Fluid Ounce – Definition, Conversion, Facts, Solved Example, FAQs

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What Is Fluid Ounce?

A fluid ounce, abbreviated as fl oz, is a unit to measure the liquid capacity or liquid volume in the imperial system and the US customary system of measurement. 

  • 1 US fluid ounce = 29.574 mL
  • 1 imperial fl oz = 28.413 mL

This measurement is particularly useful for everyday tasks like measuring liquids, beverages, cooking ingredients, and personal care products. The unit fluid ounce is commonly used in many American recipes and labels.

Fluid Ounce: Definition

A fluid ounce (abbreviated as “fl oz”) is a unit of volume measurement primarily used in the United States and the Imperial system. 

A fluid ounce (fl oz) is indeed a relatively small volume measurement. As mentioned above, the conversion between fluid ounces and milliliters based on the US customary system of measurement is as follows:

1 fluid ounce (US) = 29.5735 milliliters (approx.) 

However, in the US, a food labeling fluid ounce is precisely equivalent to 30 milliliters (mL), and it is roughly equivalent to the amount of liquid that can fill a standard medicine cup.

A medicine cup with capacity about 1 fl oz or about 30 mL

US Fluid Ounces vs. Imperial Fluid Ounces

Fluid ounces vary in definition between the US customary and imperial systems. 

US Customary SystemImperial System
1 fl oz = 29.5735 mL1 fl oz = 28.4131 mL 
1 US cup = 8 fl oz1 Imperial cup = 10 fl oz
1 US liquid pint = 16 fl oz1 Imperial pint = 20 fl oz
1 US liquid quart = 32 fl oz1 Imperial quart = 40 fl oz
1 US gallon = 128 fl oz1 Imperial gallon = 160 fl oz

Note: In both the systems, the pint, quart, and cup have the same definitions corresponding to gallon

  • 1 gallon = 4 quarts = 8 pints = 16 cups
  • 1 gallon = 4 quarts
  • 1 quart = 2 pints
  • 1 pint = 2 cups

The US fluid ounce is tied to the US gallon, and the imperial fluid ounce relates to the imperial gallon. Both the American fluid ounce and the Imperial fluid ounce are based on different historical standards, and the liquids used for their definitions are as follows:

  • American fluid ounce: Originally defined as the volume occupied by an ounce of wine.
  • Imperial fluid ounce: Defined as the volume occupied by an ounce of water.

These definitions have led to a slight difference in the volume of a fluid ounce in the American and Imperial systems. The American fluid ounce is slightly larger than the Imperial fluid ounce due to the difference in the density of wine and water.

How to Measure Fluid Ounces

Measuring fluid ounces can be done using various tools, depending on the quantity of the liquid. Here are two common methods:

  • Liquid Measuring Cups: Use a liquid measuring cup, typically made of clear glass or plastic.

Fill the cup with the liquid until it reaches the desired fluid ounce mark on the side. Ensure you read the measurement at eye level for accuracy.

1 cup (US) = 8 fl oz

  • Measuring Spoon: For smaller quantities of fluid, measuring spoons (teaspoons and tablespoons) can be used.

6 US teaspoons = 1 US fl oz

2 US tablespoons = 1 US fl oz

Fluid ounces can also be converted into other suitable units, such as pints, by applying the appropriate conversion factors.

Difference Between Fluid Ounces and Ounces

Fluid ounces (fl oz) are distinct from ounces (oz), a unit of mass. Using these two units interchangeably is not the right practice. Context usually makes it clear but using the correct unit is extremely important.

Example: 1 fluid ounce of honey weighs about 1.5 ounces.

1 imperial fluid ounce of water weighs about 1 ounce.

Fluid OuncesOunces 
Measurement type: VolumeMeasurement type: Weight (Mass)
Abbreviation: fl oz Abbreviation: oz
Typically used for measuring liquids like water, milk, or juice, etc.Typically used for measuring solids such as fruits, vegetables, or packaged foods, etc.

