Fluid ounce facts for kids
A fluid ounce is a unit used to measure how much liquid something holds. It's part of two main measurement systems: the imperial system (used in places like the UK) and the U.S. customary system (used in the USA). Even though they share the same name, the amount of liquid a fluid ounce holds is a little different in each system!
Imperial Fluid Ounce
In the imperial system, one fluid ounce is a small part of an imperial gallon. Imagine an imperial gallon divided into 160 equal parts; one of those parts is an imperial fluid ounce. It's also equal to 8 fluid drams.
To give you an idea of its size, an imperial fluid ounce is about 28.413 milliliters (mL). If you had this much water, it would weigh almost exactly one avoirdupois ounce. This measurement is officially defined in UK laws.
U.S. Fluid Ounce
In the U.S. customary system, a U.S. fluid ounce is a part of a U.S. fluid gallon. Here, a U.S. fluid gallon is divided into 128 equal parts. So, one U.S. fluid ounce is a bit larger than an imperial fluid ounce.
Specifically, a U.S. fluid ounce is about 29.573 milliliters (mL). If you had this much water, it would weigh a little more than one avoirdupois ounce, about 1.04 ounces.
Fluid Ounce in Nutrition Labels
Sometimes, for things like nutrition labels on food and drinks, the U.S. has a special definition for a fluid ounce. For these labels, a fluid ounce is considered to be exactly 30 mL. This makes it easier to measure and understand serving sizes on food packaging.
See also
In Spanish: Onza líquida para niños
