Moissanite Carat vs Millimetre: The Size Guide That Matters

Carat is a weight, not a size. That sounds pedantic until you open two listings for a "1ct moissanite" ring and find that one stone is 6.5mm across and the other is 7mm. They are both honestly described, they are both about one carat, and they look different on the hand.

Why carat is a slippery number for moissanite

Moissanite has a specific gravity of about 3.21, diamond about 3.52. A cubic millimetre of moissanite simply weighs less. Worse, moissanite is sold in "diamond equivalent" carats — a round brilliant moissanite that measures the same across the table as a 1ct diamond is called a 1ct moissanite, even though it weighs nearer 0.9ct on a scale.

So the carat figure on a moissanite listing is a size proxy borrowed from diamond, then rounded. Millimetres are not rounded. Millimetres are the measurement.

The conversion table you actually need

These numbers are for a round brilliant cut, measured across the girdle at the widest point. Fancy shapes do not follow this table.

Round numbers are approximate by nature. Expect half a tenth of a millimetre of variation either way on a production stone, and treat any listing that quotes 6.50mm exactly as aspirational.

Fancy shapes break the table

An oval, pear or marquise spreads weight lengthways. A 1ct-equivalent oval typically measures about 7.7mm by 5.7mm, which covers a longer strip of finger than a 6.5mm round and reads bigger than its weight. A cushion is the opposite: a 6.5mm cushion looks smaller than a 6.5mm round because the corners eat into the visible spread.

Emerald and radiant cuts sit somewhere between. For a step-cut emerald, judge the length-to-width ratio rather than the millimetres. Ratios from 1.40 to 1.50 give the classic emerald look; below 1.35 the stone reads square and loses the shape's whole point.

For a band, this is a practical question rather than an aesthetic one. A 9mm stone on a UK size J finger will overhang the sides of the finger on most hands. Past about 8.5mm, finger coverage becomes a conversation with whoever is going to wear it.

Why the depth matters as much as the width

Millimetre spread is measured across the girdle, and it says nothing about how deep the stone is. A shallow-cut stone with a wide spread and a thick girdle can measure 7mm and sit low, catching less light and looking flat. A well-proportioned stone of the same width will be deeper and brighter.

Ask for the full measurement set: width, length and depth. On a round brilliant the depth percentage should land somewhere near 61 to 62.5 per cent of the width. On a good oval or pear, expect 60 to 63 per cent. If a listing gives only carat, you cannot judge any of this, which is exactly why so many listings give only carat.

What this means for the price you pay

Moissanite pricing tracks millimetres far more than carats. In the UK and EU, a loose colourless round at 6.5mm runs roughly €35 to €80 depending on grade and cut quality. At 8mm the same grade is nearer €95 to €180, and at 10mm you are looking at €250 and up. The step is not linear, because large clean rough is scarce.

On a finished ring, that shows up as roughly €250 to €450 for a well-made 7.5mm solitaire in silver with gold plating, and €600 to €1,200 once you move into 18ct gold or platinum with a GRA report and a UK hallmark.

How to order without regret

Cut a paper circle to the millimetre size you are considering and tape it to your finger. It takes two minutes and it is more accurate than any online ring visualiser.

1. Find the mm width in the listing, not just the carat.
2. Check whether the carat figure is diamond equivalent or actual moissanite weight. Ask if it is unclear.
3. For fancy shapes, get length and width separately, then calculate the ratio.
4. Confirm the return window before ordering an unusual size.

If a seller cannot tell you the millimetres of the stone in the setting, they do not have the stone in front of them. That is worth knowing before you pay a deposit.