A carat (ct.) is the unit of measurement specifically used to describe the weight of a diamond (or other gemstones). A single carat is equivalent to 0.2 grams, or 200 milligrams, and is divided into 100 points. Don't confuse "carat weight" with "karat," the method of determining the purity of gold.
AA certified diamond that comes with a grading report, indicates the exact carat weight to the nearest hundredth of a carat, in decimal format. A 1-carat diamond has 100 points (1.00) while a ¾ carat stone has 75 points (0.75). Carat weight in pre-set jewelry is typically described as a fraction (e.g. ¾ carats) and has an equivalent decimal range (¾ carats = 0.69 – 0.82 points).
The table below correlates fractional sizes with their decimal equivalent:
| Carat Fraction | Decimal Equivalent |
| 1/10 | .09 – .11 |
| 1/8 | .12 – .13 |
| 1/7 | .14 – .15 |
| 1/6 | .16 – .17 |
| 1/5 | .18 – .22 |
| ¼ | .23 – .28 |
| 1/3 | .29 – .36 |
| 3/8 | .37 – .44 |
| ½ | .45 – .58 |
| 5/8 | .59 – .68 |
| ¾ | .69 – .82 |
| 7/8 | .83 – .94 |
| 1.0 | .95 – 1.05 |
Carat Weight and Price
Diamonds are mined from deep within the earth's crust. Rough diamonds are either too small or inferior in quality to produce glimmering and polished products. The rarity of larger, high-quality diamonds has resulted in significant price escalations between sizes. For example, a one-carat diamond can command four to six times more than a half-carat diamond.
Selecting a Carat Weight
- Price of a Diamond is determined by its carat weight, and broken down in half-carat increments. Thus, assuming factors like cut and clarity are equal, a diamond weighing one carat will cost more than a diamond weighing slightly less than one carat.
- While the diamonds carat measurement indicates the weight of a diamond, its length and width measurements indicate the actual physical size, or how big the diamond will appear when viewed from top. Diamonds can vary in size even though they are of the same carat weight. For example, a round diamond of one-carat could measure 5.9 mm in diameter, on the other hand a round diamond of one-carat could be 6.5 mm wide. Broader diamonds with larger millimeter measurements create greater visual impact.
- The price of a diamond is affected by the carat weight, hence higher the carat weight, higher the price of the diamond.