Carats to Grams Converter — Convert ct to g (Exact: g = ct × 0.2)
Accurate carats (ct) to grams (g) converter using the exact metric carat definition 1 ct = 0.2 g (200 mg). Ideal for jewelry, gemstones, assay reports, auctions, and lab documentation. Includes formula, step-by-step examples, precision/rounding guidance, expanded quick tables, and in-depth FAQs.
Exact factor: 1 ct = 0.2 g (200 mg) ⇒ g = ct × 0.2. See all free weight converters.
About Carats to Grams Conversion
The carat (ct) is the standard unit of mass for gemstones. The modern metric carat is defined exactly as 200 milligrams, i.e., 0.2 grams. Converting carats to grams links jewelry listings, appraisals, and lab certificates to broader SI workflows—inventory, shipping, customs, and analytics.
Because 1 ct = 0.2 g is exact, you can maintain a single canonical store in grams or milligrams and compute carats for UI and certificates. This avoids round-trip drift between storefronts, invoices, and exports. For multi-region stacks, keep SI canonical (g/mg) and generate ct, points, and imperial units only at the edges.
Remember: carat is mass—not size. Stone dimensions (mm) depend on density and cut. For mounts and designs, always confirm calibrated sizes even when carat weight is known.
Carats to Grams Formula
Exact relationship
Use either expression:
g = ct × 0.2
// reverse
ct = g ÷ 0.2Example:
1.58 ct × 0.2 = 0.316 gRelated Weight Converters
What is a Carat (ct)?
A carat is a gemstone mass unit equal to 0.2 g. In trade, it is subdivided into points (100 points = 1 ct). Certifications, auction catalogs, and retail listings use carats and points because they are familiar to buyers and align with gemological standards.
What is a Gram (g)?
The gram is an SI unit of mass widely used in shipping, customs, lab documentation, and manufacturing. Storing mass in grams (or milligrams) provides clean rollups to kilograms and straightforward conversion to carats for jewelry-facing surfaces.
Step-by-Step: Converting ct to g
- Read the gemstone mass in carats (ct).
- Multiply by 0.2 to convert to grams (g).
- Apply your rounding policy once at presentation (e.g., g to 3–4 d.p., ct to 2–3 d.p.).
Example walkthrough:
Input: 2.75 ct
Compute: 2.75 × 0.2 = 0.55 g
Output: 0.550 g (UI, 3-decimal rounding)Common Conversions
Everyday quick checks (ct → g)
| ct | g | ct | g |
|---|---|---|---|
| 0.10 | 0.02000 | 0.25 | 0.05000 |
| 0.33 | 0.06600 | 0.50 | 0.10000 |
| 0.75 | 0.15000 | 1.00 | 0.20000 |
| 1.25 | 0.25000 | 1.50 | 0.30000 |
| 2.00 | 0.40000 | 2.50 | 0.50000 |
| 3.00 | 0.60000 | 5.00 | 1.00000 |
| 7.50 | 1.50000 | 10.00 | 2.00000 |
| 20.00 | 4.00000 | 50.00 | 10.00000 |
| 100.00 | 20.00000 | 250.00 | 50.00000 |
Precision, Rounding & Significant Figures
Operational rounding
Retail: 0.01 ct and 3 d.p. in grams are common. Lab/certificates: 0.001 ct and 3–4 d.p. in grams. Always store raw values with full precision and round once at the UI/export boundary to maintain auditability across invoices, certificates, and dashboards.
Consistent documentation
Name fields clearly (e.g., mass_ct, mass_g, mass_mg) and add a methods note: “Conversion uses 1 ct = 0.2 g (exact); g = ct × 0.2; ct = g ÷ 0.2. UI shows ct to 2–3 decimals; g to 3–4 decimals.”
Where This Converter Is Used
- 💎 Jewelry retail & auctions: Listing stones in ct while warehousing and shipping in g.
- 🧪 Gem labs & appraisals: Certificates reporting ct with gram/mg precision internally.
- 🏭 Manufacturing: Casting, settings, and QC workflows that reconcile gem ct to SI units.