Fluid Ounces Conversions

Here’s a list of common fluid ounces conversions:

  • 1 US fl oz = 1/128 US liquid gallon = 0.0078125 US liquid gallon
  • 1 US fl oz = 1/32 US liquid quart = 0.03125 US liquid quart
  • 1 US fl oz = 1/16 US liquid pint = 0.0625 US liquid pint
  • 1 US fl oz = ⅛ US liquid cup = 0.125 US cup
  • 1 US fl oz = 2 US tablespoons
  • 1 US fl oz = 6 US teaspoons
  • 1 US fl oz = 29.5735 mL
  • 1 Imperial fl oz = 1/160 imperial liquid gallon = 0.00625 imperial liquid gallon
  • 1 Imperial fl oz = 1/40 imperial liquid quart = 0.025 imperial liquid quart
  • 1 Imperial fl oz = 1/20 imperial liquid pint = 0.05 imperial liquid pint
  • 1 Imperial fl oz = 1/10 imperial liquid cup = 0.1 imperial liquid cup
  • 1 Imperial fl oz = 28.4131 mL

Facts about Fluid Ounce

  • In the US, a typical serving size for a soft drink is 12 fluid ounces.
  • Fluid ounce abbreviation: fl oz
  • The term “fluid ounce” distinguishes it from the avoirdupois ounce, which measures mass.
  • For approximate conversion between US fluid ounces and milliliters, multiply the volume by 30.


In this article, we learned about fluid ounces, how to measure liquid volume in fluid ounces, difference between ounces and fluid ounces, and formulas for common conversions. Let’s solve a few examples for better comprehension. 

Solved Examples on Fluid Ounce

1. A bottle contains 10.5 fl oz of medicine. If the doctor has instructed you to consume 0.25 fluid ounces of the liquid twice a day, how many days will the medicine last?

A 10.5 fl oz medicine bottle


Volume of liquid in the bottle = 10.5 fl oz

Volume of liquid consumed daily = 0.50 fl oz

If you consume 0.25 fluid ounces of the medicine twice daily, then you are consuming 0.50 fl. Oz. of the medicine in a day.

Number of days the bottle will last $= \frac{10.5}{0.50}$

Thus, the medicine will last for 21 days.

2. If a bottle of shampoo contains 20 fl oz of product, how many cups of shampoo are in the bottle?


1 US cup = 8 fl oz

Thus, 20 fl oz $= \frac{20}{8}$ cups = 2.5 cups

There are 2.5 cups of shampoo in the bottle.

3. How will you convert 64 fluid ounces into milliliters using the US Customary system?


1 fl oz = 29.5735 mL

To convert 64 fl oz into milliliters, we multiply the volume by 29.5735. 

(Note that for approximate results, you can multiply the volume by 30.)

64 fl oz = 29.574 mL × 64 = 1892.736 mL

Practice Problems on Fluid Ounce

Fluid Ounce – Definition, Conversion, Facts, Solved Example, FAQs

Attend this quiz & Test your knowledge.


How many milliliters are approximately equal to one US fluid ounce?

10.29 milliliters
29.57 milliliters
100 milliliters
500 milliliters
Correct answer is: 29.57 milliliters
1 US fl oz ≈ 29.57 mL

Identify the largest unit of volume measurement among the following options.

Fluid ounce
Correct answer is: Gallons
A gallon is the same as 16 cups or 8 pints, or 4 quarts, making it the largest unit of measurement for liquids.

How many US fluid ounces make one US cup?

Correct answer is: 8
1 US cup = 8 fl oz

If 1 barrel of oil is equal to 42 US gallons, what will be its equivalent measurement in fluid ounces?

Correct answer is: oz.
1 US gallon = 128 fluid ounces
42 gallon $= 128 \text{fl.oz.} \times 42 = 5376 \text{fl oz}$

Frequently Asked Questions about Fluid Ounce

One US fluid ounce of water weighs approx. 29.57 g. 

One Imperial fluid ounce of water weighs about 1 oz.

The avoirdupois ounce, often simply referred to as an ounce, is a common unit of weight or mass in the customary system used in the United States. It is equal to approximately 28.35 grams.

No. Fluid ounces are not the same as ounces. They are two different units of measurements, where fluid ounces is the unit of liquid volume and ounce is the unit of weight or mass.