Convert between diamond carats, grams, milligrams, and points. Calculate price per carat and compare diamond sizes with visual scale references.
The carat is the standard unit of weight for diamonds and gemstones, where one carat equals exactly 0.2 grams (200 milligrams). The word "carat" comes from the carob seed, historically used as a balance weight because of its remarkably uniform mass. Today, the metric carat is precisely defined and universally used in the gem trade.
This calculator converts between carats, grams, milligrams, grains, and points (1 point = 0.01 carat). It also estimates price based on per-carat pricing, which is how diamonds are valued in the wholesale and retail markets. Diamond pricing is nonlinear—a 2-carat diamond costs significantly more than twice a 1-carat diamond of equal quality because larger stones are rarer.
The tool includes visual size comparisons showing how diamonds of different carat weights appear when viewed from the top (face-up), which is how they're seen when set in jewelry. Face-up size depends on both weight and cut quality—a well-cut 1-carat diamond appears larger than a poorly cut one of the same weight because less mass is hidden in excessive depth.
Understanding carat weight and its relationship to physical size, visual appearance, and price is essential for anyone buying, selling, or appraising diamonds.
Use this calculator when you want to move cleanly between carats, points, grams, and simple price-per-carat math without juggling separate charts. It is useful for comparison shopping, appraisal notes, and translating seller language into the actual weight units used in the trade. That helps turn a listed stone size into a number you can actually compare.
1 carat = 0.2 grams = 200 milligrams = 100 points = 3.086 grains. Approximate round diamond diameter (mm) = 6.5 × (carats)^(1/3) for well-cut stones. Total Price = carats × price_per_carat.
Result: 0.3g / 300mg / 150 points / $12,000
A 1.50-carat diamond weighs 0.3 grams (300 milligrams, 150 points). At $8,000 per carat, the total price is $12,000. The estimated face-up diameter is approximately 7.4mm.
The carat unit was standardized in 1907 at the Fourth General Conference on Weights and Measures as exactly 200 milligrams. Before standardization, the "carat" varied by region—from about 188mg in Cyprus to 215mg in Livorno. The modern metric carat provides universal consistency for international diamond trading.
Diamond wholesale pricing is based on the Rapaport Price List, a weekly publication that provides benchmark prices per carat based on shape, color, clarity, and weight. Retail prices are typically 20–50% above Rapaport. Prices per carat increase exponentially with weight—a 1-carat diamond may be $5,000/ct while a 3-carat of equal quality might be $15,000/ct.
The visual size of a diamond when set in a ring is its face-up appearance. This depends on the table width and depth ratio. An ideal-cut round brilliant has about 57% table width and 61% total depth. Stones cut too deep hide weight in the pavilion, appearing smaller face-up. Stones cut too shallow may appear larger but suffer from light leakage and reduced brilliance.
Carat (ct) measures gemstone weight. Karat (K or kt) measures gold purity—24K is pure gold, 18K is 75% gold. They are completely different units despite the similar name.
One point equals 1/100 of a carat (0.01 ct). A 50-point diamond is 0.50 carats. Jewelers commonly use points for small stones.
Prices jump significantly at "magic sizes"—0.50, 0.75, 1.00, 1.50, 2.00, 3.00, and 5.00 carats—because of market demand. A 0.99ct diamond costs notably less than 1.00ct.
No. Weight scales with volume (cubic), but visual size scales with area (squared). A 2-carat round diamond is about 8.2mm vs 6.5mm for 1 carat—only 26% wider.
Professional gem scales measure to 0.001 carats (1/10 of a point). Industry standard is to round down to the nearest point if the third decimal is not 9.
TCW is the combined weight of all diamonds in a piece of jewelry. A ring might be described as "1.50 TCW" with a 1.00ct center stone and 0.50ct of accent stones.