What Is a Carat? Diamond Carat Weight Explained
Every diamond ever sold has been weighed in carats — a tiny unit named after a tree seed once used as nature's standard for measuring gems. Here's exactly what a carat is, how it relates to grams and millimetres, and how it shapes both the appearance and the price of a diamond.
If you've ever shopped for an engagement ring, a pair of stud earrings, or any piece of fine jewellery, the very first number you encountered was almost certainly the carat weight. It's the figure printed on every grading certificate, the basis for every price quote, and — somewhat misleadingly — the closest thing the gemstone world has to a single measure of "how big" a stone is.
This guide covers what a carat actually is, how to convert it to grams, why a 2 carat diamond costs much more than two 1 carat diamonds, and how the unit became the global standard for weighing precious stones. If you only need the arithmetic, jumping straight to carats to grams or grams to carats is the fastest route.
Carat, gram & milligram converter
Convert instantly between carats, grams, milligrams and points.
What is a carat?
A carat is a unit of mass used exclusively for weighing gemstones — diamonds, sapphires, rubies, emeralds, and every other precious and semi-precious stone you can think of. The symbol is ct, and the unit was internationally standardised in 1907 at the General Conference on Weights and Measures in Paris.
The defining number is small and clean:
1 carat = 0.2 grams = 200 milligrams — exactly. This is sometimes called the metric carat to distinguish it from older, slightly different national variants now obsolete.
The carat name comes from the Greek keration, meaning "carob seed". For centuries, gem traders used carob seeds as a natural counterweight on their balance scales — the seeds were thought to be uniformly small and consistent in weight. Modern measurements have shown carob seeds actually vary more than the legend suggests, but the name and the rough magnitude stuck.
In modern lab paperwork, the same weight may be written in carats, grams, or milligrams, so it helps to keep carats to grams, grams to carats, and milligrams to grams close at hand.
The exact conversion
Two formulas — and one is just the inverse of the other. For exact single-value lookups, use carats to grams or grams to carats.
Carats and points
Diamonds are rarely sold at exactly whole-number carat weights, so jewellers use a finer subdivision called the point. There are 100 points to a carat, which means each point equals 0.01 ct, or 2 milligrams.
That also means tiny accent stones often move back and forth between grams and milligrams on workshop notes, making grams to milligrams and milligrams to grams useful alongside the carat calculation.
You'll often hear shop staff describe stones in terms of points: "a fifty-pointer" is 0.50 ct, "a twenty-five-pointer" is 0.25 ct. The system makes life easier when discussing the small stones — accents, side stones, or melee — that make up the supporting cast of most fine jewellery.
Carat to grams conversion chart
| Carats | Points | Grams | Milligrams |
|---|---|---|---|
| 0.10 ct | 10 pt | 0.020 g | 20 mg |
| 0.25 ct | 25 pt | 0.050 g | 50 mg |
| 0.33 ct | 33 pt | 0.066 g | 66 mg |
| 0.50 ct | 50 pt | 0.100 g | 100 mg |
| 0.75 ct | 75 pt | 0.150 g | 150 mg |
| 1.00 ct | 100 pt | 0.200 g | 200 mg |
| 1.50 ct | 150 pt | 0.300 g | 300 mg |
| 2.00 ct | 200 pt | 0.400 g | 400 mg |
| 3.00 ct | 300 pt | 0.600 g | 600 mg |
| 5.00 ct | 500 pt | 1.000 g | 1,000 mg |
| 10.00 ct | 1,000 pt | 2.000 g | 2,000 mg |
Diamond size chart: carats and millimetres
Carats measure weight, not size. But because round brilliant diamonds are cut to relatively standard proportions, you can roughly map carat weight to physical diameter. The chart below applies to a typical, well-cut round brilliant — fancy shapes (oval, marquise, emerald) will appear different sizes for the same weight.
Most people overestimate the size of a 1 ct diamond. In reality, it's about 6.5 mm across — slightly smaller than a pencil eraser, slightly larger than a pinhead. The visual impact comes from the brilliance and fire of the cut, not the raw dimensions.
Carat vs karat: the most confused unit pair in jewellery
"Carat" and "karat" sound identical, but they measure entirely different things — and they appear together on almost every piece of fine jewellery. Mixing them up is the single most common rookie error.