- 📦 Logistics & customs: Declarations and harmonized codes that prefer SI mass (g/kg).
- 📊 Analytics: Price-per-carat datasets normalized in grams for cross-category analysis.
Quick Reference Table
Common gram values (g → ct)
| g | ct | g | ct |
|---|---|---|---|
| 0.020 | 0.10 | 0.050 | 0.25 |
| 0.066 | 0.33 | 0.100 | 0.50 |
| 0.150 | 0.75 | 0.200 | 1.00 |
| 0.250 | 1.25 | 0.300 | 1.50 |
| 0.400 | 2.00 | 0.500 | 2.50 |
| 0.600 | 3.00 | 1.000 | 5.00 |
| 1.500 | 7.50 | 2.000 | 10.00 |
| 4.000 | 20.00 | 10.000 | 50.00 |
| 20.000 | 100.00 | 50.000 | 250.00 |
Frequently Asked Questions
What is the exact formula to convert carats to grams?
Use the metric carat definition adopted internationally: 1 carat (ct) = 200 milligrams = 0.2 grams. The conversion is exact. Multiply carats by 0.2 to get grams: g = ct × 0.2. Example: 1.58 ct × 0.2 = 0.316 g. Because the constant is exact, you can standardize rounding for retail (e.g., 0.01 ct) while keeping full precision internally for lab notes and invoices. Document the constant and rounding so exports, PDFs, and UI are consistent.
Is carat (ct) the same as karat (kt)?
No—these are different. Carat (ct) is a unit of MASS for gemstones (1 ct = 0.2 g). Karat (kt or K) is a measure of gold PURITY (24 kt = pure gold). Don’t mix them: carat affects weight-based pricing of gems, while karat affects alloy composition of gold jewelry. Keep the symbols distinct in your UI and data fields to avoid costly misinterpretations.
Do carats measure size or weight? Why do two 1 ct stones look different?
Carats measure mass, not size. Apparent size depends on gemstone density (specific gravity) and cut proportions. Two stones of the same carat can have different dimensions if they’re different materials (e.g., diamond vs. sapphire) or are cut differently. Use carats for weight and certification; use calibrated dimensions (mm) for mounting and design.
What precision should I use—retail vs. laboratory?
Retail listings commonly show carat to two decimals (e.g., 1.23 ct). Appraisals, certificates, and labs often use three decimals (0.001 ct). For grams, 3–4 decimal places are typical for lab reports; 2–3 may be enough for storefronts. Best practice: store raw mass with maximum fidelity (e.g., grams with at least 0.0001 g), round once at the output boundary, and publish your rounding policy.
Give a worked example with clear rounding.
Suppose a stone weighs 0.73 ct. Convert to grams: 0.73 × 0.2 = 0.146 g exactly. If your UI displays grams to three decimals, show 0.146 g. If you also show carats to two decimals, present 0.73 ct. Keep the exact internal value and round only once when rendering to avoid tiny mismatches between labels, receipts, and CSV exports.
How does carat relate to milligrams and points?
1 ct = 200 mg exactly. Jewelers also use “points,” where 1 ct = 100 points. That means 0.01 ct = 1 point, 0.25 ct = 25 points, 1.50 ct = 150 points, etc. In data models, keep grams or milligrams canonical, and derive carats and points for UI. This simplifies analytics and avoids float rounding issues.
Any QA tips to prevent rounding drift and audit issues?
Centralize conversions, write unit tests for known pairs (e.g., 1 ct = 0.2 g; 2.5 ct = 0.5 g; 5 ct = 1.0 g), and ensure rounding occurs only once at display/export time. Include unit symbols in every table and document a short methods note: “Exact: 1 ct = 0.2 g (200 mg); g = ct × 0.2; ct = g ÷ 0.2.”
Tips for Working with Carats & Grams
- Keep SI canonical (g/mg); compute ct and points at the edge for storefronts and certificates.
- Publish a single rounding policy (e.g., ct: 0.01–0.001; g: 3–4 d.p.) and use it consistently.
- Don’t confuse carat (mass) with karat (purity); enforce distinct field names and symbols.