Carat to Millimetre Converter
Instantly convert diamond and gemstone carat weight to millimetre dimensions
Diamond Carat to MM Converter
Conversion Result
Popular Carat to MM Conversions
Round Brilliant Cut Diamonds
| Carat Weight | Diameter (mm) | Visual Size |
|---|---|---|
| 0.25 ct | 4.0 mm | Small, delicate appearance |
| 0.50 ct | 5.1 mm | Noticeable presence |
| 0.75 ct | 5.9 mm | Elegant size |
| 1.0 ct | 6.4 mm | Classic engagement ring size |
| 1.25 ct | 6.9 mm | Substantial appearance |
| 1.5 ct | 7.4 mm | Notable presence |
| 2.0 ct | 8.1 mm | Statement piece |
| 2.5 ct | 8.8 mm | Luxury size |
| 3.0 ct | 9.2 mm | Exceptional size |
Princess Cut Diamonds
| Carat Weight | Length × Width (mm) |
|---|---|
| 0.25 ct | 3.5 × 3.5 mm |
| 0.50 ct | 4.4 mm |
| 0.75 ct | 5.0 mm |
| 1.0 ct | 5.5 mm |
| 1.5 ct | 6.4 mm |
| 2.0 ct | 7.0 mm |
Oval Cut Diamonds
| Carat Weight | Length × Width (mm) |
|---|---|
| 0.25 ct | 5.0 × 3.0 mm |
| 0.50 ct | 6.0 × 4.5 mm |
| 0.75 ct | 7.0 × 5.0 mm |
| 1.0 ct | 7.5 × 5.5 mm |
| 1.5 ct | 8.5 × 6.5 mm |
| 2.0 ct | 9.0 × 6.0 mm |
Conversion Formula & Method
Core Principle
One carat equals 200 milligrammes (0.2 grammes). However, converting carat weight to millimetre dimensions isn’t a simple mathematical formula because it depends on the diamond’s density, cut, and proportions.
The relationship between carat weight and millimetre size varies significantly based on several factors. For round brilliant cut diamonds, jewellers use established conversion tables based on ideal cut proportions. A well-cut 1.0 carat round diamond typically measures approximately 6.4 mm in diameter, whilst a poorly cut stone of the same weight might measure differently due to depth variations.
Step-by-Step Conversion Process
Step 1: Identify Shape
Determine the diamond shape (round, princess, oval, etc.) as each shape has different carat-to-mm ratios.
Step 2: Reference Chart
Consult the appropriate conversion chart for your specific diamond shape and cut style.
Step 3: Consider Cut Quality
Account for cut quality—ideal cuts provide the most accurate conversions, whilst shallow or deep cuts may vary.
Step 4: Verify Proportions
Check the diamond’s depth percentage and table percentage to confirm it matches standard proportions.
Important Note
Carat weight measures mass, not size. Two diamonds of identical carat weight can appear different sizes based on their cut proportions. A shallow cut spreads weight across a larger surface area, whilst a deep cut concentrates weight in the pavilion, making it appear smaller from the top view.
Quick Reference: Common Sizes
Shape Comparison Guide
Different diamond shapes distribute weight differently, which affects how large they appear relative to their carat weight. Elongated shapes such as marquise, oval, and pear cuts often appear larger than round diamonds of the same carat weight because they spread surface area across a longer dimension.
Surface Area by Shape
| Diamond Shape | 1 Carat Dimensions | Perceived Size |
|---|---|---|
| Round Brilliant | 6.4 mm diameter | Standard reference |
| Princess | 5.5 × 5.5 mm | Appears slightly smaller |
| Oval | 7.5 × 5.5 mm | Appears 10-15% larger |
| Marquise | 9.0 × 6.0 mm | Appears 15-20% larger |
| Pear | 8.0 × 5.0 mm | Appears 10-15% larger |
| Emerald | 7.0 × 5.0 mm | Appears slightly larger |
Gemstone Variations
Whilst diamond conversions are standardised, coloured gemstones have different densities, which significantly affects their carat-to-millimetre ratios. A 1.0 carat sapphire will measure differently than a 1.0 carat diamond, even in the same shape.
Density Comparison
| Gemstone | Specific Gravity | 1 ct Round Size |
|---|---|---|
| Diamond | 3.52 | 6.4 mm |
| Ruby | 4.00 | 6.0 mm |
| Sapphire | 4.00 | 6.0 mm |
| Emerald | 2.72 | 6.5 mm |
| Aquamarine | 2.69 | 6.5 mm |
| Amethyst | 2.65 | 6.5 mm |
Gemstones with higher specific gravity (like ruby and sapphire) will measure smaller than diamonds of the same carat weight, whilst lower-density gemstones (like emerald and aquamarine) will measure larger.
Frequently Asked Questions
How many millimetres is a 1 carat diamond?
A well-cut 1 carat round brilliant diamond typically measures approximately 6.4 mm in diameter. However, this can vary slightly based on the diamond’s cut proportions and depth. Princess cut 1 carat diamonds measure about 5.5 mm square, whilst oval 1 carat diamonds measure approximately 7.5 × 5.5 mm.
Why do two diamonds of the same carat weight look different sizes?
Cut proportions significantly affect a diamond’s face-up appearance. A shallow cut diamond spreads its weight across a larger diameter, making it appear bigger, whilst a deep cut diamond carries more weight in the pavilion (bottom portion), making it look smaller from above. Additionally, different shapes distribute weight differently.
Does carat weight equal size?
No, carat measures weight (mass), not physical dimensions. Carat weight indicates how much a diamond weighs (1 carat = 200 milligrammes), whilst millimetres measure the actual physical size. The relationship between weight and size depends on the diamond’s density, cut, and shape.
Which diamond shape looks largest for its carat weight?
Elongated shapes like marquise, oval, and pear typically appear largest for their carat weight because they maximise surface area. A marquise cut can appear 15-20% larger than a round brilliant of the same carat weight, making these shapes popular choices for those seeking maximum visual impact.
Are carat to mm conversions exact?
No, these conversions are approximations based on ideal cut proportions. Individual diamonds may vary slightly due to differences in depth percentage, table size, and overall cut quality. Conversion charts provide estimates for well-proportioned stones within standard parameters.
How do I measure a diamond’s size at home?
Whilst carat weight requires precision scales, you can estimate a diamond’s millimetre size with a calliper or a printable diamond size chart. However, for accurate measurements, consult a professional jeweller with proper gemological equipment. Visual estimation from above provides the face-up dimensions.
Do coloured gemstones convert the same as diamonds?
No, different gemstones have varying densities (specific gravity). For example, a 1 carat sapphire measures approximately 6.0 mm (smaller than diamond), whilst a 1 carat emerald measures about 6.5 mm (larger than diamond). Always use gemstone-specific conversion charts for accurate estimates.
What’s the difference between carat and karat?
Carat (ct) measures gemstone weight, whilst karat (kt or K) measures gold purity. A 1 carat diamond weighs 0.2 grammes, whereas 24 karat gold means 100% pure gold. These are entirely different measurement systems despite the similar spelling.