Carat to Grams Made Simple, With a Diamond Size Chart

A single carat weighs 0.2 grams. That's one fifth of a gram, and it holds true whether the stone is a diamond, a sapphire, or a piece of cut glass. The carat to grams math never bends, which is why every jeweler leans on it.

So a two carat diamond is 0.4 grams, and a five carat showstopper weighs a full gram. Nobody prices gemstones in ounces at the counter, they work in carats and grams because the numbers stay small and exact. Get the carat to grams conversion wrong and you can misjudge a stone by hundreds of dollars.

The Carat to Grams Formula, and a Full Chart

The formula is short. Multiply carats by 0.2 to get grams, or divide by five if that feels easier. A 0.75 carat stone is 0.15 grams, a three carat stone is 0.6 grams, and you can run most of it in your head.

How many grams in a carat, exactly?

One carat is 0.2 grams or 200 milligrams, a value the United States fixed in 1913 when it adopted the metric carat. Before that, carat weights drifted from one trading city to the next. That steady carat in grams number is why gems trade cleanly across borders, and diamond carat weight is read on a calibrated digital scale, per the GIA.

One carat equals 0.2 grams, and that figure is identical in every country, on every scale, for every gemstone.

Carat vs Karat, Two Words People Always Mix Up

Here's where buyers trip. Carat with a C measures a gemstone's weight. Karat with a K measures how pure gold is, on a scale that tops out at 24. They sound the same out loud, so people ask for the wrong one constantly.

Carat is weight. Karat is purity. Swapping the two is the most common mistake in the whole jewelry case.

A 24 karat chain is essentially pure gold, while a 14 karat chain is about 58 percent gold, since 14 of its 24 parts are the metal and the rest is a tougher alloy. None of that touches carat weight. You can hold a heavy 10 karat ring or a featherlight 22 karat one.

Why a Diamond Carat Looks Bigger Than a Ruby

Two stones can share a carat weight and still look different sizes. Weight tracks density, so a denser stone folds its grams into a smaller footprint. Gem weight and gem size are simply not the same measurement.

Measure a 5 millimeter round stone and a diamond lands near 0.5 carats. The same 5 millimeter ruby or sapphire weighs about 0.6 carats because it is denser, while a 5 millimeter opal comes in at only about 0.34 carats, per Fire Mountain Gems. So gemstone weight always depends on what the stone is made of.

A one carat diamond and a one carat opal weigh the same, yet the opal looks noticeably larger on the hand.

A Diamond Size Chart From Millimeters to Carats

Because width and weight rise together within one gem type, a diamond size chart lets you read carats straight off a millimeter measurement. For round brilliant diamonds the mapping is well established.

A quarter carat round diamond spans about 4 millimeters, a half carat about 5 millimeters, and a full carat roughly 6.5 millimeters, according to published jeweler size charts. Notice how the width barely grows as weight climbs, which is exactly why an mm to carat conversion surprises people the first time they see it.

What Carat Weight Means for the Price You Pay

Carat weight moves price more than color, cut, or clarity, and it does not move in a straight line. Diamond prices leap at whole numbers, so a 1.00 carat stone can cost far more than a 0.95 carat one that looks the same to the eye.

That 0.05 carat gap is just 10 milligrams, lighter than a single grain of rice, yet it can swing the price sharply. Shoppers who understand the carat to grams reality often buy just under a round weight and keep the difference in their pocket.

Ten milligrams of extra weight, the jump from 0.99 to 1.00 carat, can carry a large price premium all on its own.

How to Weigh Gems and Diamonds at Home

You don't need a jeweler's loupe to check a stone's weight, you need a scale that reads to a hundredth of a gram. Zero it with the empty tray, set the loose stone down, and convert. A reading of 0.20 grams means a one carat stone.

If you'd rather skip the hardware, Scale for Grams estimates weight from a photo, and you can run the figure through a grams to ounces or milligrams to grams converter to double check. For heavier metals, our troy ounce guide covers the other unit jewelers reach for.

Common Questions About Carat Weight

How much does a carat weigh in grams?

A carat weighs 0.2 grams, which is 200 milligrams or one fifth of a gram. The figure is fixed worldwide and applies to every gemstone, from diamond to emerald. Multiply any carat number by 0.2 and you have the weight in grams.

Is a carat the same as a karat?

No. A carat is a weight unit for gemstones, while a karat measures the purity of gold on a 24 part scale. They sound alike but describe different things, so a 14 karat setting can easily hold a 2 carat stone.

How many milligrams are in one carat?

One carat holds 200 milligrams, and it divides into 100 points of 2 milligrams each. A 25 point diamond, written 0.25 ct, weighs 50 milligrams. Jewelers use points for anything under a full carat.

Does a bigger carat always look bigger?

Within a single gem type, yes, but the width grows slowly. A one carat round diamond is about 6.5 millimeters wide, and stepping up to two carats adds only a couple of millimeters, not double. Across different stones, a denser gem can weigh more yet look smaller.

Need to weigh a loose stone or double check a carat to grams figure fast? Point Scale for Grams at it and get an AI powered weight estimate from a photo in seconds, then convert to carats yourself. Get the app here: iOS.