Carat vs Karat: Gemstone Weight (ct to Grams) vs Gold Purity

The short answer

Carat (ct) is a unit of weight for gemstones: 1 carat is exactly 200 milligrams, or 0.2 grams. Karat (K) is a measure of gold purity: 24K is pure gold and 22K is 22 parts gold out of 24. The two sound the same, but one tells you how heavy a diamond is and the other tells you how much of a ring is real gold. To convert gemstone carats to grams or milligrams, use the Weight Converter and pick Carat (ct) as the unit.

Carat: the weight of a gemstone

The metric carat is defined as exactly 200 mg. That makes conversion simple: multiply carats by 0.2 to get grams, or by 200 to get milligrams.

Carats (ct)	Grams (g)	Milligrams (mg)
0.25 ct	0.05 g	50 mg
0.5 ct	0.1 g	100 mg
1 ct	0.2 g	200 mg
1.5 ct	0.3 g	300 mg
2 ct	0.4 g	400 mg
5 ct	1 g	1,000 mg

Going the other way, 1 gram is 5 carats, and 10 grams is 50 carats.

Jewellers often split a carat into 100 points. A "50-pointer" diamond is 0.50 ct (0.1 g). Points are a sales convention, not a separate unit in the converter, so divide points by 100 to get carats first.

Why carat weight is not the same as size

Carat measures mass, not diameter. Two 1-carat stones can look different sizes because:

• Different gemstones have different densities. A 1 ct ruby or sapphire is denser than a 1 ct diamond, so it looks slightly smaller.
• The cut matters. A deep cut hides weight underneath the stone, so it can look smaller from the top than a shallower stone of the same weight.

If you are comparing stones, compare carat weight and measured dimensions in millimetres together.

Karat: the purity of gold

Karat describes how much of an alloy is pure gold, on a scale of 24. Pure gold is too soft for most jewellery, so it is mixed with other metals such as silver, copper or zinc.

The arithmetic is karat ÷ 24. In India, hallmarked jewellery carries a three-digit fineness number (parts per thousand) next to the karat. The Bureau of Indian Standards lists these grades for gold hallmarking:

Hallmark grade	Karat	Purity by karat ÷ 24	Fineness mark
24KS995	24K	100% (pure)	995
23K958	23K	95.83%	958
22K916	22K	91.67%	916
20K833	20K	83.33%	833
18K750	18K	75.00%	750
14K585	14K	58.33%	585

Two details are worth knowing:

• 24KS995 means at least 99.5% gold. Commercial "pure" gold is not 100.0%.
• 14K585 is slightly higher than the 58.33% you get from 14 ÷ 24. The fineness number is the standard grade used for hallmarking, so go by the number stamped on the piece rather than your own calculation.

Since 1 July 2021, a BIS hallmark on gold jewellery has three marks: the BIS logo, the purity grade (for example 22K916) and a six-digit alphanumeric HUID code.

Carat vs karat at a glance

Feature	Carat (ct)	Karat (K)
Measures	Weight (mass)	Purity of gold
Used for	Diamonds and other gemstones	Gold jewellery and coins
Value	1 ct = 200 mg = 0.2 g	22K = 22 parts gold in 24
Higher number means	A heavier stone	More gold, less alloy
Converted with	A weight converter	Simple division (K ÷ 24)

Spelling note: in British English and Indian usage, "carat" is often used for both meanings, as in "22 carat gold". That is correct usage in those places. The context tells you which one is meant: "carat" with gold describes purity, and "carat" with a diamond describes weight. In American English the two are usually spelled differently (karat for gold, carat for stones).

How to convert carats with the Weight Converter

Open the Weight Converter.

Enter the weight, for example 1.5.

In From, choose Carat (ct).

In the result box, choose Gram (g) or Milligram (mg). For 1.5 ct you get 0.3 g.

The All Weight / Mass conversions list underneath shows the same value in every unit at once. Click a row to copy it.

To go from grams to carats, set From to Gram (g) and pick Carat (ct) as the result. Very small results, such as 1 ct expressed in metric tons, are shown in scientific notation (for example 2.0000e-7).

Worked examples

A ring with a 0.75 ct diamond. 0.75 × 0.2 = 0.15 g, or 150 mg. The diamond adds very little to the total weight of the ring.

How much pure gold is in 10 g of 22K jewellery? Use the fineness mark: 10 g × 0.916 = 9.16 g of gold. Using 22 ÷ 24 instead gives 9.17 g. The small difference is why jewellers quote fineness.

How much pure gold is in an 8 g 18K chain? 8 × 0.750 = 6 g of gold. The other 2 g is alloy.

A loose stone weighs 2.6 g. How many carats? 2.6 ÷ 0.2 = 13 ct.

Common mistakes

• Using karat purity to convert weight. "22 carat gold" says nothing about how heavy a chain is. You still need its weight in grams.
• Adding stone weight to gold weight. A jeweller's bill usually lists gross weight and net gold weight separately. Diamond weight in carats should be listed on its own.
• Confusing carats with troy ounces. International gold prices are often quoted per troy ounce, which is a different unit from the ordinary (avoirdupois) ounce in the converter. Indian retail gold rates are normally quoted per gram or per 10 grams.
• Rounding 916 to 92% or 91%. On a large purchase, small rounding differences add up. Use the three-digit mark.

Related tools and guides

• Converting kg, pounds, stone, ounces and tola: How to Convert kg to lbs, Stone, Ounces and Tola
• Working out making charges or price differences as a percentage: Percentage Calculator
• Checking GST on a jewellery bill: GST Calculator

This guide explains the general rules and is not financial advice. For purchases, rely on the hallmark and the bill issued by the jeweller.