💎 Gemstone Carat Weight Calculator
Estimate the carat weight of a cut stone from its measured length, width, and depth and a shape factor — a fast reckoning for a mounted gem you can measure but cannot weigh.
📐 Measure the Stone
How the estimate works
When a gemstone is mounted in a setting you cannot easily place it on a scale, so jewellers estimate its weight from its measured dimensions. Multiply length by width by depth in millimetres, then by a factor that captures the stone’s density and its cut shape, and you get an approximate carat weight — a carat being exactly 0.2 grams.
Use the round-brilliant diamond factor of about 0.0061 as a default, or the published factor for your stone’s shape and material. Treat the result as a planning estimate for cataloguing or insurance; for a precise figure, weigh the loose stone or have it laboratory-graded.
❓ Frequently Asked Questions
How is carat weight estimated from dimensions?
The calculator multiplies the stone's length × width × depth (in millimetres) by a factor that bundles the material's specific gravity with a cut-shape constant. For a round brilliant diamond the factor is about 0.0061; other shapes and gem materials use their own published factors. The result approximates the weight in carats.
What is a carat?
A carat (metric) is exactly 0.2 grams, or 200 milligrams — the standard unit of weight for gemstones. It is unrelated to the karat that measures gold purity, despite the similar name. Carats are further divided into 100 points, so a 0.25-carat stone is a 'twenty-five pointer.'
Why estimate instead of just weighing the stone?
Because you often can't get a stone onto a scale — it may be mounted in a ring, brooch, or historical setting you don't want to disturb. Estimating from measured dimensions lets you approximate the carat weight without unmounting the piece, which is useful for cataloguing and insurance ballparks.
How accurate is the estimate?
It's an approximation. The real weight depends on the exact cut, girdle thickness, bulge, and the specific material's density, so the figure can be off by several percent. For a precise carat weight, have the stone weighed on a calibrated gem scale or graded by a laboratory.