Carat calculator
The carat is a unit of weight, not size. One carat is 0.2 grams, and a one-carat diamond looks noticeably different from a one-carat sapphire, ruby, emerald, or pearl because each of those stones has a different density.
That is why a buyer who knows only a center stone's millimeter measurements cannot directly compare it against a quoted carat weight without a calculation step. The Gemstone Rush carat calculator will handle that step and will also estimate carat weight from cut, depth, and stone family.
Until the widget ships, this page explains what it will do and links to the stone pages where densities and typical sizes are already listed.
What the calculator will do
The carat calculator takes two common inputs and returns the third. A reader can input stone dimensions (length and width in millimeters, plus depth for round or oval cuts) and select a stone family, and the calculator will return an estimated carat weight.
Alternatively, a reader can input a target carat weight and a stone family, and the calculator will return the typical finished dimensions. Because density varies between gem families, the tool will display the difference: a one-carat diamond is about 6.5 millimeters round.
A one-carat ruby in the same cut is closer to 6 millimeters because ruby is denser.
Those numbers matter for anyone shopping a center stone with a specific size in mind.
The user problem it solves
Shoppers comparing listings between retailers frequently see the same stone described by carat on one site and by millimeter dimensions on another. Without a calculator, the reader cannot tell whether a 2-carat sapphire and a 2-carat tanzanite will look similar on the hand.
They will not: tanzanite is less dense and the same weight produces a physically larger stone. The calculator closes that gap with a single field entry per stone.
The questions the calculator will ask
- Stone family. A dropdown covering the major gem families (diamond, corundum including ruby and sapphire, beryl including emerald and aquamarine, topaz, tourmaline, quartz family, spinel, tanzanite, opal, pearl, and others).
- Cut. Round, oval, cushion, princess, emerald, pear, marquise, heart, radiant, asscher, or cabochon.
- Known dimensions, if dimensions-to-carat. Length, width, depth in millimeters.
- Known weight, if carat-to-dimensions. Carat weight in decimal form.
- Optional: depth percentage, for precision estimates in round and oval cuts.
Why density matters
The shortlist below shows typical specific gravity values for the stones most readers are calculating between. Higher density means a smaller stone at the same carat weight. It is a short list worth keeping in mind.
The stones the calculator will cover
Stones supported in the calculator
- All formulas used by the calculator will be estimates, not substitutes for a laboratory weight. Actual carat weight depends on cut precision, inclusions, and the individual stone's specific gravity, which varies slightly within each family. Use the calculator for shopping comparison, not for resale valuation.
In the meantime: check each stone page
Every stone page on Gemstone Rush lists the stone's specific gravity, typical carat-to-millimeter relationships, and the price ranges buyers encounter at common sizes. For a shopper who wants to verify a specific listing today, those stone pages carry the data the calculator will eventually automate.