The ounce
The avoirdupois ounce (oz) is a unit of mass in the imperial and US customary systems, defined as exactly 28.349523125 grams (1/16 of an avoirdupois pound). It is the standard small-mass unit in the United States for food packaging, recipes, and postal weights. Do not confuse it with the troy ounce (used for precious metals, 31.1035 g) or the fluid ounce (a volume).