Carat weight and millimetre diameter are two different ways of describing a stone's size, and they are not interchangeable — they measure different things. Carat measures weight; millimetres measure physical dimensions. For a buyer choosing a moissanite online without seeing it in person, the millimetre measurement is often the more useful number: it tells you exactly how large the stone will appear on the finger. This guide provides a complete MM-to-carat reference for every major cut shape.
Carat vs Millimetre: What Each Measures
- Carat (ct): A unit of weight. 1 carat = 0.2 grams. Two stones of the same carat weight can have different face-up sizes depending on how they are cut — a shallower cut stone spreads its weight wider; a deep cut concentrates weight below the surface.
- Millimetre (mm): A unit of physical dimension. For a round stone, this is the diameter. For fancy shapes, it is typically length × width. The mm measurement tells you exactly how much of the finger the stone will cover.
- Diamond equivalent weight (DEW): Sometimes used in moissanite listings. Moissanite is slightly less dense than diamond, so a moissanite of the same physical dimensions as a 1ct diamond weighs approximately 0.88ct. DEW states what a diamond of the same size would weigh, which is typically the number buyers recognise from diamond comparisons.
Round Brilliant Moissanite: MM to Carat
| Diameter (mm) | Approx. Moissanite Carat | Diamond Equivalent (DEW) |
|---|---|---|
| 4.0mm | 0.21ct | 0.25ct |
| 4.5mm | 0.30ct | 0.35ct |
| 5.0mm | 0.44ct | 0.50ct |
| 5.5mm | 0.57ct | 0.66ct |
| 6.0mm | 0.72ct | 0.84ct |
| 6.5mm | 0.88ct | 1.00ct |
| 7.0mm | 1.07ct | 1.25ct |
| 7.5mm | 1.32ct | 1.50ct |
| 8.0mm | 1.67ct | 2.00ct |
| 8.5mm | 2.00ct | 2.43ct |
| 9.0mm | 2.44ct | 2.75ct |
| 9.5mm | 2.79ct | 3.35ct |
| 10.0mm | 3.28ct | 3.87ct |
Oval Moissanite: Common Sizes
| Dimensions (mm) | Approx. DEW |
|---|---|
| 6 × 4mm | ~0.50ct |
| 7 × 5mm | ~1.00ct |
| 8 × 6mm | ~1.50ct |
| 9 × 7mm | ~2.00ct |
| 10 × 8mm | ~3.00ct |
| 12 × 8mm | ~4.00ct |
Cushion Moissanite: Common Sizes
| Dimensions (mm) | Approx. DEW |
|---|---|
| 5 × 5mm | ~0.75ct |
| 6 × 6mm | ~1.00ct |
| 7 × 7mm | ~1.75ct |
| 8 × 8mm | ~2.50ct |
| 9 × 9mm | ~3.50ct |
Emerald Cut Moissanite: Common Sizes
| Dimensions (mm) | Approx. DEW |
|---|---|
| 6 × 4mm | ~0.50ct |
| 7 × 5mm | ~1.00ct |
| 8 × 6mm | ~1.50ct |
| 9 × 7mm | ~2.50ct |
| 10 × 8mm | ~3.50ct |
How to Use This Chart
- Measure a ring you already own on the same finger, or a coin/object of a known size, to calibrate your sense of how large a given mm measurement is in person.
- Use the mm, not the carat as your primary size reference when buying online. The carat number alone does not tell you how large the stone will appear on your hand.
- Note the DEW stated in listings and confirm whether the product page is listing actual moissanite carat weight or diamond equivalent weight — these can differ by approximately 10–15%.
Related reading:
How to Choose the Right Moissanite Carat Size for Your Hand
What Carat Size Should I Buy?
Moissanite Shapes & Cuts: Find Your Perfect Style