Carat Weight Calculator
Accurately estimate gemstone weight and value based on dimensions and shape
Estimation Settings
Diamond (SG: 3.52)
Ruby (SG: 4.00)
Sapphire (SG: 4.00)
Emerald (SG: 2.72)
Amethyst/Quartz (SG: 2.65)
Topaz (SG: 3.53)
Spinel (SG: 3.72)
Cubic Zirconia (SG: 5.80)
Specific Gravity (SG) determines density relative to water.
Round Brilliant
Oval
Princess (Square)
Emerald Cut (Rectangular)
Marquise
Pear
Heart
Cushion
Different shapes have different volume coefficients.
Please enter a valid positive number.
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Enter current market rate to estimate total value.
Estimated Carat Weight
0.00 ct
Formula: L × W × D × Shape Factor × SG Adjustment
Weight in Grams
0.000 g
Total Estimated Value
$0.00
Length-to-Width Ratio
1.00
Gem Points (100pt = 1ct)
0
Comparative Density Analysis
| Material | Specific Gravity | Weight for these dimensions |
|---|
Comparison: Selected Gemstone vs. Diamond vs. Cubic Zirconia (CZ)
Carat Weight Calculator: A Comprehensive Guide to Gemstone Estimation
Whether you are a jewelry appraiser, a gemstone enthusiast, or an investor looking to verify a stone's credentials, accurate weight estimation is critical. This carat weight calculator helps you estimate the weight of a mounted stone or loose gem using precise millimetric dimensions, specific gravity, and shape coefficients.
What is a Carat Weight Calculator?
A carat weight calculator is a mathematical tool used to estimate the weight of gemstones when a direct scale measurement is impossible—such as when a diamond is already mounted in a ring setting. By inputting the physical dimensions (length, width, and depth), the calculator applies a volumetric formula adjusted for the stone's density (Specific Gravity) and cut style.
Common misconceptions include believing that carat refers to size. In reality, "carat" is a unit of weight (mass), where 1 carat equals exactly 0.2 grams. Two stones of the same dimensions may have vastly different carat weights if their densities differ, which is why this calculator adjusts for material types like Diamond, Ruby, or Sapphire.
Carat Weight Formula and Mathematical Explanation
The core logic behind any accurate carat weight calculator involves calculating volume and multiplying it by density. The industry-standard formula used by gemological laboratories is:
Weight (ct) = Length × Width × Depth × Shape Coefficient × SG Adjustment
Here is a breakdown of the variables used in our calculation:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Length (L) | Longest dimension | Millimeters (mm) | 2.00 – 15.00 mm |
| Width (W) | Shortest dimension | Millimeters (mm) | 2.00 – 15.00 mm |
| Depth (D) | Height from table to culet | Millimeters (mm) | 1.00 – 10.00 mm |
| Shape Coeff | Volume correction factor | Constant | 0.0057 – 0.0092 |
| Specific Gravity (SG) | Density of the mineral | Ratio | 2.65 (Quartz) – 4.00 (Ruby) |
Practical Examples (Real-World Use Cases)
Example 1: The Round Brilliant Diamond
Imagine you have a mounted round diamond that you cannot weigh directly. You measure the diameter as 6.50 mm and the depth as 3.90 mm.
- Input L/W: 6.50 mm
- Input Depth: 3.90 mm
- Shape: Round Brilliant (Factor 0.0061)
- Gem: Diamond (SG 3.52)
- Calculation: 6.5 × 6.5 × 3.9 × 0.0061 ≈ 1.00 ct
This matches the "standard" proportions for a 1-carat diamond, validating the cut quality.
Example 2: The Deep Ruby
A Ruby is denser than a diamond. If you have an Oval cut Ruby measuring 7.00 x 5.00 x 3.50 mm:
- Volume Factor: Oval (0.0062)
- SG Adjustment: Ruby (4.00) vs Diamond (3.52). Factor = 4.00/3.52 = 1.136
- Result: A diamond of this size would be ~0.75 ct, but the Ruby will weigh roughly 0.85 ct due to higher density.
How to Use This Carat Weight Calculator
- Select Gem Material: Choose the specific stone (e.g., Diamond, Sapphire). This sets the Specific Gravity.
- Select Shape: Choose the cut (e.g., Round, Emerald, Cushion). This applies the correct volume coefficient.
- Measure Dimensions: Use a digital caliper to measure Length, Width, and Depth in millimeters. Be as precise as possible (2 decimal places).
- Enter Price (Optional): If you know the price per carat, enter it to see the total estimated value.
- Analyze Results: View the estimated carat weight, weight in grams, and compare it against other minerals in the chart below.
Key Factors That Affect Carat Weight Results
While this carat weight calculator provides a high-accuracy estimate, several real-world factors can influence the final scale weight:
- Girdle Thickness: A thick girdle adds "dead weight" that increases caratage without increasing face-up size. This calculator assumes a Medium girdle.
- Cut Proportions: Extremely deep or shallow stones may deviate slightly from standard volumetric coefficients.
- Measurement Error: A discrepancy of just 0.1mm can significantly alter the estimated weight, especially for shapes like the Emerald cut.
- Bulge Factor: For shapes like Cushion or Emerald cuts, the curvature of the pavilion (belly) can vary, affecting volume.
- Mounting Obstructions: Prongs or bezels often make it difficult to measure the exact depth or width of a mounted stone.
- Specific Gravity Variations: While chemically pure stones have constant SG, natural inclusions or treatments can slightly alter density.
Frequently Asked Questions (FAQ)
Is this calculator accurate for mounted stones?
Yes, this is the primary use case for a carat weight calculator. However, accuracy depends entirely on your ability to measure dimensions (especially depth) while the stone is in the setting.
Why does my 1-carat sapphire look smaller than my 1-carat diamond?
Sapphire (SG 4.00) is denser than Diamond (SG 3.52). Therefore, a 1-carat sapphire packs more weight into a smaller volume, making it physically smaller than a 1-carat diamond.
Does shape affect price per carat?
Yes. Round brilliants are generally more expensive because they require removing more rough material during cutting. Fancy shapes like Ovals or Cushions retain more weight from the rough crystal.
How do I measure depth if the stone is set low?
You may need a specific tool called a "Leveridge Gauge" which can reach under mountings. If the culet is closed off, depth can only be estimated, reducing the accuracy of the carat weight calculator.
What is "Total Carat Weight" (TCW)?
TCW refers to the combined weight of all stones in a piece of jewelry. This calculator estimates the weight of a single center stone.
Can I use this for Cubic Zirconia (CZ)?
Yes, simply select "Cubic Zirconia" in the dropdown. Note that CZ is much heavier (SG 5.80) than diamond, so a CZ of the same size will weigh roughly 1.7x more.
What is the difference between a carat and a karat?
"Carat" (ct) measures gemstone weight. "Karat" (K) measures gold purity. They are not interchangeable.
Why are points used in weight?
For diamonds under 1 carat, jewelers often use "points". 100 points = 1 carat. A "50 pointer" is a 0.50 ct diamond.
Related Tools and Internal Resources
Explore more financial and gemological tools to assist your investments:
- Diamond Price Calculator Estimate the market value of loose diamonds based on the 4Cs.
- Gemstone Specific Gravity Chart A complete reference guide for densities of over 50 gem types.
- Gold Value Calculator Calculate scrap gold value based on Karat purity and current spot prices.
- Jewelry Valuation Guide Learn how professional appraisers determine replacement value vs. resale value.
- International Ring Size Converter Convert ring sizes between US, UK, and European standards.
- Understanding the 4Cs Deep dive into Cut, Color, Clarity, and Carat weight.