Most pearl size charts include pearls ranging in size from 5 or 6 millimeters up to 13 millimeters and as high as 18 to 20 millimeters. Pearl size charts reflect the diameter of a pearl measured in millimeters, perpendicular to the drill hole.
An average pearl is between 4 and 8 millimeters in diameter, ranging in size from a kernel of unpopped popcorn to the size of a green pea. A 20-millimeter pearl is mid-way in size between a penny and a nickel.
Pearls 7 to 7.5 millimeters in diameter, the size of mini-dice, are among the most popular for jewelry.
Pearl size varies by type of pearl. Most Akoya pearls, farmed in the oceans of Japan and China, range from 2 to 8 millimeters in size. Tahitian pearls, originating in the oceans of Polynesia from larger oysters, average 8 to 10 millimeters, though a Tahitian pearl of 16 millimeters diameter is possible. South Seas pearls, most commonly farmed in waters off Australia, are the world's largest, ranging from its typical size, 10 to 12 millimeters in diameter, to as much as 20 millimeters.
Pearls vary not only in size, but in color, shape and lustre. To match pearls for a 16-inch necklace, a jeweler typically inspects more than 10,000 pearls to arrive at the 47 needed for the necklace.