Carat (ct)
0.2 g per carat- Measures: weight of a gemstone
- Used for: diamonds, sapphires, rubies, emeralds
- From Greek keration ("carob seed")
- 1 carat = 100 points = 0.2 g
Karat (K)
24K = pure gold- Measures: purity of gold
- Used for: gold alloys (rings, chains, bars)
- 24-part scale, where 24K = 100% gold
- 18K = 75% gold, 14K = 58.3% gold
Confusing the two is so common that some countries (notably the UK) use "carat" for both, just to keep things interesting. So a British "18 carat gold" ring contains 75% pure gold; an "18 carat diamond" would be a very large stone weighing 3.6 grams. Context tells you which is which. For more on gold purity, see our Troy Ounce Gold Guide.
If a jeweller gives you the gemstone weight in grams but the market comparison in carats, switch between grams to carats and carats to grams first, then keep the karat discussion separate.
Why a 2 carat diamond costs much more than two 1 carat diamonds
Here's the counterintuitive bit: diamond price is not linear with weight. A 2 ct diamond can cost three to four times as much as a 1 ct diamond of comparable quality, despite being only twice the weight. The reason is rarity — large rough diamonds are vastly less common in nature than small ones, and faceting a large stone wastes more material than faceting two smaller ones.
This non-linear pricing creates two practical effects worth knowing:
- Magic sizes. Prices jump at psychologically significant thresholds: 0.50 ct, 1.00 ct, 1.50 ct, 2.00 ct. A 0.99 ct stone is often noticeably cheaper than a 1.00 ct stone of identical quality, simply because the round number commands a premium.
- Buying just-under sizes. Savvy buyers often look for stones at 0.90 ct, 1.40 ct or 1.90 ct — visually almost identical to the next-up "magic size", but priced significantly lower.
Carat is one of four Cs
Carat weight is just one factor in a diamond's value. The Gemological Institute of America's Four Cs framework is the global standard for diamond grading:
Carat
Weight in carats. Bigger means rarer, but only one of four factors.
Cut
The proportions and craftsmanship. Cut quality determines brilliance and fire.
Colour
Graded D (colourless) to Z (light yellow). Colourless stones are most valued.
Clarity
Internal flaws (inclusions). FL is flawless; I3 has visible imperfections.
A 0.75 ct diamond with excellent cut, D colour and VVS1 clarity will often outshine — and outprice — a 1.50 ct stone with poor cut, J colour and SI2 clarity. Carat alone tells you very little.
Where the carat came from
Long before electronic balances and laser micrometers, gem merchants in the Mediterranean and Middle East needed a small, reliably consistent weight to put on the other side of a balance scale. Carob seeds — the dried fruit of the carob tree, Ceratonia siliqua — became the unofficial standard. The seeds are roughly 0.2 g each, small enough to weigh single gemstones with reasonable precision.
By the 19th century, different countries had drifted to slightly different "carat" definitions, ranging from about 0.188 g in some Italian states to 0.213 g in parts of Britain. This was a problem: a stone bought in Antwerp could weigh a different "carat" by the time it reached London. In 1907, the Fourth General Conference on Weights and Measures fixed the metric carat at exactly 0.2 g, and by the 1930s, virtually every diamond-trading nation had adopted the standard.
Practical buying tips
- Don't fixate on round numbers. A 0.92 ct stone looks essentially identical to a 1.00 ct stone, but can cost 20–30% less.
- Always check the cut grade. An "Excellent" or "Ideal" cut on a smaller stone will out-sparkle a "Good" cut on a larger stone every time.
- Verify the certificate. For any significant diamond purchase, insist on a grading report from GIA, AGS or another reputable lab. The carat weight on the certificate is measured to four decimal places.
- Understand "total carat weight" (TCW). A ring described as "1 ct TCW" might contain one 0.5 ct centre stone plus two 0.25 ct side stones, or twenty 0.05 ct accents. The total adds up, but the visual impact is very different.
- Ask whether the weight is the centre stone or the ring as a whole. Marketing language is sometimes deliberately ambiguous.
For perspective, even a large diamond still weighs very little in ordinary units. If you want to sanity-check a listing against a scale reading, compare it with grams to ounces or ounces to grams.