Overview
The fluid ounce is a unit for measuring volume traditionally used in English-speaking countries. It appears in both the U.S. customary system and the British imperial system, but the two are not identical in size. The term is commonly abbreviated as "fl oz" and is intended only for liquids and other volumetric measures; it should not be confused with the ounce that measures weight.
Definitions and common conversions
Each system defines the fluid ounce relative to its gallon. In the U.S. customary system one fluid ounce equals one one-hundred-and-twenty-eighth of a U.S. gallon (1/128), whereas in the imperial system one fluid ounce equals one one-hundred-and-sixtieth of an imperial gallon (1/160). In practical metric terms:
- 1 U.S. fluid ounce ≈ 29.5735 millilitres
- 1 imperial fluid ounce ≈ 28.4131 millilitres
These values are frequently used for converting recipes, nutrition labels, and laboratory quantities. For reference, a standard U.S. can of soda is 12 U.S. fluid ounces (≈355 mL).
History and development
The fluid ounce developed from earlier English measuring practices tied to different types of gallons (such as the wine gallon and the ale gallon). During the 18th and 19th centuries the United Kingdom moved to an imperial definition that altered the size of the gallon and consequently the fluid ounce. The United States retained a separate gallon standard, which is why the two fluid ounces remain distinct today.
Uses, notation and practical distinctions
Fluid ounces are widely used in cooking, bartending, beverage labeling and everyday dispensing of liquid products in countries that use customary measures. Recipe books and food labels often mix abbreviations such as "fl oz" or "oz fl"; some style guides prefer writing "fluid ounce(s)" in full for clarity. When precision is required—laboratories, pharmaceuticals, international trade—metric volumes in millilitres and litres are preferred.
Notable points and further reading
- Always distinguish fluid ounces (volume) from ounces (mass). A fluid ounce of water does not weigh one ounce by mass except under specific conditions.
- When converting recipes or measurements between regions, check whether values are U.S. or imperial; the difference is small but can matter in precise contexts.
- For definitions and official standards see relevant metrology references.