How Many Ounces in a Gallon?

Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

Expert Contributor
Gretchen Graef

Gretchen has a Ph.D in Materials Science and Engineering. She has been an engineer, technical writer, and a teacher teaching physics, chemistry, biology, and mathematics.

This lesson offers step by step instructions for finding how many ounces (oz) are in a gallon. You'll also learn how to convert from the Imperial (U.K.) ounce to gallon measurements as well.

Ounces to Gallons

The most important thing to remember about changing ounces to gallons is that there are 128 fluid ounces in every gallon.

Table converting ounces to gallons
Changing fl oz to gallon

Another way to think of it is 'for every 128 ounces you have 1 gallon'. Let's use x to represent the number of ounces and y to represent the number of gallons. (Notice that this formula divides the number of ounces into groups of 128 to determine the number of gallons).

Formula for calculating how many fluid ounces are in a gallon

Let's try an example: A farmer measures his cows' milk production by the ounce but sells it by the gallon. If the farmer collected 19,200 ounces of milk in a week, how many gallons of milk will he be able to sell?

Step 1:

Remember the formula for finding how many fluid ounces are in a gallon.

  • Formula for calculating how many fluid ounces are in a gallon

Step 2:

Substitute known values into the formula. Here the number of ounces is 19,200.

  • Substituting known values to find out how many ounces are in a gallon

Step 3:

Calculate the answer.

  • Solution for equation to find oz in a gallon

The equivalent number of gallons is 150. It is most appropriate to answer word problems with full sentences, so the answer to this problem is best written as 'The farmer will be able to sell 150 gallons of milk this week'. Remember that ounces can be abbreviated as oz and gallons can be abbreviated as gal in your answers.


19,200 ounces converts to 150 gallons.

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Additional Activities

SI and US Customary Units

The United States did not adopt the metric system when the rest of the world did in the 1960s. Because of this, most items in the US are not measured in the same way as they are in the rest of the world. The United States kept the old system from Britain that was called the British Imperial system and renamed it the US Customary System (USCS). While the rest of the world measures mass and volume in kilograms and liters, in the United States, pounds and gallons are still used. To get an idea of how the two systems compare, consider this.

1 kilogram (kg) = 2.2 pounds (lb) or 1 lb = 0.454 kg = 454 g

1 liter (L) = 0.995 quarts or 1 quart = 1.005 L

US Customary Unit Conversions

1 cup = 8 fluid ounces (fl oz)

1 pint = 2 cup (c)

1 quart = 2 pints (pt)

1 gallon = 4 quarts (qt)

The volume for one cup is specified in fluid ounces, which is used for volume. A fluid ounce has a volume of about 30 mL. The USCS system also uses ounces for mass. In this case, the unit is the avoirdupois ounce and is 1/16th of a pound or about 28 grams (g).


So now we can calculate the number of ounces in a gallon.

8 fl oz/c x 2 c/pt x 2 pt/qt x 4 qt/gallon = 128 fl oz/gallon

Test Your Knowledge (fill in the blanks)

1. There are _____ fluid ounces in a US gallon.

2. The United States uses the _______________ system of measurement.

3. The standard measuring system used by the rest of the world is the _____ system.

4. A quart is a little _____ than a liter.

5. A kilogram has a _____ mass than a pound.

Think and Write

Which system of measurement do you think is easier for doing calculations? Write one to three sentences explaining your choice.


1. 128

2. US Customary

3. SI, systeme international, or international system

4. smaller (less)

5. greater (larger)

There are many possible answers. The SI system is easier to perform calculations because everything is based on multiples of ten. It is accepted everywhere in the world. A student could say that the USCS is easier because it is a system with which they are already familiar. Accept any reasonable answer, but not "I don't care," "I don't know," and the like.

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