Carat Weight Calculator — Gemstone Weight Converter
Convert a gemstone's carat weight to grams, milligrams, troy ounces and grains, and multiply by a price per carat to estimate total value. One metric carat equals exactly 0.2 grams.
ct
1 carat = 0.2 g exactly (metric carat, international standard)
- 1
Weight in milligrams
1 × 200 = 2001 metric carat = 200 mg exactly (international standard since 1907). - 2
Weight in grams
200 ÷ 1000 = 0.2000
How does this calculator work?
One metric carat = 0.2 g = 200 mg exactly (international standard since 1907). To get grams: multiply carats by 0.2. To estimate value: multiply carats by price per carat — but note that real gemstone prices per carat are non-linear and depend on cut, colour and clarity, not just weight.
Formula
How this is calculated
The metric carat (abbreviated ct) has been an internationally agreed unit since 1907, defined as exactly 0.2 grams or 200 milligrams. It is used worldwide for gemstones — diamonds, rubies, sapphires, emeralds and others — and is distinct from the karat (k) used to measure the purity of gold alloys.
Converting to other mass units is a simple multiplication. One troy ounce equals 31.1035 grams, so 1 carat equals 0.2 / 31.1035 ≈ 0.006430 troy ounces. One avoirdupois grain equals 64.79891 milligrams, so 1 carat (= 200 mg) equals 200 / 64.79891 ≈ 3.0865 grains. These are exact physical unit definitions, not estimates.
Value estimation multiplies the carat weight by a price per carat. Gemstone pricing per carat is highly non-linear in practice — a 2 ct diamond does not cost twice the price of a 1 ct diamond of the same quality; it typically costs far more per carat because large gem-quality stones are rarer. The value here is a simple linear estimate useful for rough budgeting. Real appraisal accounts for the 4 Cs (cut, colour, clarity, carat) and current market conditions.
Frequently asked questions
No — they are completely different units. A carat (ct) for gemstones is a unit of weight equal to 0.2 grams. A karat (k) for gold is a measure of purity: 24k is pure gold, 18k is 75% gold. The similar names come from a shared historical root (the carob seed), but the units are unrelated.
Large gem-quality diamonds are exponentially rarer than small ones. Price per carat increases steeply at key size thresholds (0.5 ct, 1 ct, 2 ct, etc.) because the supply of large stones is so limited. This calculator uses a flat price per carat for simplicity — for accurate valuation, consult a certified gemologist.
This calculator uses the metric carat for gemstone weight. Gold is typically measured in troy ounces or grams, not carats. If you need to convert gold purity (karats), use a gold karat calculator instead.
Also known as
TG we-Calculate Editorial Team. (2026). Carat Weight Calculator — Gemstone Weight Converter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/carat-weight-calculator
TG we-Calculate Editorial Team. "Carat Weight Calculator — Gemstone Weight Converter." TG we-Calculate. 2026. https://we-calculate.com/calculator/carat-weight-calculator.
TG we-Calculate Editorial Team, "Carat Weight Calculator — Gemstone Weight Converter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/carat-weight-calculator
@misc{wecalculate_carat_weight_calculator, title = {Carat Weight Calculator — Gemstone Weight Converter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/carat-weight-calculator}}, year = {2026}, note = {TG we-Calculate} }
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