Gem Weight Calculator
Estimate the weight of a cut gemstone in carats based on its dimensions (Length, Width, Depth), shape, and the specific gravity (density) of the material.
Enter the gemstone's dimensions in millimeters (mm) and select a common Gem Type or enter its Specific Gravity (SG) directly. Select the correct Shape for an accurate estimate.
Enter Gemstone Details
Understanding Gemstone Weight Calculation
How Gem Weight is Estimated
Unlike weighing a finished piece of jewelry, loose gemstones are often weighed precisely on a gemological scale to determine their carat weight. However, if a gem is already set in jewelry or needs to be estimated from its size, calculation based on dimensions is used.
The principle is simple: **Volume × Density = Mass**. The mass is then converted to carats (1 gram = 5 carats).
Since measuring the exact volume of a complex cut gem is difficult, the volume is *estimated* from the gemstone's length, width, and depth (L×W×D) using shape-specific formulas or correction factors. These factors account for how the particular cut shape fills the space defined by its maximum dimensions.
Specific Gravity (SG)
Specific Gravity is the ratio of the density of a substance to the density of a reference substance (usually water at a specific temperature). For gemstones, it's essentially how dense the gem material is. Different gem types have different, relatively consistent, specific gravities.
- A gem with a *higher* SG will weigh *more* than a gem of the same size and shape but with a *lower* SG.
- For example, a Ruby (SG ~4.00) will weigh more than an Emerald (SG ~2.72) of the exact same dimensions and cut.
Accurate weight calculation requires knowing the gem's specific gravity. This calculator uses standard SG values for common gem types or allows you to input a known SG.
Shape Correction Factors (Approximate)
The calculator uses empirical factors applied to the Length × Width × Depth product to estimate the gem's volume based on its shape. These factors are derived from observations of many stones and are approximations. Common factors used in estimation formulas:
- Round Brilliant: ~0.0018
- Oval: ~0.0020
- Emerald Cut: ~0.0025
- Princess Cut: ~0.0025
- Pear Shape: ~0.0020
- Marquise: ~0.0018
- Cushion Cut: ~0.0023
- Radiant Cut: ~0.0025
- Asscher Cut: ~0.0024
- Heart Shape: ~0.0020
These factors are typically applied in a formula like: Weight (Carats) ≈ Length (mm) × Width (mm) × Depth (mm) × Specific Gravity × Shape Factor.
Note: The specific factors can vary slightly between different gemological sources. This calculator uses commonly accepted approximate factors.
What is a Carat?
The carat is a unit of mass used for weighing gemstones. One carat is defined as exactly 200 milligrams (0.2 grams). The term comes from the carob seed, which was historically used as a small unit of weight.
Example: A 1.00 carat diamond weighs 0.2 grams (or 200 milligrams).
Gem Weight Calculation Examples
Here are some examples of how gem weight is estimated using common dimensions, shapes, and specific gravities. Click to expand.
Example 1: Standard Round Diamond
Scenario: Estimate the weight of a round brilliant diamond.
Known Values: Type: Diamond (SG 3.52), Shape: Round, Dimensions: 6.5 mm L x 6.5 mm W x 4.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Round)
Calculation: Weight ≈ 6.5 × 6.5 × 4.0 × 3.52 × 0.0018
Result: Weight ≈ 1.06 Carats.
Conclusion: A round diamond with these dimensions is typically around 1 carat.
Example 2: Oval Sapphire
Scenario: Estimate the weight of an oval blue sapphire.
Known Values: Type: Sapphire (SG 4.00), Shape: Oval, Dimensions: 8.0 mm L x 6.0 mm W x 4.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Oval)
Calculation: Weight ≈ 8.0 × 6.0 × 4.0 × 4.00 × 0.0020
Result: Weight ≈ 1.54 Carats.
Conclusion: This oval sapphire would estimate to about 1.54 carats.
Example 3: Emerald Cut Emerald
Scenario: Estimate the weight of an emerald cut emerald.
Known Values: Type: Emerald (SG 2.72), Shape: Emerald Cut, Dimensions: 7.0 mm L x 5.0 mm W x 4.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Emerald)
Calculation: Weight ≈ 7.0 × 5.0 × 4.0 × 2.72 × 0.0025
Result: Weight ≈ 0.95 Carats.
Conclusion: The estimated weight of this emerald is about 0.95 carats.
Example 4: Princess Cut Garnet
Scenario: Estimate the weight of a princess cut garnet.
Known Values: Type: Garnet (SG 4.00), Shape: Princess Cut, Dimensions: 5.0 mm L x 5.0 mm W x 3.5 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Princess)
Calculation: Weight ≈ 5.0 × 5.0 × 3.5 × 4.00 × 0.0025
Result: Weight ≈ 0.88 Carats.
Conclusion: This princess cut garnet estimates to about 0.88 carats.
Example 5: Pear Shape Aquamarine
Scenario: Estimate the weight of a pear shape aquamarine.
Known Values: Type: Aquamarine (SG 2.72), Shape: Pear Shape, Dimensions: 9.0 mm L x 6.0 mm W x 4.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Pear)
Calculation: Weight ≈ 9.0 × 6.0 × 4.0 × 2.72 × 0.0020
Result: Weight ≈ 1.17 Carats.
