Quick answer: carat is a unit of weight, not size: 1 carat = 0.2 grams. A well-cut round diamond of 0.25 ct is about 4.1 mm across, 0.5 ct about 5.2 mm, 0.75 ct about 5.8 mm, 1 ct about 6.5 mm, 1.5 ct about 7.4 mm and 2 ct about 8.2 mm. Price per carat rises with weight, with jumps at 0.5 and 1 carat, so a 0.9 ct stone costs noticeably less than a 1 ct one that looks almost the same. Value depends on the 4Cs: carat, cut, color and clarity, with cut having the biggest effect on sparkle.
Diamond carat weight
Diamond weight is measured in carats: 1 carat equals 200 milligrams, or 0.2 grams. The carat was adopted as the standard unit for weighing gemstones in 1907 by the Fourth General Conference on Weights and Measures in Paris. The name comes from the carob tree, whose seeds were once used as counterweights. The United States adopted the metric carat in 1913, and Great Britain and Europe soon followed.
Size matters
Size plays an important role in the diamond's appearance. In addition to the carat weight, the diameter of the stone should be taken into account. A common misconception is that half a carat is half the size of one carat. Half a carat is half the weight of one carat, but the difference in diameter for a round brilliant is only about 1.3 mm: a well-cut 0.50-carat stone is about 5.2 mm across, while a 1.00-carat stone is about 6.5 mm.
Carat weight by itself does not determine the value of a diamond. Two one-carat diamonds can be significantly different in price, because value also depends on clarity, color and quality of cut. Large, flawless diamonds are rare.
Carat weight also looks different across shapes such as round, princess, pear, oval, cushion, marquise, emerald, radiant or heart. A diamond can carry more weight without looking bigger, and two diamonds of the same weight can look different in size if one is cut deeper than the other.
By size, diamonds are often grouped as:
- Melee: 0.01 to 0.09 carats
- Small stones: up to 0.29 carats
- Medium stones: 0.30 to 0.99 carats
- Large stones, often set as solitaires: over 1 carat
- Exceptional stones over 3 to 6 carats, often sold at auction
A loose diamond can be weighed to 0.01 carat on a carat scale. To estimate the weight of a diamond already set in an engagement ring, jewelers use a formula for round stones: carat weight ≈ D² × H × 0.0061, where D is the diameter and H the depth in millimeters. The estimate can be off by up to 10 percent.
The value of diamonds
Large diamonds are rarer, so the price per carat rises with weight. An old rule of thumb, the "Tavernier rule", says the value grows roughly with the square of the carat weight.
In practice prices jump at "magic" weights such as 0.50, 0.75, 1.00 and 2.00 carats, so a 0.48-carat or 0.95-carat stone can cost noticeably less than one just over the line while looking almost the same. A 2-carat diamond of the same quality typically costs several times as much as a 1-carat stone.
Round diamond size chart
| Carat weight | Approx. diameter | Typical use |
|---|---|---|
| 0.01–0.03 ct | 1.3–2.0 mm | Accent stones, dainty bands |
| 0.10 ct | 3.0 mm | Small solitaire, promise ring |
| 0.25 ct | 4.1 mm | Petite engagement ring |
| 0.50 ct | 5.2 mm | Classic engagement ring |
| 0.75 ct | 5.8 mm | Engagement ring |
| 1.00 ct | 6.5 mm | Statement engagement ring |
| 1.50 ct | 7.4 mm | Statement engagement ring |
| 2.00 ct | 8.2 mm | Large statement ring |
Want the look of a big stone for less? Moissanite of the same diameter costs a fraction of a diamond: read moissanite vs diamond. Shop diamond engagement rings, solitaire engagement rings and moissanite engagement rings, all handcrafted to order.
