Carat to Gram Converter

Convert metric carats (ct) to grams (g) for gemstones, diamonds, and pearls. The metric carat is the standard unit of mass for measuring precious stones, with 1 carat equal to 200 milligrams or 0.2 grams. This precise conversion is essential for jewellers, gemologists, and anyone working with fine gemstones.

Quick Conversions

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Conversion Formula

The conversion between carats and grams follows this simple relationship:

Grams = Carats × 0.2

Or alternatively:

Grams = Carats ÷ 5

Since 1 metric carat equals exactly 200 milligrams (0.2 grams), you simply multiply the carat value by 0.2 to obtain the mass in grams. Conversely, to convert from grams to carats, multiply by 5.

Conversion Steps

  1. Identify the weight in carats that you wish to convert (for example, 3.5 ct)
  2. Multiply the carat value by 0.2 (the conversion factor): 3.5 × 0.2 = 0.7
  3. The result is the equivalent weight in grams: 3.5 ct = 0.7 g
  4. For greater precision, retain additional decimal places as needed for your application

Common Conversions

Carats (ct) Grams (g) Milligrams (mg)
0.25 ct 0.05 g 50 mg
0.5 ct 0.1 g 100 mg
0.75 ct 0.15 g 150 mg
1 ct 0.2 g 200 mg
1.5 ct 0.3 g 300 mg
2 ct 0.4 g 400 mg
3 ct 0.6 g 600 mg
5 ct 1 g 1,000 mg
10 ct 2 g 2,000 mg
25 ct 5 g 5,000 mg
50 ct 10 g 10,000 mg
100 ct 20 g 20,000 mg

Gemstone Weight Comparison

0.5 ct
0.1 g
1 ct
0.2 g
1.5 ct
0.3 g
2 ct
0.4 g
2.5 ct
0.5 g

About the Metric Carat

Definition

The metric carat is a unit of mass specifically designated for measuring gemstones and pearls. Adopted internationally in 1907 at the Fourth General Conference on Weights and Measures, it provides a standardised measurement that replaced various historical carat systems used across different regions.

Carat Points

Each carat is subdivided into 100 points, allowing gemologists to express weight with precision. For instance, a 0.75-carat diamond can also be described as 75 points. This subdivision system enables detailed grading and valuation of gemstones at fractions of a carat.

Historical Context

Before the metric carat was standardised, different countries used varying definitions. The current metric carat was chosen for its convenience and compatibility with the metric system, facilitating international trade and gemstone certification.

Carat Weight in Gemstones

Diamond Weight Categories

Diamonds are often categorised by carat weight ranges that impact their value significantly:

  • Melee diamonds: Under 0.2 ct (0.04 g) – often used as accent stones
  • Quarter carat: 0.25 ct (0.05 g) – popular for delicate jewellery
  • Half carat: 0.5 ct (0.1 g) – common engagement ring size
  • One carat: 1 ct (0.2 g) – classic benchmark for fine diamonds
  • Two carats: 2 ct (0.4 g) – premium category with significant presence
  • Five carats+: 5+ ct (1+ g) – rare and highly valuable specimens

Other Gemstone Conversions

Gemstone Type Typical Range (ct) Typical Range (g)
Diamond (engagement ring) 0.5 – 2 ct 0.1 – 0.4 g
Ruby 1 – 3 ct 0.2 – 0.6 g
Sapphire 1 – 5 ct 0.2 – 1 g
Emerald 1 – 3 ct 0.2 – 0.6 g
Pearl 2 – 10 ct 0.4 – 2 g
Aquamarine 5 – 15 ct 1 – 3 g
Topaz 5 – 20 ct 1 – 4 g

Frequently Asked Questions

How many grams are in 1 carat?
One metric carat equals exactly 0.2 grams or 200 milligrams. This standardised definition has been used internationally since 1907 for measuring precious gemstones and pearls.
Why is carat weight important for gemstones?
Carat weight is one of the fundamental factors in determining a gemstone’s value, alongside cut, colour, and clarity (the “Four Cs” for diamonds). Larger stones are generally rarer and more valuable per carat, though quality characteristics also play crucial roles in pricing.
Is a gemstone carat different from gold carat?
Yes, these are entirely different measurements. Gemstone carats measure weight (mass), whilst gold carats (often spelled “karat” in some regions) measure purity, with 24-carat gold being pure gold. The two should not be confused despite sharing similar names.
Does carat weight affect gemstone size?
Carat weight measures mass, not physical dimensions. Two gemstones of identical carat weight may appear different in size depending on their density, cut, and proportions. For example, a 1-carat diamond will look smaller than a 1-carat emerald due to density differences.
How precise should carat measurements be?
Professional gemological laboratories typically measure carat weight to the nearest hundredth (0.01 ct) or two decimal places. For certification purposes and valuable stones, this precision is essential as even small differences in carat weight can significantly impact value.
What are carat points?
Points are subdivisions of a carat, with 100 points equalling 1 carat. A 0.50-carat stone is referred to as 50 points, whilst a 0.75-carat stone is 75 points. This terminology allows precise communication about gemstone weights within the trade.
Can I convert carats to other mass units?
Yes, once you have converted carats to grams, you can further convert to other metric units such as milligrams (multiply grams by 1,000) or kilograms (divide grams by 1,000). For imperial units, 1 gram equals approximately 0.035 ounces.
Why was 200 milligrams chosen for the metric carat?
The 200-milligram standard was selected because it closely approximated the average of various historical carat weights used in different countries, whilst also being a convenient round number within the metric system. This facilitated international adoption and trade.

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