Gemstone Size to Carat Calculator – From Millimeters to Carats
The Gemstone Size to Carat Calculator on MyTimeCalculator converts gemstone dimensions in millimeters into an estimated carat weight. Instead of relying only on lookup tables, this tool uses shape-based volume formulas and gemstone specific gravity values to approximate how much a stone should weigh.
This is useful when you know the size of a stone you want to order or design around and need a reasonable estimate of the carat weight, or when you are comparing how different shapes and gemstones behave at similar millimeter measurements.
The Core Volume to Carat Formula
The calculation works in two stages. First, the tool approximates the gemstone’s volume using a formula that matches its shape. Second, it converts that volume into weight using specific gravity and the relationship between grams and carats.
Volume is computed in cubic millimeters. To convert that to carats, the calculator uses the following sequence:
- Convert mm³ to cm³: 1 cm = 10 mm, so 1 cm³ = 1000 mm³. This means Volume_cm³ = Volume_mm³ ÷ 1000.
- Convert volume to grams using specific gravity SG: Weight_g = Volume_cm³ × SG.
- Convert grams to carats: 1 carat = 0.2 g, so Carat Weight = Weight_g ÷ 0.2 = Weight_g × 5.
Combining these steps gives a compact formula in terms of cubic millimeters:
This is the main formula used by the calculator after the geometric volume has been estimated from the shape and dimensions.
Shape-Based Volume Formulas
Different gem cuts approximate different three-dimensional forms. The calculator uses common gemological volume approximations for the main commercial shapes. Let L be length, W be width, D be depth or height and Dm be diameter, all measured in millimeters.
Round Brilliant
For round stones, the model is a cylindrical-type volume based on diameter and depth:
Once volume is found, the carat weight uses the main formula:
Oval
For oval stones, the face outline is treated as an ellipse, and the stone is given depth D:
The calculator then applies Carat = Volume_oval × SG × 0.005.
Pear and Marquise
The pear and marquise shapes use related formulas with proportional factors to reflect their outlines.
Volume_marquise ≈ (π × L × W × D) ÷ 4
These factors simulate the fact that a pear is roughly half to two-thirds of an ellipse and a marquise occupies more of the elongated outline.
Emerald, Cushion, Princess, Radiant and Asscher
Rectangular and square faceted cuts are often modeled as length × width × depth multiplied by a proportion factor to account for cut corners, steps and pavilion structure.
Volume_cushion ≈ L × W × D × 0.85
Volume_princess ≈ L × W × D × 0.75
Volume_radiant ≈ L × W × D × 0.80
Volume_asscher ≈ L × W × D × 0.83
The different constants represent average proportion differences between these cuts.
Heart and Trillion
Heart and triangular stones are more complex, so the calculator uses standard approximations.
Volume_trillion ≈ 0.34 × L × W × D
The extra factor 1.12 for hearts models the lobes at the top of the stone, and the 0.34 factor for trillions reflects the tapered triangular shape.
Cabochon
Round cabochons can be modeled as a spherical cap or domed cylinder using diameter and height D:
This volume is then converted to carats using the same Carat = Volume_mm³ × SG × 0.005 formula.
Specific Gravity Values For Common Gemstones
Specific gravity is a measure of density relative to water. Gemstones with higher SG weigh more than gems with lower SG at the same size. The calculator provides typical SG values, such as:
- Diamond: about 3.52
- Ruby and sapphire: about 4.00 to 4.02
- Emerald: about 2.70 to 2.80
- Aquamarine: about 2.68
- Amethyst and quartz: about 2.65
- Garnet: often around 3.8 to 4.2 depending on variety
- Spinel: around 3.60
- Tourmaline: about 3.06
- Topaz: around 3.53
- Zircon: typically about 4.65
- Peridot: about 3.34
- Tanzanite: about 3.35
- Morganite: around 2.85 to 3.10
- Opal: generally around 2.10 to 2.20
You can choose a preset gemstone type and then fine-tune the SG field if your stone’s lab data suggests a slightly different value.
