Carats to Grams Converter

How to use the carats to grams converter

1. Enter the gemstone weight in carats in the input field.
2. The converter multiplies by 0.2 to calculate the weight in grams.
3. Review the results in grams, milligrams and troy ounces below.

How the carats to grams conversion works

The metric carat was standardized in 1907 at the Fourth General Conference on Weights and Measures. It is defined as exactly 200 milligrams, or 0.2 grams. To convert carats to grams, multiply the carat value by 0.2:

grams = carats x 0.2

For example, a 3.5-carat ruby weighs 3.5 x 0.2 = 0.7 grams. Because the factor is exact, there is no rounding error in the gram result regardless of how many decimal places the carat weight has. If you need to go the other direction, divide the gram weight by 0.2, or equivalently multiply by 5, to get back to carats.

This exactness makes the carat a convenient unit for international trade. A jeweler in Tokyo and a buyer in New York both know that 1.00 ct means precisely 0.200 g, with no ambiguity about which local standard applies.

Common gemstone weights reference table

This table shows frequently encountered diamond and gemstone sizes converted from carats to grams and milligrams. Use it as a quick lookup when evaluating stones without a calculator.

Notice that five carats equal exactly one gram. This round number is easy to remember and serves as a useful mental checkpoint when estimating larger parcels of stones.

Points, carats and precision

In the jewelry trade, one carat is divided into 100 points. A 50-point diamond weighs 0.50 carats, which is 0.1 grams. When pricing stones, even a single point can change the value significantly, so precise weight matters. Scales used by gemologists typically read to 0.001 carats (one thousandth), which corresponds to 0.0002 grams or 0.2 milligrams.

Because prices per carat jump at whole-number thresholds (1.00 ct, 1.50 ct, 2.00 ct), cutters aim to retain just enough material to cross those marks. A stone weighing 0.99 ct might be re-cut to reach 1.00 ct even if some material is lost, because the per-carat price at one full carat is substantially higher than at 99 points. Understanding the exact gram equivalent helps buyers verify that a certificate weight matches what their own scale reads.

If you need to convert larger quantities of precious metals rather than gemstones, you might also find the lbs to grams converter useful for bulk shipments.

Carat versus karat

A common source of confusion is the difference between carat and karat. A carat (abbreviated ct) measures the mass of gemstones and pearls. A karat (abbreviated K or kt) measures the purity of gold alloys, where 24 karat is pure gold. They sound identical but measure completely different properties. This tool handles only gemstone carat weight.

Mixing the two up on an invoice or appraisal can lead to costly misunderstandings. An 18-karat gold ring setting does not weigh 18 carats. The setting's mass would be measured in grams or pennyweights, while any diamond mounted in it would be described in carats. Always check whether the abbreviation is ct or K to avoid errors.

Why gemstones are weighed in carats instead of grams

The carat system predates the widespread use of the metric system. Historically, traders used carob seeds as counterweights on balance scales because the seeds were believed to have uniform mass. The word carat derives from the Greek keration, meaning carob seed. When the metric carat was adopted internationally in 1907, it preserved continuity with centuries of trade practice while fixing the unit to an exact metric value of 0.2 grams.

Before standardization, different trading centers used slightly different carat values. The London carat, the Paris carat and the Florence carat all varied by small fractions, which caused disputes in international transactions. The 1907 agreement eliminated these discrepancies by tying the carat directly to the kilogram, ensuring that converting carats to grams produces the same answer everywhere.

Converting carats to troy ounces

Precious metals are traditionally weighed in troy ounces, and some gemstone dealers also reference this unit for large parcels. One troy ounce equals exactly 31.1034768 grams. Since one carat is 0.2 grams, you can convert carats to troy ounces by dividing the gram result by 31.1034768. For example, a 10-carat stone weighs 2 grams, which is approximately 0.0643 troy ounces.

Do not confuse troy ounces with avoirdupois ounces, the everyday ounce used in US grocery stores. One avoirdupois ounce equals 28.349523125 grams. The difference is roughly ten percent, which matters when valuing expensive materials. This converter displays both ounce types so you can pick the correct one for your context.

Worked examples

Each result below was produced by the same code the tool runs, so you can check your own numbers against them.