Carats to Grams Converter

What this converter does

It converts gemstone carats to grams, multiplying by exactly 0.2. The carat here is the metric carat used for diamonds and coloured stones the world over — a unit of mass, and specifically not the karat that describes gold purity. That distinction is the single most useful thing this page has to offer, and it is covered in full below.

The formula

Multiply carats by exactly 0.2, or equivalently by 200 to get milligrams. The metric carat was fixed at 200 milligrams by international agreement in the early twentieth century, so the factor is a definition rather than a measurement. Five carats is exactly one gram; a one-carat stone is exactly a fifth of a gram. Nothing rounds anywhere in the conversion.

Worked examples

A one-carat diamond is 0.2 g. A half-carat stone is 0.1 g. A 1.25 ct stone is 0.25 g. A parcel totalling 62 ct weighs 12.4 g, which is the figure a customs declaration or a laboratory balance would record. And a 0.35 ct stone — described in the trade as a 35-pointer — is 0.07 g, which shows how quickly the numbers become awkward in grams and why the trade keeps its own unit.

Carat and karat: not the same word

Carat, with a c, measures the mass of a gemstone. Karat, with a k, measures the purity of gold and is not a mass at all: 24-karat gold is pure, 18-karat is 18 parts gold in 24 by mass, or 75 percent. A one-carat diamond set in 18-karat gold uses both terms for entirely unrelated quantities, and the spellings diverged precisely so that the two could be told apart in writing. British usage sometimes spells both with a c, which makes context the only guide — if the number is 9, 14, 18 or 24 and the subject is gold, it is purity; if it has decimals and the subject is a stone, it is mass.

Points, and why the trade uses them

A point is a hundredth of a carat, so it is exactly 2 milligrams. Stones under one carat are usually described in points: a 25-pointer is 0.25 ct, a 50-pointer is half a carat. The convention exists because whole numbers misread far less often than two-decimal figures, the same reasoning that keeps medication doses in milligrams rather than decimal grams. Converting points to grams means dividing by 500, or converting to carats first and using this page.

Why carat weight is not stone size

Two stones of equal carat weight can look noticeably different, for two reasons. Density varies between species: sapphire and ruby are denser than diamond, so a one-carat sapphire is physically smaller than a one-carat diamond. And cut proportions redistribute mass — a stone cut with a deep pavilion hides weight below the girdle where it does nothing for appearance, while a shallow cut spreads the same mass across a wider face. Carat is a mass, this converter treats it as one, and any inference from mass to appearance needs the cut and the species as well.

The history behind the 200 milligram carat

Before standardisation the carat varied by market, generally somewhere between 0.19 and 0.21 grams depending on where you were trading. That variation was tolerable when stones changed hands locally and intolerable once the trade became international, since a parcel could gain or lose several percent of its stated weight simply by crossing a border. The metric carat of exactly 200 milligrams resolved the problem by tying the unit to the kilogram, and it has been universal in the trade ever since. The name itself is generally traced to the carob seed, whose reputed uniformity made it a convenient counterweight in antiquity — though modern measurements of carob seeds show them to be no more uniform than any other seed, which makes the story better etymology than metrology.

Precision in gem weighing

Laboratories weigh stones to a thousandth of a carat, which is 0.2 milligrams, on balances with draught shields and level benches. Trade quotes two decimal places. The gap between those two precisions matters commercially because prices step sharply at round carat weights: a stone at 0.99 ct and one at 1.00 ct differ by 2 milligrams of mass and often by a substantial margin in price. That is a market convention rather than a physical fact, and it is the reason the third decimal place exists on certificates at all — to make the rounding to two places defensible rather than arbitrary.

Common mistakes

Reading a karat purity as a carat weight leads, and produces nonsense: an 18-karat ring is not an 18-carat stone. Assuming carat weight predicts size is second, for the density and cut reasons above. Third is using the carat for gold: gold is weighed in troy ounces and grams, never carats, and a gold weight arriving in carats has almost certainly been confused with purity somewhere upstream.

Benchmarks

0.25 ct is 0.05 g. 0.5 ct is 0.1 g. 0.75 ct is 0.15 g. 1 ct is 0.2 g. 1.5 ct is 0.3 g. 2 ct is 0.4 g. 3 ct is 0.6 g. 5 ct is exactly 1 g. 10 ct is 2 g. The five-carats-to-the-gram relationship is the one to hold, since every other value follows from it by proportion.

Accuracy and limits

Exact factor, exact decimal arithmetic, display-only rounding, and refusal of ambiguous input with an explanation rather than a guess. The tool converts mass. It cannot tell you a stone value, its species, its cut quality or its purity, and it deliberately does not treat karat as though it were a unit of weight.

Total weight and stone weight are different numbers

A piece of jewellery set with many stones is often described by its total carat weight, abbreviated in the trade to a figure covering every stone together. That is not the weight of any individual stone, and the distinction is commercially large: a ring described as one carat total weight holding twenty small stones is worth far less than a ring holding a single one-carat stone, because larger rough is rarer and prices rise disproportionately with size. Converting a total carat weight to grams gives the mass of all the stones combined and says nothing about how that mass is distributed.

Converting a parcel rather than a stone

Rough and small goods trade by the parcel, and the carat figure describes the lot. Converting 340 ct to 68 g gives the shipping and customs mass directly, which is what a declaration needs, while the carat figure is what the invoice uses. Keeping both on the paperwork is standard practice and removes an entire category of dispute, since the two numbers verify each other and neither has to be re-derived by a reader.