A carat (ct) is a unit of weight for gemstones equal to exactly 200 milligrams (0.2 grams). A separate term, 'karat' (K or kt), measures the purity of gold on a scale of 24, where 24K is pure gold.

What is the difference between carat and karat?

Carat (ct) measures gemstone weight (1 ct = 200 mg). Karat (K) measures gold purity (24K = 100% gold, 18K = 75% gold, 14K = 58.3% gold). In British English, 'carat' is used for both.

How big is a 1-carat diamond?

A 1-carat round brilliant diamond is approximately 6.5mm in diameter. However, two diamonds of equal carat weight can appear different sizes depending on their cut proportions and shape.

What Is a Carat? Gemstone Weight & Gold Purity Explained

Gemstone Carats (ct)

A carat is the standard unit of weight for gemstones and pearls:

1 carat = 200 milligrams = 0.2 grams

The word comes from the carob seed, which was historically used as a balance weight due to its supposedly uniform mass.

Diamond Size Reference

Carats	Diameter (round)	Weight (mg)
0.25 ct	~4.1 mm	50 mg
0.50 ct	~5.2 mm	100 mg
0.75 ct	~5.9 mm	150 mg
1.00 ct	~6.5 mm	200 mg
1.50 ct	~7.4 mm	300 mg
2.00 ct	~8.2 mm	400 mg
3.00 ct	~9.4 mm	600 mg

Gold Karats (K)

Karat	Gold Content	Use
24K	99.9%	Investment gold, some Asian jewelry
22K	91.7%	Indian/Middle Eastern jewelry
18K	75.0%	Fine jewelry (European standard)
14K	58.3%	Most US jewelry
10K	41.7%	Minimum for "gold" in US

History

The metric carat (200 mg) was adopted internationally in 1907. Before standardization, the carat varied by region (197-215 mg). The carob seed origin is partly myth; carob seeds actually vary significantly in weight, but the name stuck.

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