Carat Weight Converter

Convert between carats, grams, milligrams, points, and grains. 1 carat = 0.2 grams = 200 mg = 100 points.

Calculator

How to Use

  1. 1
    Enter the gem's weight in any unit

    Input the carat weight, gram weight, or milligram weight of your gemstone. One metric carat equals exactly 0.2 grams or 200 milligrams, a standard adopted internationally by the Fourth Extraordinary General Conference on Weights and Measures in 1907.

  2. 2
    Select the target unit for conversion

    Choose the unit you want to convert to: carats, grams, milligrams, grains, or troy ounces. For pearls and coral, the tool also provides momme (Japanese unit) conversions used in the Japanese pearl trade.

  3. 3
    Record the converted weight and point value

    Note both the converted weight and the point value (1 carat = 100 points), which is commonly used by jewelers when discussing stones smaller than one carat. A 0.75-carat stone is described as a “75-pointer” in trade terminology.

About

The metric carat, standardized as exactly 0.2 grams, is the universal unit of gemstone weight in the modern jewelry industry. Before international standardization in the early twentieth century, carat values varied by region—the English carat differed from the French and German carats by small but commercially significant amounts. The adoption of the 0.2-gram metric carat eliminated these discrepancies and enabled consistent international trade in precious stones.

For the diamond trade, carat weight is recorded to two decimal places, and certain weights carry price premiums known as “magic sizes” or “barrier weights.” Stones at or just above 0.50, 1.00, 1.50, and 2.00 carats command higher prices than those just below these thresholds, because many buyers specifically seek these landmark weights. This phenomenon means a 0.99-carat diamond typically sells for less per carat than a 1.00-carat stone of identical cut, color, and clarity, even though the size difference is imperceptible to the naked eye.

For colored stones, carat weight interacts differently with value because density varies widely among gem species. Buyers of fine rubies, sapphires, and emeralds often think in terms of both physical size (millimeter dimensions) and carat weight, since the relationship between the two depends on density. Pearl weight in the Japanese trade is historically measured in momme (3.75 grams), while other pearl markets use millimeter size as the primary size descriptor rather than weight. Understanding these unit conventions is essential for anyone navigating international gem and jewelry markets.

FAQ

Why is carat weight important in gem valuation?
Carat weight is one of the four main value determinants (the Four Cs: cut, color, clarity, carat) established by the GIA for diamonds, and the principle extends to all precious gems. Larger stones are exponentially rarer than smaller ones of equal quality, causing price-per-carat to increase disproportionately with size. A 2-carat diamond of a given quality typically sells for more than four times the price of a 1-carat diamond of the same quality, because fine 2-carat diamonds are much scarcer. Precise carat weight is therefore critical to accurate valuation.
How does carat weight translate to physical size?
The physical size of a given carat weight depends on the gem's specific gravity (density). A 1-carat diamond measures approximately 6.5 mm in diameter when cut as a round brilliant, while a 1-carat emerald (lower density at SG 2.67–2.78 vs. diamond's 3.52) measures approximately 6.7–6.9 mm. A 1-carat ruby (SG ~4.0) measures only about 5.5 mm because of its greater density. This is why “carat” and “mm size” are not interchangeable, and jewelers specify both weight and dimensions for important stones.
What is the difference between carat and karat?
These are entirely different measurement systems despite similar spellings. Carat (ct) measures gemstone weight, where 1 carat = 0.2 grams. Karat (k or kt) measures gold purity: 24 karats is pure gold (99.9%), 18 karats is 75% gold, and 14 karats is 58.3% gold. The word roots are related—both derive from the Arabic “qirat” (carob seed), used historically as a small weight standard—but in modern usage they are completely separate systems. Writing “2-karat diamond” or “18-carat gold” are common errors in non-specialist writing.
How do jewelers weigh gemstones precisely?
Professional gem weighing uses calibrated electronic scales accurate to at least 0.01 carat (0.002 grams), with laboratory-grade scales accurate to 0.001 carat. Weighing is done with the gem cleaned and dry, as surface moisture adds detectable weight at this precision level. Gemological laboratories report weights to two decimal places (e.g., 1.23 ct). For mounted stones, experienced gemologists can estimate weight by measuring length, width, and depth with calipers and applying formula-based calculations specific to each gem species and cut shape, since removing a set stone for weighing risks damage.
What is a total carat weight (TCW) versus a center stone weight?
Total carat weight (TCW) refers to the combined weight of all gemstones in a piece of jewelry, including side stones, accent diamonds, and any secondary gems. Center stone weight refers only to the primary gem. A ring described as “1.50 TCW” might have a 1.00-carat center diamond and 0.50 carats of surrounding accent stones, or it might be composed entirely of smaller stones—the description alone does not specify. Buyers seeking a specific center stone size should confirm individual stone weights, as TCW can be misleading when comparing pieces with different stone configurations.
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