Mm to Carat Calculator

MM to Carat Gemstone Weight Calculator

Use this calculator to estimate the carat weight of a round brilliant cut gemstone based on its dimensions in millimeters and specific gravity. This is a common approximation used in the jewelry industry.

If left blank, depth will be estimated as 61% of the diameter.

Diamond (SG: 3.52) Sapphire / Ruby (SG: 4.00) Emerald (SG: 2.72) Custom Specific Gravity

Depth (mm) was not provided or invalid. Estimating depth as " + depthMM.toFixed(2) + " mm (approx. 61% of diameter).

Calculated Carat Weight:

" + carats.toFixed(2) + " ct

(This is an approximation for a round brilliant cut gemstone. Actual weight may vary based on precise cut proportions, girdle thickness, and specific gravity variations.)

Understanding Gemstone Weight: From Millimeters to Carats

When buying or selling gemstones, you'll often encounter two primary measurements: dimensions in millimeters (mm) and weight in carats (ct). While millimeters describe the physical size of a stone, carats represent its weight. Converting between these two isn't as straightforward as you might think, as it depends on several factors unique to each gemstone. Our MM to Carat Gemstone Weight Calculator helps you make an informed estimate for round brilliant cut stones.

What is a Carat?

A carat is a unit of mass equal to 200 milligrams (0.2 grams). It's the standard unit of weight for diamonds and other gemstones. The word "carat" originates from the carob seed, which was historically used as a standard for weighing precious stones due to its consistent weight.

Why MM to Carat Conversion Isn't Simple

Unlike converting inches to centimeters, converting a gemstone's dimensions (mm) to its weight (carats) is not a direct linear conversion. This is because weight is a function of both volume and density. Two main factors influence this conversion:

1. Specific Gravity (Density): Different gemstones have different densities. For example, a sapphire is denser than an emerald. This means that a sapphire and an emerald of the exact same dimensions will have different carat weights – the sapphire will be heavier.
2. Shape and Cut Proportions: The way a gemstone is cut significantly affects its volume for a given set of face-up dimensions. A deep cut will result in a heavier stone than a shallow cut of the same diameter. Our calculator focuses on the common "round brilliant" cut, which has standardized proportions, allowing for a more reliable estimation.

Specific Gravity Explained

Specific gravity (SG) is a measure of a material's density relative to the density of water. For gemstones, it tells us how much heavier a stone is compared to an equal volume of water. A higher specific gravity means a denser, and thus heavier, stone for the same size. Here are some common specific gravities:

• Diamond: ~3.52
• Sapphire/Ruby (Corundum): ~4.00
• Emerald (Beryl): ~2.72
• Topaz: ~3.53
• Garnet: ~3.80
• Quartz: ~2.65

Our calculator includes pre-set specific gravities for popular gemstones, and also allows you to input a custom value for other types of stones.

How the Calculator Works

Our MM to Carat Gemstone Weight Calculator uses an industry-standard approximation formula for round brilliant cut gemstones:

Carats = (Diameter_mm)^2 × Depth_mm × Specific_Gravity × 0.0018

The 0.0018 factor is an empirical constant that accounts for the typical volume of a round brilliant cut and the conversion from cubic millimeters to carats. This formula provides a reliable estimate for stones with standard proportions.

You'll need to input the gemstone's diameter in millimeters. The depth in millimeters is optional; if you don't provide it, the calculator will estimate it based on a common depth-to-diameter ratio (approximately 61% for round brilliant cuts). You then select your gemstone type, or enter a custom specific gravity.

Using the Calculator: Examples

Let's look at some practical examples:

Example 1: A 6.5mm Round Diamond

• Diameter: 6.5 mm
• Depth: (Let's leave it blank, so it estimates 6.5 * 0.61 = 3.965 mm)
• Gemstone Type: Diamond (SG: 3.52)
• Calculation: (6.5 * 6.5) * 3.965 * 3.52 * 0.0018 ≈ 1.05 ct
• Result: Approximately 1.05 carats. This aligns with the common knowledge that a 6.5mm round brilliant diamond is typically around 1 carat.

Example 2: A 7.0mm Round Sapphire

• Diameter: 7.0 mm
• Depth: (Let's leave it blank, so it estimates 7.0 * 0.61 = 4.27 mm)
• Gemstone Type: Sapphire (SG: 4.00)
• Calculation: (7.0 * 7.0) * 4.27 * 4.00 * 0.0018 ≈ 1.51 ct
• Result: Approximately 1.51 carats.

Example 3: An 8.0mm Round Emerald

• Diameter: 8.0 mm
• Depth: (Let's leave it blank, so it estimates 8.0 * 0.61 = 4.88 mm)
• Gemstone Type: Emerald (SG: 2.72)
• Calculation: (8.0 * 8.0) * 4.88 * 2.72 * 0.0018 ≈ 1.22 ct
• Result: Approximately 1.22 carats. Notice how an 8mm emerald is lighter than a 7mm sapphire due to its lower specific gravity.

Important Considerations and Limitations

While this calculator provides a valuable estimate, it's crucial to remember that it's an approximation. Actual carat weight can vary due to:

• Exact Cut Proportions: The formula assumes ideal or very good proportions for a round brilliant. Variations in crown height, pavilion depth, and girdle thickness will affect the actual weight.
• Specific Gravity Variations: The specific gravity values used are averages. Natural gemstones can have slight variations in density.
• Other Shapes: This calculator is specifically designed for round brilliant cuts. Other shapes (e.g., oval, princess, emerald cut) require different formulas or more complex calculations involving length, width, and depth, as their volume-to-dimension ratios differ significantly.

For precise carat weight, especially for high-value gemstones, always rely on a professional gemological appraisal using calibrated scales.