What a point is
A point is one hundredth of a carat. GIA puts it in the same terms a jeweller would: just as a dollar is divided into 100 pennies, a carat is divided into 100 points, so a 50-point diamond weighs 0.50 carats. The carat itself is a fixed unit of mass, exactly 0.2 grams, which makes a point exactly 2 milligrams. Every conversion on this page follows from those two definitions and is exact.
Points are used mainly for stones under a carat, where talking in hundredths is easier than talking in decimals. A 0.25 ct stone is called a twenty-five pointer, and a ring set with stones of 2 points each is described by the size of the stones rather than by a long decimal. Above one carat, weights are usually given in carats and decimals instead: GIA’s example is a 1.08 ct stone, read as "one point oh eight carats".
Total carat weight
Jewellery set with several stones is usually tagged with a total carat weight, often written ct TW or ct tw. It is the sum of the weights of every stone in the piece. A ring with one 50-point centre stone and ten 2-point side stones has a total of 50 + 20 = 70 points, or 0.70 ct TW.
The total says nothing about how that weight is shared out. A ring of 0.70 ct TW could hold a single 70-point stone or seventy 1-point stones, and those are very different pieces to look at. The calculator therefore takes one row per size of stone, so the total it shows is built from the stones themselves, and the per-stone sizes stay visible beside it.
How GIA rounds a weight
A laboratory weighs more finely than it reports. GIA’s FAQ says diamonds are weighed to the thousandth of a carat and then rounded to the nearest hundredth, and that the weight is only rounded up if the thousandth digit is a nine. Its own examples are a stone weighing 1.768 ct, reported as 1.76 ct, and one weighing 1.769 ct, reported as 1.77 ct.
This is not the rounding taught in school, where 1.765 would round up. Under GIA’s rule, 1.765, 1.766, 1.767 and 1.768 all become 1.76, and only 1.769 becomes 1.77. The effect is that a reported weight is very rarely above the weight measured. The third box in the calculator applies the rule to any figure you enter to the thousandth.
Points, carats and grams side by side
Because a carat is 0.2 g, 1 point is 0.002 g or 2 mg, 10 points is 20 mg, 25 points is 50 mg, 50 points is 100 mg and 100 points, one carat, is 200 mg. Five carats make one gram. The carat to gram converter on this site handles larger weights and the reverse conversion from grams.
Carat with a C, the weight of a gemstone, is a different word from karat with a K, the purity of gold, even though both are often spelled "carat" outside the US. A 2-point diamond in an 18 karat setting uses both units at once, and they measure unrelated things.
What weight does not tell you
Points measure weight only. Two stones of equal weight can face up at different sizes depending on how they are cut, and a stone of a given weight in one shape spreads differently from the same weight in another. The diamond size calculator converts a weight into millimetres for each shape. No figure on this page is a price: the price of a diamond depends on far more than its weight, and this site publishes none.