Free Tool
Gemstone Carat Converter
Instantly convert between carats, grams, milligrams, and points. Includes a round brilliant cut mm size reference.
Convert Carat Weight
Enter a value in any field to convert
1 carat = 0.2 grams
1 gram = 5 carats
1 carat = 200 milligrams
1 carat = 100 points
Conversion Summary
Carats
Grams
Milligrams
Points
Size Reference
Round Brilliant Cut - Size Chart
Approximate mm diameter for round brilliant cut diamonds and gemstones. Actual sizes may vary slightly depending on cut proportions and gemstone type.
| Carat Weight | Diameter (mm) | Visual Size |
|---|---|---|
| 0.25 ct | 4.1 mm | |
| 0.33 ct | 4.4 mm | |
| 0.50 ct | 5.2 mm | |
| 0.75 ct | 5.9 mm | |
| 1.00 ct Popular | 6.5 mm | |
| 1.25 ct | 6.9 mm | |
| 1.50 ct | 7.4 mm | |
| 1.75 ct | 7.8 mm | |
| 2.00 ct | 8.2 mm | |
| 2.50 ct | 8.8 mm | |
| 3.00 ct | 9.3 mm | |
| 4.00 ct | 10.2 mm | |
| 5.00 ct | 11.0 mm |
Sizes shown are approximate for round brilliant cut diamonds. Other gemstone types may have slightly different size-to-weight ratios.
Quick Reference
Key conversion relationships at a glance.
= 0.2 grams
= 200 milligrams
= 100 points
= 5 carats
= 1000 milligrams
= 500 points
= 0.01 carats
= 0.002 grams
= 2 milligrams
Origin of the Carat
The word "carat" comes from the carob seed, which ancient gem traders used as counterweights on balance scales because of their remarkably consistent weight of about 200 milligrams.
Carat vs Karat
A "carat" (ct) measures gemstone weight, while "karat" (K) measures gold purity. A 1-carat diamond weighs 200mg. An 18-karat gold piece is 75% pure gold. These are entirely different measurements despite sounding similar.
Learn
Understanding Carat Weight
What Is a Carat?
A carat is a unit of mass used for measuring gemstones and pearls. One carat is defined as exactly 200 milligrams (0.2 grams). This standard was adopted internationally in 1907 at the Fourth General Conference on Weights and Measures and is now used worldwide.
Does Carat Equal Size?
Carat measures weight, not physical size. Two gemstones of the same carat weight can appear different in size depending on how they are cut and their density. For example, a 1-carat sapphire is physically smaller than a 1-carat diamond because sapphire is denser. The cut also affects how large a gemstone appears face-up.
The Point System
Jewellers often use "points" to describe stones under 1 carat. One carat equals 100 points, so a 0.50-carat diamond is commonly referred to as a "50-point diamond" or a "half-carat." This system allows for precise communication about very small weight differences that significantly affect pricing.
Why Carat Matters for Pricing
Diamond prices increase exponentially with carat weight, not linearly. A 2-carat diamond costs significantly more than twice the price of a 1-carat diamond of similar quality. This is because larger rough diamonds are rarer in nature. Prices also jump at "magic sizes" like 0.5ct, 1.0ct, 1.5ct, and 2.0ct.
Total Carat Weight (TCW)
When a piece of jewellery contains multiple gemstones, the "Total Carat Weight" represents the combined weight of all stones. For example, a ring with a 1-carat centre diamond and twenty 0.05-carat side stones has a TCW of 2 carats. TCW should not be confused with the weight of the centre stone alone.
Buying Tip
Consider choosing a diamond just under a popular carat weight (e.g. 0.90ct instead of 1.00ct). The visual size difference is negligible, but the price difference can be substantial - sometimes 15-20% less. The same principle applies at the 0.50ct, 1.50ct, and 2.00ct thresholds.
Beyond Carat Weight
The 4Cs of Diamond Quality
Carat weight is just one of four factors that determine a diamond's value. Together they are known as the 4Cs.
Carat
The weight of the diamond. Larger diamonds are rarer and more valuable, but size alone does not determine worth.
This ToolCut
How well the diamond is cut affects its brilliance. A well-cut diamond reflects light beautifully, maximising sparkle.
Colour
Graded from D (colourless) to Z (light yellow). Less colour means a higher grade and greater rarity.
Clarity
Natural inclusions and blemishes are graded from Flawless to Included. Most flaws are invisible to the naked eye.
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