How many fluid ounces are in 1 gallon of liquid?

Quick Answer

There are 128 fluid ounces in 1 gallon of liquid. A gallon is a unit of volume equal to 4 quarts or 3.785 liters. A fluid ounce is a unit of volume equal to 1/128 of a gallon or about 29.57 ml. So dividing a gallon into 128 equal parts gives 128 fluid ounces.

Measuring Volume in Gallons and Fluid Ounces

In the United States, gallons and fluid ounces are commonly used units for measuring the volume of liquids. Understanding the relationship between gallons and fluid ounces is useful for cooking, mixing drinks, calculating fuel economy, and other applications.

A gallon is a relatively large unit of volume. For reference, a standard gallon is equivalent to:

  • 4 quarts
  • 8 pints
  • 16 cups
  • 128 fluid ounces
  • 3.785 liters
  • 0.133680555 cubic feet
  • 231 cubic inches

A fluid ounce is 1/128 of a gallon, making it a much smaller unit of volume. Some key equivalents for a fluid ounce include:

  • 1/8 of a cup
  • 2 tablespoons
  • 1.8046875 cubic inches
  • 29.5735 milliliters

So 128 fluid ounces make up a gallon, because a fluid ounce is defined as precisely 1/128 of a gallon.

Converting Between Gallons and Fluid Ounces

The conversion between gallons and fluid ounces is straightforward using the fact that there are 128 fluid ounces in a gallon. Some examples:

  • 1 gallon = 128 fluid ounces
  • 2 gallons = 256 fluid ounces
  • 5 gallons = 640 fluid ounces
  • 0.5 gallons (one half gallon) = 64 fluid ounces
  • 1.25 gallons = 160 fluid ounces

Going the other direction:

  • 128 fluid ounces = 1 gallon
  • 256 fluid ounces = 2 gallons
  • 640 fluid ounces = 5 gallons
  • 64 fluid ounces = 0.5 gallons
  • 160 fluid ounces = 1.25 gallons

To convert any number of gallons to fluid ounces, simply multiply the number of gallons by 128. For example:

  • 7.5 gallons x 128 fl oz/gallon = 960 fluid ounces

To go from fluid ounces to gallons, divide the number of fluid ounces by 128. For example:

  • 384 fluid ounces / 128 fl oz/gallon = 3 gallons

Converting Fluid Ounces to Milliliters and Liters

Fluid ounces can also be converted to milliliters or liters using the density of water. At room temperature, 1 fluid ounce of water has a volume of approximately:

  • 29.5735 milliliters
  • 0.0295735 liters

So to convert fluid ounces to milliliters, simply multiply by 29.5735. For example:

  • 128 fl oz x 29.5735 ml/fl oz = 3,785.28 ml
  • 16 fl oz x 29.5735 ml/fl oz = 473 ml

To convert fluid ounces to liters, multiply by 0.0295735. For example:

  • 128 fl oz x 0.0295735 L/fl oz = 3.78528 L
  • 8 fl oz x 0.0295735 L/fl oz = 0.236588 L

Going the opposite way, from milliliters to fluid ounces:

  • 3,785 ml / 29.5735 ml/fl oz = 128 fluid ounces

From liters to fluid ounces:

  • 3.785 L / 0.0295735 L/fl oz = 128 fluid ounces

So the conversions are:

  • 1 fluid ounce = 29.5735 milliliters
  • 1 fluid ounce = 0.0295735 liters
  • 1 milliliter = 0.033814 fluid ounces
  • 1 liter = 33.814 fluid ounces

Examples of Gallons and Fluid Ounces in Real Life

Understanding gallon and fluid ounce conversions allows us to measure volumes precisely for recipes, fuel efficiency, and other everyday uses.

Cooking and Recipes

In cooking:

  • 1 gallon of milk contains 128 fluid ounces
  • A 16 fl oz pint of blueberries is 0.125 gallons
  • A 64 fl oz (0.5 gallon) carton of ice cream equals 4 pints
  • A half cup of liquid is equivalent to 4 fluid ounces

Recipes may call for ingredients like 2 tablespoons of milk, which is 1 fluid ounce. Or 1/4 cup olive oil, which equals 2 fluid ounces.

Mixing Drinks

Some examples with mixing drinks:

  • A 1.75 L bottle of liquor contains 59.2 fluid ounces
  • A 750 ml wine bottle holds 25.4 fluid ounces
  • A standard shot is 1.5 fluid ounces
  • A 12 fl oz can of soda equals 0.09375 gallons

This allows you to measure out exact cocktail recipes by fluid ounces or convert a volume between different units.

Fuel Efficiency

For fuel efficiency:

  • A 15 gallon gas tank can hold 1,920 fluid ounces of fuel
  • Mileage of 30 mpg equates to 0.0416 gallons per mile
  • Driving 450 miles at 25 mpg uses 18 gallons of gas, or 2,304 fluid ounces

Converting mileage into gallons helps calculate trips and gas budgets.


For aquariums, knowing gallons and fluid ounces helps with tank capacity, filtration, amount of chemicals to add, and determining fish capacity.

For example, a 55 gallon aquarium holds:

  • 55 gallons x 128 fl oz/gallon = 7,040 fluid ounces
  • Can potentially hold a water volume of 7,040 fluid ounces

Why There are 128 Ounces in a Gallon

The reason there are 128 fluid ounces in a gallon traces back to the definition of these units.

A fluid ounce was historically defined as 1/160 of an imperial gallon. There were 8 imperial pints in an imperial gallon, and 20 imperial fluid ounces in an imperial pint.

So an imperial gallon contained:

  • 8 pints
  • 20 fluid ounces per pint
  • 160 fluid ounces total

When the gallon was later redefined in the United States as 231 cubic inches, the fluid ounce kept its ratio as 1/128 of this new US gallon.

So the volume relationship of 128 fluid ounces to 1 gallon carried over to the US customary units, even after the gallon was redefined.

Other Facts About Gallons and Fluid Ounces

A few other interesting facts when working with gallons and fluid ounces:

– A US gallon is only about 83.3% of an imperial gallon. So 128 imperial fluid ounces is equivalent to about 154 US fluid ounces.

– Since a gallon is defined as 231 cubic inches, 1 US fluid ounce works out to be exactly 1.8046875 cubic inches.

– Fluid ounces can be further subdivided into minims, drams, and cubic centimeters for more precision.

– Water weighs about 8.34 pounds per gallon, or about 1.04 ounces per fluid ounce.

– Gasoline and other liquids may weigh slightly different per gallon. Fuel is often measured as pounds per gallon.

– The gallon was originally defined as the volume occupied by 10 pounds of distilled water at 62 °F. Measurements were then standardized using prototypes.

So while gallons and fluid ounces can seem arbitrary, they have an interesting history and scientific origin!


In summary, a gallon consists of 128 fluid ounces. This conversion comes from the definition of a fluid ounce as 1/128 of a US gallon.

Knowing the relationship between gallons and fluid ounces allows precise volume measurements for cooking, mixing drinks, fuel efficiency, aquarium capacity, and other everyday situations involving liquid volumes. Converting between gallons, fluid ounces, liters, and milliliters is straightforward using the ratios and equivalents discussed here.

The origin of the 128 ounces per gallon has its roots in the earlier imperial system, carried over to the US customary system even after the gallon was redefined. So the next time you use fluid ounces or gallons for measuring liquids, you’ll know exactly where the 128 fluid ounces in a gallon standard comes from!

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