Diamond Size Chart: Carat to Millimetres for Every Shape

A diamond size chart converts carat weight into millimetres. Carat is a unit of mass, one fifth of a gram, and it says nothing directly about how large a stone looks. Millimetres across the face are what the eye reads. The chart is the bridge between the number on the price tag and the object on the hand.

The three tables below cover eleven shapes from half a carat to five carats. Round and square outlines are given as a single width. Elongated outlines are given as length by width, because a single figure would hide the thing that makes them elongated. Every figure is the midpoint of a published reference band for a well proportioned stone.

Read down any column and the first pattern appears immediately. Going from 1 carat to 2 carats, a round grows from 6.5 to 8.2 millimetres, about 26 percent wider, while the weight has doubled. From 2 to 4 carats it grows from 8.2 to 10.3, another 26 percent, for another doubling. Weight scales with volume. Size, as the eye reads it, scales with area. The chart is the arithmetic of that gap.

Read across the 1 carat row and the second pattern appears: the same weight produces outlines from 5.6 millimetres to 10.5 millimetres along the longest axis. That is not a rounding difference. It is the difference between a stone that sits inside the width of a finger and one that spans it.

The order is consistent at every weight. Marquise is longest, then pear, oval, emerald and trillion. Round sits in the middle of the group. Heart, cushion, asscher and princess read smallest, because their outlines are compact. The princess also hides a large share of its weight below the girdle, in a deep pavilion where it cannot be seen.

This is the whole reason a shape decision cannot be made on carat weight alone. A buyer who wants a stone that reads long chooses from the top of the chart. A buyer who wants a stone that reads bright and compact chooses from the bottom, and pays for depth rather than spread. Neither is wrong. The chart makes the trade visible before the money moves.

Compare three published size charts and the round column agrees almost to the tenth of a millimetre: 6.4 to 6.6 at 1 carat in every source consulted for this page. The oval column does not. One reference lists a 1 carat oval at 7.7 by 5.7 millimetres. Another lists it at 8.4 by 5.7. Neither is wrong.

The difference is the length to width ratio the publisher assumed and did not state. An oval cut at 1.35 to 1 is short and full. An oval cut at 1.47 to 1 is longer and slimmer. Both weigh a carat. The longer one measures longer, and a chart built from one population of stones will disagree with a chart built from another.

This chart states its ratios in the table above so that the figures can be checked and adjusted. If the stone in front of you is squarer than the stated ratio, expect it to run shorter than the table at the same weight. If it is longer, expect the reverse. The round column has no ratio to argue about, which is why it is the one column every chart agrees on.

Depth is the other variable. A stone cut deep hides weight below the girdle and measures smaller face up than the chart. A stone cut shallow spreads wider than the chart and gives up light return to do it. The chart describes a well proportioned stone; the grading report describes the actual one.

The round column gets its own table because most people calibrate everything else against it. Half millimetre steps are also the scale at which buying decisions turn.

Three rows are worth memorising. Five millimetres is half a carat. Six and a half millimetres is one carat. Eight millimetres is two carats. Everything else on the chart can be estimated from those three anchors, and they are the three sizes that appear most often in stud earrings, pendants and tennis bracelets.

Notice how the carat figure accelerates as the diameter climbs. From 5 to 6.5 millimetres the weight doubles. From 8 to 10 millimetres it nearly doubles again. This is why a stone that looks only slightly larger than another can cost considerably more: the price follows the weight, and the weight is growing faster than the outline.

Stud earrings are the place a size chart is most often misread, because they are sold by total weight across the pair. A 2 carat total weight pair is not a 2 carat stone. It is two 1 carat stones, each about 6.5 millimetres across, one in each ear.

Halve the total weight and read the round table. A 1 carat total weight pair is two half carat stones of about 5.2 millimetres. A 3 carat total weight pair is two 1.5 carat stones of about 7.4 millimetres. A 4 carat total weight pair is two 2 carat stones of about 8.2 millimetres. At that size a stud starts to read as a statement rather than a detail.

At Draco the 1 carat total weight pair is CAD 711 at the 10K gold base. The 2 carat pair is CAD 999 and the 3 carat pair CAD 1,299. Every pair is IGI certified at E to F colour and VS2 or better. The step from 1 carat to 2 carats total weight adds 1.4 millimetres to each stone; the step from 2 to 3 adds 0.9. The chart shows why the first step is the one people notice.

The same logic applies to a tennis bracelet, where the chart describes one stone in the line rather than the total. A 5 carat round line on a 7 inch bracelet uses about 62 stones of roughly 0.08 carat, each about 2.8 millimetres across. Quarter carat rounds, about four millimetres each, would fill the same length at about 11 carats. Draco lists its 5 carat round brilliant tennis bracelet at CAD 2,920 and the elongated cuts at CAD 3,201. On that piece the outline is close to a free choice, and the width follows from it. See the earring collection and the bracelet collection.

Every Draco diamond is IGI certified at E to F colour and VS2 or better and ships with its certificate. The report can be provided on request before purchase and verified free at IGI.org. It states the stone's measured dimensions in millimetres.

Every piece carries 30 day returns, the Lifetime Authenticity Guarantee, free resizing, and free insured worldwide shipping to 25 markets, delivered duty paid with nothing collected at the door, and signature on delivery.

A stone at a millimetre size or ratio outside the catalogue is a custom order, quoted and confirmed before production. Start one at create your piece.