Conclusion: The estimated weight of this pear shape aquamarine is around 1.17 carats.
Example 6: Marquise Amethyst
Scenario: Estimate the weight of a marquise amethyst.
Known Values: Type: Amethyst (SG 2.65), Shape: Marquise, Dimensions: 10.0 mm L x 5.0 mm W x 3.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Marquise)
Calculation: Weight ≈ 10.0 × 5.0 × 3.0 × 2.65 × 0.0018
Result: Weight ≈ 0.72 Carats.
Conclusion: This marquise amethyst estimates to weigh about 0.72 carats.
Example 7: Round Peridot
Scenario: Estimate the weight of a round peridot.
Known Values: Type: Peridot (SG 3.34), Shape: Round, Dimensions: 7.0 mm L x 7.0 mm W x 4.5 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Round)
Calculation: Weight ≈ 7.0 × 7.0 × 4.5 × 3.34 × 0.0018
Result: Weight ≈ 1.32 Carats.
Conclusion: A round peridot with these dimensions would estimate to about 1.32 carats.
Example 8: Custom SG - White Topaz (Specific Cut)
Scenario: Estimate the weight of an oval white topaz with a specific, slightly different cut.
Known Values: Type: Custom SG, Specific Gravity: 3.50, Shape: Oval, Dimensions: 7.0 mm L x 5.0 mm W x 3.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Oval)
Calculation: Weight ≈ 7.0 × 5.0 × 3.0 × 3.50 × 0.0020
Result: Weight ≈ 0.74 Carats.
Conclusion: Using the custom SG, the oval topaz estimates to weigh about 0.74 carats.
Example 9: Cushion Cut Spinel
Scenario: Estimate the weight of a cushion cut red spinel.
Known Values: Type: Spinel (SG 3.60), Shape: Cushion, Dimensions: 6.0 mm L x 6.0 mm W x 4.0 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Cushion)
Calculation: Weight ≈ 6.0 × 6.0 × 4.0 × 3.60 × 0.0023
Result: Weight ≈ 1.19 Carats.
Conclusion: This cushion cut spinel estimates to weigh about 1.19 carats.
Example 10: Small Round Zircon
Scenario: Estimate the weight of a small round blue zircon.
Known Values: Type: Zircon (SG 4.65), Shape: Round, Dimensions: 4.0 mm L x 4.0 mm W x 2.5 mm D.
Formula (Approx): Weight (ct) ≈ L × W × D × SG × Factor(Round)
Calculation: Weight ≈ 4.0 × 4.0 × 2.5 × 4.65 × 0.0018
Result: Weight ≈ 0.33 Carats.
Conclusion: Due to its high SG, this small zircon estimates to weigh about 0.33 carats.
Frequently Asked Questions about Gem Weight
1. Why is calculating gem weight based on dimensions only an estimate?
Calculation is an estimate because the actual volume depends heavily on the gem's exact proportions, cut style, and variations within that style (e.g., belly vs. flat pavilion). The formulas use average correction factors for a given shape, but individual gems will vary.
2. What is Specific Gravity and why is it important for gem weight?
Specific Gravity (SG) is the ratio of a material's density to water's density. It's crucial because weight = volume × density. A gem's material density (represented by SG) directly affects its weight for a given size. Different gem types have different SGs.
3. What units should I use for the dimensions?
Dimensions (Length, Width, Depth) are standardly measured and entered in millimeters (mm). The formulas used by the calculator are based on mm inputs.
4. How does the Shape affect the weight calculation?
Different shapes (like Round, Emerald Cut, Pear) distribute the gem's mass differently within its overall length, width, and depth. Shape correction factors are used to approximate the true volume based on the typical proportions of that cut, which is then used in the weight calculation.
5. Can I use this calculator for any gemstone?
You can use it for any gemstone provided you know its Specific Gravity (SG) and can select an appropriate shape. For gems not listed in the dropdown, select "Enter Specific Gravity (SG) manually" and input the known SG for that specific material.
6. What if my gem's shape isn't listed?
If your gem has a shape not listed, try selecting the closest geometric shape (e.g., Square Cushion might use Cushion or Princess factors as approximations). The estimate will be less accurate. A direct weight measurement is best in such cases.
7. Where can I find the Specific Gravity for a gemstone not in the list?
You can find specific gravity values in gemological textbooks, reputable online gemological resources (like GIA, IGI, Mindat), or by having the gem tested by a gemologist. Be aware that some gems have a range of SG values.
8. What is the 'Depth' measurement?
Depth is the measurement from the very top surface of the gem (the table facet) down to the lowest point (usually the culet). It represents the height of the cut gemstone.
9. Is this tool suitable for appraising a gem?
No. This tool provides an *estimate* for educational or quick reference purposes. Accurate gem valuation and appraisal require physical weighing on calibrated scales, detailed examination by a certified gemologist, and consideration of many factors beyond just weight and basic dimensions.
10. What happens if I enter zero or negative dimensions?
The calculator is designed to require positive dimensions for Length, Width, and Depth. Entering zero or negative values will result in an error message because a gem cannot have zero or negative size.
