Fluid ounce
A fluid ounce (fl oz) is a unit of volume in the U.S. customary and imperial systems of measurement. In the US customary system, 1 fluid ounce is equal to 29.574 mL; in the imperial system, 1 fl oz is equal to 28.413 mL. As the name of the unit suggests, the two are mostly used to measure liquids, such as the amount of liquid in a container, like a drink or a cup of coffee.
What is a fluid ounce
A fluid ounce is a measure of volume primarily used in the US and the UK. In both systems of measurement, liquid measures are defined in terms of the gallon. A gallon in each system is defined by a different number of fluid ounces, but other liquid measures such as the pint, quart, and cup have the same definition relative to the gallon regardless of which system is used. Specifically:
- 1 gallon = 4 quarts
- 1 quart = 2 pints
- 1 pint = 2 cups
Thus, even though 1 gallon in US fluid ounces is equal to 128 fl oz and 1 gallon in imperial fluid ounces is equal to 160 fl oz, in each system, we would divide the number of fluid ounces by 4 to get quarts, 8 to get pints, and 16 to get cups.
US vs imperial fluid ounces
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.
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.
| Measure | US customary | Imperial | ||
|---|---|---|---|---|
| fl oz | mL | fl oz | mL | |
| Gallon | 128 | 3785.41 | 160 | 4546.09 |
| Quart | 32 | 946.353 | 40 | 1136.52 |
| Pint | 16 | 473.176 | 20 | 568.261 |
| Cup | 10 | 295.735 | 16 | 454.609 |
| Conversion: | 1 fl oz = 29.574 | 1 fl oz = 28.413 | ||
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
Fluid ounce vs ounce
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 talking about fluids, it is common to just say "ounce" instead of "fluid ounce." For example, a 20 oz coffee refers to its volume rather than to the weight of the coffee. Usually, based on the context, we can tell whether a person saying "ounce" is referring to the fluid ounce or the ounce representing weight.