides its weight underneath the setting, where nobody sees it. A stone cut shallow spreads wider for the same weight. Two one carat diamonds can differ by half a millimetre in width, which is visible.
Cut style. Old cuts generally spread differently from modern brilliants. Rose cuts, being flat-backed with no pavilion at all, spread dramatically wider than their weight suggests, which is one of their great practical charms and is explained in what is a rose cut diamond.
Shape. Elongated shapes look larger than round ones of the same weight because the eye reads length. A one carat marquise or oval appears noticeably bigger than a one carat round.
The setting. A halo of small stones around a centre makes it read a size larger. A wide bezel makes it read smaller. Claw settings that lift a stone up show more of its width than settings that sink it.
The total carat weight trap
This one costs people real money. Total carat weight, written tcw or ct tw, is the sum of every stone in a piece. A ring described as "1 carat total diamond weight" may hold one stone or sixty, and the difference in value is enormous, because large stones are rare and small ones are not. Sixty small diamonds weighing a carat together are worth a fraction of one diamond weighing a carat alone.
The featured piece above is exactly this: a full carat of diamond, spread through a bouquet of small stones. It is a lovely thing and it is priced as what it is. When a listing gives only a total weight, the question to ask is simply what the centre stone weighs on its own.
Nearly a carat again, and again distributed. The wings hold what a solitaire would hold in one place. Estate Butterfly Ring, 0.95 carats VS diamonds, 18K white gold
Buying by eye instead
Three pieces of practical advice I give constantly. Look at stones on a hand, not on a cloth, because scale changes completely against skin and against a finger's width. Consider the shy weights. A 0.90 carat stone costs meaningfully less than a 1.00, because the market pays a premium at round numbers, and nobody can see the difference. And consider an old cut, which for the same money will usually be larger, because antique stones are cheaper per carat for the reasons set out in the 4Cs and antique diamonds.
Frequently asked questions
How many millimetres is a one carat diamond?
About 6.4 to 6.5 mm across in a standard round brilliant. Other shapes and cutting styles vary at the same weight.
How much does a carat weigh?
Exactly 0.2 grams, or 200 milligrams. A carat is divided into 100 points, so 0.25 ct is a 25 point stone.
Why do two one carat diamonds look different sizes?
Because depth, cut style and shape all change how much of the weight sits where you can see it. Deep stones hide weight below the setting.
What does total carat weight mean?
The combined weight of every diamond in a piece. It says nothing about the size of the largest stone, and many small stones are worth far less than one large one.
Does a two carat diamond look twice as big?
No. It is about a quarter wider, because weight grows with the cube of the dimensions while width grows linearly.
Which shape looks biggest for its weight?
Elongated shapes such as marquise, oval and pear look larger than round stones of the same weight, and rose cuts spread widest of all.
Is it worth buying just under a carat?
Often yes. Prices jump at round weights, so a 0.90 carat stone costs less than a 1.00 while looking effectively identical.
Read next: Carat vs karat · The 4Cs and antique diamonds · What is a solitaire ring?
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