Diamond MM to Carat Size Chart: Every Shape, 0.25 to 10 Carats

What does a diamond mm to carat size chart show?

Carat is a unit of weight, not size. One carat (ct) is 0.2 grams, or 200 milligrams, and it is split into 100 points, so a 0.75 ct stone is often called a '75-pointer'. Our guide to diamond carat weight explains how the unit came about and how stones are weighed.

A size chart translates that weight into what you actually see: the face-up size, measured in millimetres across the top of the stone. For a round that is one number, the diameter. For fancy shapes it is two numbers, length × width, because the stone is longer one way than the other.

Carat is only one of the four Cs of diamond quality, and it is the one people most often misread. A heavier stone is not automatically a bigger-looking one. The cut proportions decide how much of the weight sits on top, where you can see it, and how much is hidden underneath. Any unfamiliar term on this page is explained in our jewellery glossary of trade terms.

Every figure below is a typical measurement for a well-cut stone. Individual diamonds vary by a few tenths of a millimetre either way, so always check the measurements printed on the stone's grading report.

How many mm is a 1 carat round diamond?

A well-cut 1 carat round brilliant measures about 6.4 mm across, give or take 0.1–0.2 mm depending on its depth. The table gives typical diameters for rounds cut to standard proportions: a depth of roughly 59–62.5% of the diameter and a medium girdle (the thin edge around the widest point).

Look at the last column. Doubling the weight from 1 ct to 2 ct adds only about 1.7 mm across the top and roughly 60% more face-up area, because the extra weight is spread through the depth of the stone as well as its width. The same works in reverse: a half-carat diamond is not half the size to the eye. At about 5.1 mm it is roughly 80% of the diameter of a carat.

The popular in-between weights follow the same curve. A 0.75 carat diamond sits at about 5.9 mm, a 1 carat round diamond at 6.4 mm and a 1.25 carat stone at about 6.9 mm. Each step is only around half a millimetre, which is why seeing stones side by side tells you more than numbers on a screen.

Diamond mm to carat size chart for every shape

Fancy shapes carry their weight differently. Long, shallow shapes such as the marquise and pear spread their weight across the top; square step cuts such as the Asscher carry more of it underneath. These are typical face-up measurements at common length-to-width ratios (length divided by width, shown in brackets). Elongated shapes are listed as length × width.

Read down a column and you can see how much shape matters. At 1 carat, a marquise is about 10 mm long while an Asscher is about 5.4 mm square, yet they weigh exactly the same. A few shape-specific points:

• Oval, pear and marquise: the longest face-up measurement per carat, and the ratio changes everything. A 1 ct oval at 1.45 looks longer and slimmer than one at 1.30, even at the same weight.
• Princess: measured edge to edge, but the corner-to-corner diagonal is about 1.4 times the side, so a 5.5 mm princess reads close to 7.8 mm across the points.
• Cushion: the widest variation of any shape. A deep 'crushed ice' cushion can measure half a millimetre less across than a shallower cushion of the same weight.
• Emerald and Asscher: step cuts with long, flat facets and often a deeper pavilion (the lower half of the stone), so they show less spread per carat than brilliant cuts.

For the mid-range weights, our pages on 1.5 carat diamonds, what a 2 carat diamond looks like, choosing a 2.5 carat diamond and 3 carat diamond sizes and grades go deeper on each size, including the colour and clarity that tend to suit it.

How big is a 4 or 5 carat diamond in mm?

Large stones follow the same rule: width grows with the cube root of the weight, so each extra carat adds less than the one before. Here are typical measurements at 4 and 5 carats.

Above 5 carats, rounds keep growing slowly: about 11.7 mm at 6 ct, 12.3 mm at 7 ct, 12.9 mm at 8 ct and 13.9 mm at 10 ct. A 10 carat round is only about 2.9 mm wider than a 5 carat, even though it weighs twice as much. Our guide to 6 to 10 carat diamonds covers what changes at those sizes, including how settings are built to carry them.

At this scale the stone shows more of everything: colour is easier to see because there is more diamond to look through, and inclusions are larger in absolute terms. Our pages on 4 carat diamonds and 5 carat diamond proportions explain which grades hold up when the stone is this big.

Why do diamonds of the same carat weight look different sizes?

