How this instrument works
The carat is the standard unit for weighing gemstones, and it has a clean, exact definition: 1 metric carat equals exactly 200 milligrams, or one-fifth of a gram. That definition was formally adopted in 1907 at the Fourth General Conference on Weights and Measures in Paris, replacing a patchwork of slightly different regional carat weights that had been used by gem traders for centuries before international standardization.
Jewelers also divide the carat into 100 'points,' so a stone described as '75 points' weighs three-quarters of a carat, and a 'half carat' stone is 50 points — this points system lets jewelers describe small weight differences (the kind that matter a lot in diamond pricing) without resorting to awkward decimal fractions of a carat. Because the carat-to-gram relationship is exact and fixed by definition, converting between carats, milligrams, and points is pure arithmetic with no rounding or approximation involved.
This is purely a unit-weight converter, distinct from tools that estimate a gemstone's carat weight from its physical dimensions (length, width, depth) — that's a separate, approximate calculation based on the stone's shape and density, whereas this instrument converts a carat figure you already know into other mass units exactly.
- Enter the weight in Carats (ct) — the standard unit gemstones are priced and described in.
- Read Grams and Milligrams for the exact metric-mass equivalent (1 ct = 0.2 g = 200 mg, by definition).
- Read Points for the jewelers' subdivision, where 100 points equal 1 carat.
- Read Ounces for the avoirdupois-ounce equivalent, useful when comparing against everyday non-metric weight references.
- All conversions are exact — there's no rounding built into the carat's own definition, only in how many decimal places you choose to read.
Worked example — converting a 1-carat diamond
A 1-carat diamond weighs exactly 0.2 grams (200 milligrams) by the international carat definition — no rounding, since 1 carat = 200 mg is the definition itself, not an approximation. In jewelers' points, that's exactly 100 points, and in avoirdupois ounces it's 0.2 / 28.349523125 ≈ 0.00705 oz, a useful comparison since most people have a better intuitive sense of ounces than of carats or milligrams.
Scale that same relationship to any other weight: a 2.5-carat stone weighs exactly 0.5 g (500 mg, 250 points), and a 0.5-carat 'half carat' stone — a very common engagement-ring center-stone size — weighs exactly 0.1 g (100 mg, 50 points). The conversion factor never changes regardless of the stone's size.
Questions
Is a carat the same as karat (as in 18-karat gold)?
No, despite the similar spelling and shared word origin, they measure completely different things. Carat (with a C) is a unit of mass used for gemstones, equal to exactly 200 mg. Karat (with a K, in US English) measures gold purity on a 24-point scale, where 24-karat is pure gold and 18-karat gold is 18/24ths (75%) pure gold by mass. This instrument converts the mass unit — carats — not gold purity.
Why do jewelers use 'points' instead of decimal carats?
Points avoid awkward-looking decimals in everyday trade language — saying a diamond is '75 points' or 'three-quarter carat' reads more naturally than '0.75 carats,' even though they're mathematically identical, and the points system directly mirrors how percentages are commonly discussed. Since 100 points equal exactly 1 carat, converting between the two is just a factor of 100, the same relationship as cents to dollars.
Does a heavier carat weight always mean a more valuable diamond?
Not on its own — carat weight is only one of the traditional '4 Cs' of diamond valuation (alongside cut, color, and clarity), and two diamonds of identical carat weight can have very different market values depending on those other three factors. This instrument only converts the weight unit itself; it doesn't estimate a stone's monetary value, which depends on quality grading a scale can't measure.
How is carat weight actually measured for a real gemstone?
Jewelers weigh loose stones on a precision gem scale calibrated in carats (or in grams, then convert), typically accurate to a hundredth of a carat or better, since even small weight differences meaningfully affect a diamond's price per carat. This instrument doesn't weigh anything itself — it converts a carat figure you already have (from a scale, a certificate, or a listing) into other units.
Is this the same as calculating carat weight from a stone's dimensions?
No — that's a different, approximate calculation that estimates weight from a stone's measured length, width, and depth combined with an assumed density and shape-specific formula, and it's meant for situations where you can't weigh the stone directly (like an already-mounted ring). This instrument instead converts a carat weight you already know precisely into other mass units, with no estimation involved.