Weight

When pearl wholesalers buy large lots of cultured pearls, they are often charged according to the weight of the pearls. The measure generally used is the momme, an ancient Japanese unit of weight which equals 3.75 grams or 18.75 carats. Kan is a Japanese unit of weight
equaling 1000 momme. Pearls are not sold by the momme or kan in retail stores.

The size of natural pearls is often expressed in pearl grains. One grain equals 0.25 carat. Natural American freshwater pearls may be sold according to their carat weight. The gram is commonly used to express the weight of cultured freshwater pearls, although carat weight is also used. One carat = 115 gram. Or in other words 5 carats = 1 gram.
 

Lenght

When pricing pearls, you should take into consideration the length of the strand as well as the millimeter size of the pearls. The pearl trade has specific names for different necklace lengths. They are as follows:

Pearl necklace lengths are summarized in the following list:

The preceding lengths are approximate. Definitions of necklace-length terms can vary from one jeweler to another. Keep in mind that pearl strands become slightly longer (about 2 inches or 5 cm) when knotted and strung with a clasp to form a necklace. The following table will help you determine approximately how many pearls there are in a 14" and 16" strand. Table 3 (Based on data from the Shima Pearl Co.)