Because weight can sit in two places: across the top, where you see it, or underneath, where you do not. A 1.00 ct round cut to a 60% depth measures about 6.5 mm across. The same weight cut to a 67% depth measures about 6.25 mm, so its face-up area is only around nine-tenths of the shallower stone's. The trade calls the face-up size for the weight a stone's spread.

• Depth percentage: total depth divided by width. Deep stones face up smaller; very shallow stones face up larger but can leak light and look glassy.
• Girdle thickness: a thick girdle adds weight around the edge without adding visible width.
• Crown height: a tall crown (the upper part of the stone) adds weight above the girdle without widening the outline.
• Shape and ratio: elongated shapes cover more length, and a longer ratio stretches the same weight further.

Gemmologists estimate weight from measurements with a simple formula, useful when a stone is already set and cannot be weighed. For a round: diameter × diameter × depth × 0.0061, all in millimetres. A stone 6.45 mm across and 3.97 mm deep works out at about 1.01 ct. Fancy shapes use length × width × depth with a different factor: roughly 0.0062 for ovals, 0.0059 for pears and hearts, 0.0080–0.0092 for emerald cuts (rising with the ratio) and 0.0083 for princess cuts. These are estimates only; a calibrated scale gives the true weight.

If face-up size matters most to you, our guide to the shapes that look largest for their weight ranks every shape by spread and explains the trade-offs in sparkle and colour.

How big does a diamond look on the hand?

A diamond's size on the hand depends as much on the finger as on the stone. A ring size around M or N in Australian letter sizes is roughly 16.5–17 mm across the inside of the band, and the top of the finger is about the same width. The table shows how much of a finger that width a round centre stone covers.

On a slimmer finger the same stone covers more and looks larger; on a broader finger it looks smaller. The setting changes the impression too. Fine claws and a slim band of about 1.8–2 mm make the centre stone look bigger by contrast, while a halo of small diamonds typically adds 2–3 mm to the overall width of the head. A 3 mm band beside a half carat can make the stone look small.

If the ring is a surprise, estimating their ring size discreetly also gives you the finger width you need to judge the stone against.

What do ct, ctw, points and TDW mean?

Carat terms on jewellery labels look similar but mean different things. The difference between carat and carat total weight matters most, because it can change the size of each stone dramatically.

Two practical points follow from this. First, a 1 ctw pair of earrings is two half-carats of about 5.1 mm each, not two 6.4 mm stones. Second, diamonds just below a magic size, such as 0.90–0.99 ct, are often priced noticeably lower than a full carat while measuring only about 0.1–0.2 mm smaller, a difference that is hard to see once set.

Are lab-grown diamonds the same size per carat as natural diamonds?

Yes. A laboratory-grown diamond is the same material as a natural diamond: carbon in the same crystal structure, with the same density of about 3.52 g/cm³. A 1 ct lab-grown round cut to the same proportions measures the same 6.4 mm as a natural one, so every chart on this page applies to both.

The real difference is price and availability. Lab-grown diamonds cost much less than natural diamonds of the same weight and grade, so many people choose a larger lab-grown stone; natural diamonds are rarer, especially above 2 carats. Proportions vary by cutter in both, so compare the measurements and depth percentage rather than assuming. Our guide to lab-grown diamond sizes from 1 to 5 carats goes through each weight.

Both should come with an independent grading report from a laboratory such as GIA or IGI. A lab-grown report states clearly that the stone is laboratory-grown, and many lab-grown stones carry a tiny laser inscription on the girdle saying so.

How to use a diamond size chart when choosing a stone: step by step

1. Start with the look, not the weight. Decide how many millimetres you want on the finger, using the paper-circle or ruler test.
2. Choose the shape. Find that size in the chart for your shape, then note the carat weight it points to.
3. Read the report's measurements. A round is listed as minimum – maximum diameter × depth, for example 6.45 – 6.48 × 3.98 mm. Fancy shapes are listed as length × width × depth.
4. Check the depth percentage. For rounds, about 59–62.5% gives a good balance of spread and brightness; deeper stones face up smaller for their weight.
5. Work out the ratio for fancy shapes. Divide length by width and compare it with the ratios in the chart, so you know what outline you are getting.
6. Look just under the magic sizes. Compare a 0.90–0.99 ct stone with a full carat; the size difference is a fraction of a millimetre.
7. Judge it in the setting. Consider band width, claws or a halo, and the wearer's finger size, before you commit to the stone.