# Fluid ounce

A fluid ounce (fl oz) is a unit of volume in the U.S. customary and imperial systems of measurement. As its name suggests, fluid ounces are mostly used to measure liquids, such as the amount of liquid in a container, like a drink or a cup of coffee.

The fluid ounce has different definitions in the US customary and imperial systems of measurement. In terms of milliliters (mL):

1 fl oz (US) = 29.574 mL

1 fl oz (imperial) = 28.413 mL

Many other definitions of the fluid ounce were used in the past, but these are the two definitions that are still in common use.

Fluid ounces should not be confused with ounces, a measurement of weight and mass. In the past, fluid ounces were defined based on the volume occupied by one ounce of some substance (water, wine etc.), which was dependent on the system of fluid measure being used. In most cases today, 1 fluid ounce of some substance will not weigh 1 oz, though 1 imperial fluid ounce of water does weigh close to 1 avoirdupois ounce. It is worth noting that when the meaning is clear, sometimes, "ounce" is used to refer to "fluid ounce." For example, a 20 oz coffee refers to its volume rather than to the weight of the coffee.

## US vs imperial fluid ounces

The US fluid ounce is defined relative to the US gallon, while the imperial fluid ounce is defined relative to the imperial gallon. The US customary and imperial systems of measurement are closely related. As a result, they both use many of the same units, such as fluid ounces, gallons, quarts, pints, cups, and gills, but these units have different measures depending on the system of measurement. Below are some of the relationships between US customary, imperial, and SI (International System of Units) measurements of volume.

US customary system:

- 128 fl oz = 1 gallon
- 32 fl oz = 1 quart
- 16 fl oz = 1 pint
- 8 fl oz = 1 cup
- 1 fl oz= 29.574 mL

Imperial system:

- 160 fl oz = 1 gallon
- 40 fl oz = 1 quart
- 20 fl oz = 1 pint
- 10 fl oz = 1 cup
- 1 fl oz = 28.413 mL mL

Within the same system of measurement, gallons, quarts, pints, and cups, etc., share the same relationship, but since the US and imperial systems have different definitions of the gallon, their actual volumes differ. Being able to convert between units of measurement in SI, the US customary system, and the imperial system can be useful.

Example

Convert 64 fl oz in both the US customary and imperial systems of measurement to milliliters.

1. US customary system:

64 × 29.574 = 1892.736 mL

2. Imperial system:

64 × 28.413 = 1818.432 mL