Worked Example: Round Diamond From Millimeter Size
Consider a round diamond with diameter Dm = 6.5 mm and depth D = 4.0 mm, using SG = 3.52.
- Step 1: Estimate volume in mm³
Compute the radius 6.5 ÷ 2 = 3.25 mm, square it and multiply by π and depth to get an approximate volume.
- Step 2: Convert volume to carats
The calculator performs both steps instantly and shows the estimated carat weight along with the equivalent weight in grams.
How To Use The Gemstone Size to Carat Calculator
- Select the gemstone type so that a typical specific gravity can be applied. Adjust SG if needed.
- Choose the shape of the stone from the list, such as round, oval, cushion, princess or cabochon.
- Measure the stone’s dimensions in millimeters and enter diameter or length, width and depth as appropriate for the chosen shape.
- Set the number of decimal places you would like in the output.
- Click the convert button to see estimated carat weight, weight in grams, volume and a summary of the inputs and formula components.
Limitations And Best Practices
The formulas used are approximations based on typical gemstone proportions. Real stones can vary due to different pavilion depths, crown heights, girdle thicknesses and cutting styles. For that reason, this calculator is best used for planning, education and comparison rather than as a replacement for actual scale measurements.
Whenever you are working with high-value stones, certification, insurance or precise inventory, always confirm weights with a calibrated scale. The calculator is designed to show how shape and density interact with size to influence carat weight, giving you a better intuitive understanding of gemstone geometry.
Gemstone Size to Carat FAQs
Frequently Asked Questions About mm to Carat Conversion
Use these quick answers to understand how gemstone dimensions, shape and specific gravity affect estimated carat weight.
Shape determines how volume grows as you change length, width and depth, while specific gravity measures how heavy the material is compared with water. Two stones of the same size but different shape and SG can differ noticeably in carat weight, so both factors are essential for realistic estimates.
Yes. You can enter the target millimeter dimensions of a planned stone and select the intended shape and gemstone type. The calculator shows an estimated carat weight, which is helpful when designing custom pieces or planning how much rough material you might need.
A stone can weigh more when it has extra depth, a very thick girdle, unusual cutting proportions or a slightly higher specific gravity than the typical value used. In these cases, you can adjust the depth measurement or SG field to better match the actual stone profile and material.
Yes. You can keep the same millimeter dimensions and shape and simply change the gemstone type and corresponding SG. This shows how a denser gem such as zircon or sapphire will weigh more than a less dense gem such as opal or quartz at the same size and cut style.
This calculator goes from size to carats, but you can reverse the process by trial and error. Adjust the dimensions until the estimated carat weight matches your target. This is often fast enough for design purposes, especially with round and oval shapes.
These formulas assume typical commercial proportions, including average girdle thickness. Extremely thick or extremely thin girdles are not modeled explicitly. If you know your stone is unusually deep or shallow, adjust the depth dimension so the approximated volume better reflects reality.
Yes. Lab-grown stones usually have the same specific gravity as their natural counterparts. You can choose the appropriate gemstone type and SG to estimate carat weight in exactly the same way as you would for natural material.
The cabochon model is most accurate for round domes, but it can still give a useful approximation for other domed shapes if you use an average diameter or a representative dimension. For very irregular cabochons, treat the result as a rough guide rather than a precise prediction.
Simple tables usually give a single carat value for a given millimeter size and shape, often assuming one gemstone type such as diamond. This calculator is more flexible because it uses formulas and specific gravity, so you can change both dimensions and material while still using the same underlying logic.
You can explore tools such as Ruby Price Calculator, Paraiba Tourmaline Price Calculator, Spinel Price Calculator and Opal Price Calculator. Together with this size to carat converter, they help you understand gemstone value from both geometric and market perspectives.