Fluid Ounce — Definition, Formula & Examples
A fluid ounce is a unit used to measure the volume of liquids in the US customary system. There are 8 fluid ounces in 1 cup and 128 fluid ounces in 1 gallon.
A fluid ounce (abbreviated fl oz) is a unit of liquid capacity in the US customary system equal to exactly 1/128 of a US gallon, or approximately 29.5735 milliliters.
How It Works
Fluid ounces measure how much space a liquid takes up, not how heavy it is. You can convert between fluid ounces and larger units using these key relationships: 8 fl oz = 1 cup, 16 fl oz = 1 pint, 32 fl oz = 1 quart, and 128 fl oz = 1 gallon. To convert from a larger unit to fluid ounces, multiply. To convert from fluid ounces to a larger unit, divide.
Worked Example
Problem: A recipe calls for 3 cups of milk. How many fluid ounces is that?
Recall the conversion: There are 8 fluid ounces in 1 cup.
Multiply: Multiply the number of cups by 8.
Answer: 3 cups of milk is 24 fluid ounces.
Visualization
Why It Matters
Fluid ounces appear on almost every drink label and in most cooking recipes in the United States. Understanding this unit helps you follow recipes correctly, compare product sizes at the store, and solve measurement problems in math class.
Common Mistakes
Mistake: Confusing fluid ounces (volume) with ounces (weight).
Correction: A fluid ounce measures how much space a liquid fills. An ounce (oz) measures how heavy something is. They are different units even though they share the word "ounce." For water, 1 fl oz happens to weigh close to 1 oz, but this is not true for all liquids.