Gemstone Weight Estimate Calculator
A gemstone's weight in carats can be estimated from its size and what it is made of, without removing it from a setting. The two inputs are the stone's volume and its specific gravity, the density of the material relative to water. This gemstone weight calculator multiplies the volume in cubic centimeters by the specific gravity to get the weight in grams, then divides by 0.2 to convert to carats, since one carat is exactly 0.2 grams. Specific gravity is a fixed property of each gem type: diamond is about 3.52, ruby and sapphire about 4.0, and emerald about 2.72, so identifying the stone fixes this input. Estimating the volume is the harder part and depends on the cut, which is why jewelers use cut-specific formulas based on measured dimensions; here you enter the volume directly. Every figure here is computed deterministically from your inputs, so the same volume and specific gravity always return the same weight. Enter the volume and specific gravity below to estimate a stone's carat weight, compare materials of the same size, or sanity-check a quoted weight, with a worked example that reconciles to the calculator defaults. This is an estimate; an accurate weight comes from weighing a loose stone.
Weight equals volume times specific gravity, and carats are grams divided by 0.2. A 0.05 cu cm stone with a specific gravity of 3.52 weighs about 0.176 g, or 0.880 carats.
Gemstone weight formula
weight (grams) = volume x specific gravity
weight (carats) = weight in grams / 0.2
volume in cubic centimeters; 1 carat = 0.2 grams
Specific gravity is the material density in grams per cubic centimeter. Multiplying by the volume gives grams, and dividing by 0.2 converts to carats, since a carat is defined as exactly one fifth of a gram.
Worked example
Take a stone with an estimated volume of 0.05 cubic centimeters and a specific gravity of 3.52, the value for diamond.
- Weight in grams: 0.05 x 3.52 = 0.176 grams
- Convert to carats: 0.176 / 0.2 = 0.880 carats
The stone weighs about 0.176 grams, or 0.880 carats, which matches the calculator's default inputs exactly. Weigh a loose stone for an exact figure.
Gemstone Weight Estimate Calculator: frequently asked questions
What is specific gravity?
Specific gravity is the density of a material compared with water, numerically equal to its density in grams per cubic centimeter. It is a fixed property of each gem type, so knowing the gem identifies this input: diamond is about 3.52, corundum (ruby and sapphire) about 4.0.
How do I estimate the volume?
For a precise estimate, jewelers use formulas tied to the cut, such as round, oval, or emerald cut, applied to measured length, width, and depth. This calculator takes the volume directly, so you can use a cut formula or a geometric approximation to find it first.
What is a carat?
A carat is a unit of gem weight equal to exactly 0.2 grams, or 200 milligrams. It is divided into 100 points, so a 0.50 carat stone is 50 points. The carat is a weight, not a size, though larger stones of the same material weigh more.
Why is this only an estimate?
The result depends entirely on how accurately you estimate the volume, which is difficult for a faceted, mounted stone. Small errors in measured dimensions translate into noticeable weight differences, so treat the carat figure as an approximation.
How do I get an exact weight?
The only exact method is to weigh a loose stone on a calibrated gem scale that reads in carats or fine grams. Once a stone is set in jewelry, its weight is usually estimated from measurements using cut-specific formulas like this one.
Official sources
- Coin metal content and precious-metal specifications: US Mint. As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.