Diamond Weight Converter

Diamond and gemstone weight uses the carat system.

Base relationships:

• 1 carat = 200 milligrams = 0.2 grams (exact, by international definition)
• 1 carat = 100 points
• 1 point = 2 milligrams = 0.002 grams

Other conversions:

• 1 carat = 0.006430 troy ounces
• 1 carat = 0.007055 avoirdupois ounces (the ordinary kitchen-scale ounce)
• 1 carat = 3.0865 grains
• 5 carats = 1 gram (exact)

Watch the two ounces. A troy ounce is 31.1035 g and is what precious metals are priced in; the everyday ounce is 28.3495 g. Charts that quote a carat as 0.00705 troy ounces have quietly used the everyday one, which overstates the troy figure by about 10 percent.

Common diamond sizes and weights:

Weight grows with the cube of the diameter, so the millimetre column climbs far more slowly than the carat column does. Doubling from 1 carat to 2 buys you 1.7 mm across the top, not another 6.5. That is the arithmetic behind the complaint that a two-carat stone does not look twice the size, and it is also why an eighth of a carat shaved off a stone is nearly invisible while the price moves a long way.

Note: The gemstone carat (weight) is different from the gold karat (purity).

The single most useful thing to understand is that carat is weight, not size. Two one-carat diamonds can look noticeably different across the top, because a deep or poorly proportioned cut hides weight down in the stone where you can’t see it, while a well-cut stone of the same weight spreads wider and faces up larger. So “bigger carat” doesn’t always mean “looks bigger,” and cut quality is what bridges the gap.

Prices behave strangely around round numbers, too. There are “magic sizes,” half-carat, three-quarter, and especially one carat, where the price jumps sharply the moment you cross the threshold. A 0.95-carat stone can cost meaningfully less than a 1.00-carat one that looks identical to the eye, which is why careful buyers shop just under the round figures. The word itself comes from carob seeds, once used as a rough weight reference in ancient gem trading, long before a carat was standardized at exactly 200 milligrams.