Carat
Carat is the standard unit of weight for diamonds and gemstones, equal to 200 milligrams or one fifth of a gram.
Carat is how the trade weighs a diamond. One carat equals exactly 200 milligrams, and each carat splits into 100 points, so a diamond described as 25 points weighs a quarter of a carat. It measures mass, not size or quality, which is a distinction that trips up a lot of first-time buyers.
How carat weight is measured and written
Diamonds are weighed on a calibrated scale to the thousandth of a carat and then rounded to the hundredth. Because the point system runs to two decimal places, jewellers often speak in points for smaller stones: a 0.50 carat diamond is a "fifty pointer", and a 0.75 is a "three-quarter carat". Certificates from GIA and IGI record the weight to the exact hundredth, for example 1.01 ct.
Weight and face-up size are related but not the same. Two one-carat diamonds can look different across the top depending on how they are cut. A shallow stone spreads its weight wider and looks bigger, while a deep stone hides weight in the pavilion and looks smaller for the money.
Why price jumps at the round numbers
Price per carat climbs steeply as weight rises, because large rough that yields large polished diamonds is far rarer. A one carat round of a given colour and clarity costs much more than two half-carat rounds of the same quality added together, even though the total weight is identical. The single larger stone is the scarcer object.
Demand clusters at the milestone weights, so there are sharp price steps right at 0.50, 0.90, 1.00, 1.50 and 2.00 carats. A stone at 0.98 ct can cost noticeably less per carat than one at 1.00 ct that looks almost the same. Buyers who care about value more than the round number often shop just below these thresholds.
- 1 carat = 200 milligrams = 100 points
- Price per carat rises with weight, so bigger is disproportionately dearer
- Milestone weights like 1.00 and 2.00 ct carry a premium
- A well cut lighter stone can look larger than a heavier deep one
Carat as one of the 4Cs
Carat is the easiest of the 4Cs to measure, but on its own it says nothing about beauty. A large diamond with a poor cut, low colour and visible inclusions can look worse than a smaller stone that is well cut and clean. Weigh carat against cut, colour and clarity together rather than chasing the biggest number, and match the choice to the setting and budget.
- Carat measures weight, not size or sparkle.
- One carat is 200 milligrams and divides into 100 points.
- Price per carat rises sharply with weight because large stones are rarer.
- Buying just under a milestone weight can save money for a similar look.
- Cut quality can make two stones of equal carat look very different.
Carat, in short.
Is a bigger carat weight always better?
No. Carat only tells you the weight. A large diamond with a weak cut or low colour can look duller than a smaller, well cut, cleaner stone. Balance carat against the other three Cs and the setting.
How much is one carat in millimetres?
Carat is weight, not length, so there is no fixed millimetre value. As a rough guide a well cut one carat round measures about 6.4 to 6.5 mm across the top, but this varies with the cut and shape.
Why does a 0.90 carat cost so much less than a 1.00 carat?
Buyers pay a premium for hitting the one carat mark, so prices step up sharply right at the round numbers. A 0.90 ct can look nearly the same face-up while costing meaningfully less per carat.
Do two half-carat diamonds equal one full carat in price?
In weight yes, in price no. A single one carat stone is rarer than two half-carat stones, so it costs considerably more than the pair combined at the same colour, clarity and cut.
Related terms
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