💎 Gemstone Weight Calculator
Estimate a stone's carat weight from its measured length, width, and depth, its gem type (specific gravity), and its cut — no need to unmount or weigh it.
📐 Measure, Choose the Gem, Estimate the Carats
What is a Gemstone Weight Calculator?
It estimates how many carats a stone weighs from its dimensions, without taking it out of the setting. Enter the length, width, and depth, pick the gem (which sets its specific gravity) and the cut (which sets a shape factor), and it gives the estimated carats and the mass in grams.
Use it to sense-check a listing, note down an heirloom you can't unmount, or compare stones by size. It's an approximation shaped by the exact proportions and girdle, not an appraisal and not an identification: for a precise weight, weigh a loose stone on a carat scale, and for what the stone is, ask a qualified gemologist.
❓ Frequently Asked Questions
How is carat weight estimated from measurements?
It uses the International Gem Society's faceted-gem formulas: length × width × depth (in mm) × specific gravity × a shape factor, which gives carats directly. For a round stone the factor is 0.0018, so a 6.5 mm round diamond 4.0 mm deep estimates at 6.5 × 6.5 × 4.0 × 3.52 × 0.0018 = 1.07 ct. Mass in grams is carats × 0.2, since one carat equals 0.2 grams.
What is specific gravity and why does it matter?
Specific gravity is a gem's density — how much a given volume weighs. Diamond is about 3.52, ruby and sapphire about 4.00, and emerald about 2.72. A denser stone of the same size weighs more, so using the right value (or your own measured one) is key to an accurate carat estimate.
Why does the shape or cut change the estimate?
A cut stone never fills its bounding box: a round brilliant fills far less of it than a step-cut rectangle, so each shape has its own factor (round 0.0018, oval 0.0020, square cushion 0.0018, rectangular cushion 0.0022, rectangular step cut 0.0025, rectangle 0.0026). Choosing the correct shape keeps the estimate from over- or under-stating the weight.
How accurate is this compared with weighing the stone?
It's an approximation. The formulas assume a well-proportioned stone with a medium or thin girdle; the International Gem Society adds 1% to 10% for a thicker girdle, and a deep or shallow cut moves the weight too. It cannot identify the stone or say whether it is natural, lab-grown or a simulant; that takes a qualified gemologist. For an exact figure, weigh a loose stone on a carat scale.