How carat conversion works
The metric carat is defined as an exact fraction of a gram, so converting to or from carats is a simple multiplication.
- ct — carats — gemstone weight unit
- g — grams
- mg — milligrams
Common values
| Value (ct) | Result |
|---|---|
| 0.1 ct | 0.02 g |
| 0.25 ct | 0.05 g |
| 0.5 ct | 0.1 g |
| 1 ct | 0.2 g |
| 1.5 ct | 0.3 g |
| 2 ct | 0.4 g |
| 3 ct | 0.6 g |
| 5 ct | 1 g |
| 10 ct | 2 g |
More detail
Why the carat is exactly 0.2 g
Before 1907, the carat's value varied slightly between countries because it was historically based on the weight of a carob seed. The 4th General Conference on Weights and Measures (CGPM) standardized it internationally as the "metric carat," fixed at exactly 200 mg (0.2 g), which is still the definition used in gem trading worldwide today.
Carat (ct) vs. karat (K) — don't confuse them
A gemstone's carat (ct) measures mass — 1 ct = 0.2 g, as converted above. Gold's karat (K) measures purity on a 24-part scale (24K = pure gold, 18K = 75% gold). The similar spelling is a common source of confusion, but the two units measure completely different things.
Practical tip. A 1-carat diamond weighs the same 0.2 g regardless of its cut or shape — carat measures mass, not size, so two 1-carat stones can look different sizes depending on how they're cut.
Frequently asked questions
How many grams is 1 carat?
Exactly 0.2 grams (200 milligrams). This is the metric carat definition adopted by the CGPM in 1907 and used worldwide in the gem trade — enter 1 above with From = ct and To = g to confirm.
How many milligrams is a 1-carat diamond?
1 carat = 200 milligrams exactly. Select From = ct and To = mg above with value 1 to see this conversion.
How much does a 3-carat ring stone weigh in grams?
3 ct × 0.2 g = 0.6 g — this is the weight of the gemstone alone, not the full ring including its metal band.
Why is the carat defined as exactly 200 mg?
The carat originally derived from the weight of a carob seed and varied slightly by country. In 1907 the CGPM standardized it as the "metric carat," fixed at 200 mg, to make gem weights consistent internationally.
Is carat (ct) the same as gold karat (K)?
No. Carat (ct) is a unit of mass for gemstones (1 ct = 0.2 g), while karat (K) measures gold purity out of 24 parts (24K = 100% pure). They share a similar name but measure entirely different properties.