Gemstone Carat Weight Calculator

Gemstone Carat Weight Calculator

Diamond Ruby / Sapphire (Corundum) Emerald Amethyst / Citrine (Quartz) Topaz Aquamarine Tanzanite Garnet (Almandine) Fluorite Peridot Tourmaline Zircon Generic (SG 1.0)

Round Brilliant Oval Emerald / Rectangular Step Cut Princess / Square Pear Shape Marquise Heart Cushion (Standard)

(Add % for thick girdles)

Estimated Weight

Grams: g

function calculateGemWeight() { var length = parseFloat(document.getElementById('gemLength').value); var width = parseFloat(document.getElementById('gemWidth').value); var depth = parseFloat(document.getElementById('gemDepth').value); var specificGravity = parseFloat(document.getElementById('gemType').value); var shapeMultiplier = parseFloat(document.getElementById('gemShape').value); var adjustment = parseFloat(document.getElementById('weightAdjustment').value) || 0; if (isNaN(length) || isNaN(width) || isNaN(depth) || length <= 0 || width <= 0 || depth <= 0) { alert("Please enter valid positive dimensions for the gemstone."); return; } // Standard Estimation Formula: L x W x D x SG x Shape Constant var baseWeight = length * width * depth * specificGravity * shapeMultiplier; // Apply percentage adjustment for girdle thickness/bulge var finalWeight = baseWeight * (1 + (adjustment / 100)); // Conversion: 1 carat = 0.2 grams var gramWeight = finalWeight * 0.2; document.getElementById('caratOutput').innerHTML = finalWeight.toFixed(3) + " ct"; document.getElementById('gramOutput').innerHTML = gramWeight.toFixed(4); document.getElementById('resultArea').style.display = "block"; }

How to Calculate Gemstone Carat Weight

Estimating the weight of a mounted gemstone requires precise measurements and an understanding of the stone's physical properties. Since you cannot place a set stone on a scale, gemologists use a mathematical formula based on dimensions (length, width, depth), shape, and Specific Gravity (SG).

The Carat Weight Formula

Weight (ct) = Length × Width × Depth × Specific Gravity × Shape Multiplier

Why Specific Gravity Matters

Not all stones are created equal. A 1-carat diamond is physically smaller than a 1-carat emerald because diamonds are denser (higher specific gravity). If you have a round sapphire and a round diamond with the exact same dimensions, the sapphire will weigh significantly more because it has an SG of 4.00 compared to the diamond's 3.52.

Gemstone	Specific Gravity (SG)
Diamond	3.52
Ruby/Sapphire	4.00
Emerald	2.72
Topaz	3.53
Amethyst	2.65

Practical Example

Suppose you have a Round Brilliant Diamond with the following measurements:

• Length: 6.50 mm
• Width: 6.50 mm
• Depth: 4.00 mm

Using the formula: 6.50 × 6.50 × 4.00 × 3.52 × 0.0018 = 1.07 carats. This provides a highly accurate estimate for stones that cannot be weighed directly on a scale.

Factors Affecting Accuracy

• Girdle Thickness: A thick or "bulky" girdle adds weight that isn't captured by basic L/W/D measurements. Use the adjustment field to add 2-5% for thick girdles.
• Pavilion Bulge: Especially in colored stones like rubies, a "belly" or bulge on the pavilion increases the weight significantly.
• Measurement Precision: Use digital calipers for the most accurate results. Even a 0.1mm error can result in a significant carat difference.