About the Carat to Grams
This carat to grams converter changes gemstone weights between metric carats, grams, milligrams, points, grains, troy ounces and ounces. Choose the unit you have — a 1.5 ct diamond, a 3 g sapphire rough or a 50-point stone — and you get every equivalent, with the headline answer in grams (or in carats if you started from another unit).
Jewellers, gem buyers and students use it to check certificate weights, compare stones sold by the gram with stones sold by the carat, or estimate the weight of loose stones for mounting and shipping.
The metric carat is exactly 200 mg, agreed internationally in 1907, so these conversions are exact. Note that carat (ct) measures gem weight while karat (K) measures gold purity — they are different things despite sounding alike.
How to use the carat to grams
- 1Enter the weight of the stone.
- 2Choose the unit it is in — carats, grams, milligrams or points.
- 3Read the headline answer in grams (or carats if you started from another unit).
- 4Use the chart for common diamond sizes.
Formula and method
The metric carat was fixed at exactly 200 milligrams (0.2 g), so grams are carats × 0.2 and carats are grams × 5. Each carat is divided into 100 points, so a 0.5 ct stone is 50 points and weighs 100 mg.
For other units the converter goes through grams: a grain is 64.79891 mg (about 0.324 ct), a troy ounce is 31.1034768 g (155.52 ct) and an avoirdupois ounce is 28.349523125 g (141.75 ct).
- ct
- Metric carat = 0.2 g
- pt
- Point = 0.01 ct = 2 mg
- g
- Grams
Worked examples
A 1-carat diamond
One metric carat is defined as 200 mg, which is 0.2 g and 100 points.
2.5 ct sapphire
2.5 × 0.2 = 0.5 g. Dividing 500 mg by 64.79891 mg gives about 7.72 grains.
5 grams of rough gemstone
Grams × 5 = carats, so 5 g is 25 ct. Rough is often sold by the gram and cut stones by the carat.
A 75-point stone
75 points ÷ 100 = 0.75 ct, and 0.75 × 0.2 = 0.15 g.
Frequently asked questions
How many grams is 1 carat?+
One metric carat is exactly 0.2 grams (200 milligrams). So 5 carats make one gram.
How do I convert grams to carats?+
Multiply grams by 5. For example, 1.2 g × 5 = 6 carats, and 0.1 g × 5 = 0.5 ct.
What is a point in diamond weight?+
A point is one hundredth of a carat (2 mg). A 0.25 ct diamond is a 25-pointer and a 0.10 ct stone is a 10-pointer.
Is carat the same as karat?+
No. Carat (ct) is a unit of mass for gemstones, 0.2 g. Karat (K or kt) measures gold purity out of 24 parts, so 18K gold is 75% pure gold.
Does carat weight tell you the size of a diamond?+
Only roughly. Carat is weight, not dimensions, so a well-cut 1 ct round diamond is about 6.4–6.5 mm across, but different shapes and cuts spread the same weight differently.