Liquid measurements have everyday significance in our lives: from cooking to hydration. One of the questions, and common enough to be frequently asked, is “how many ounces in a gallon?”
Even though this seems to be rather a simple question, it is an important question with a difference in the answer depending on where you ask the question in the world.
In this article, we will take a look at converting gallons to ounces, we will compare the two systems of measurement (US Customary System and Imperial – UK), and we will cement your understanding with many examples of practical applications.
The Quick Answer: 128 Ounces (Usually)
If you are one of the many residents of the U.S.A., the quick answer for the question “how many ounces in a gallon,?” lies at – 128 ounces, or 128 fl oz.
This measurement follows the US Customary System of measurement, which is widely understood in the United States, and its territories, and certain Caribbean countries.
U.S. Fluid Ounces vs. Imperial Fluid Ounces
When trying to get an answer for an calculations of measurement, the answer will with be logical when you refer to the terms fluid ounce and gallon as measured by that system of measurement.
The US Customary System: The Standard in the US
In the US Customary System, the definitions are:
- 1 US Gallon = 4 US Quarts
- 1 US Quart = 2 US Pints
- 1 US Pint = 2 US Cups
- 1 US Cup = 8 US Fluid Ounces
So we can arrive at this simple conversion:
- 1 US Gallon = 4 Quarts * (2 Pints/Quart) * (2 Cups/Pint) * (8 Fluid Ounces/Cup)
- 1 US Gallon = 128 US Fluid Ounces
This is the conversion you will need 99% of the time when you are following recipes (state-side) or referencing nutrition labels or buying liquid goods (seriously if you try to make something with a US gallon you will almost always find it is 128 US fluid ounces). This is how people will tend to answer.
The Imperial System: The UK and Beyond
The Imperial System is what is used in the United Kingdom and some Commonwealth countries, however, the definition of their units is a little different:
- 1 Imperial Gallon = 4 Imperial Quarts
- 1 Imperial Quart = 2 Imperial Pints
- 1 Imperial Pint = 20 Imperial Fluid Ounces (this is where the first big difference comes in!)
If we calculate out the conversion for the Imperial System:
- 1 Imperial Gallon = 4 Quarts * (2 Pints/Quart) * (20 Imperial Fluid Ounces/Pint)
- 1 Imperial Gallon = 160 Imperial Fluid Ounces
As you can see, an Imperial gallon is larger than a US gallon with there being 32 fluid ounces difference. This difference does matter when you are cooking internationally and different parts of the world use different liquid measurements.
This table shows a quick comparison of both:
| Unit | USA Fluid Ounces | Imperial Fluid Ounces |
| 1 Gallon | 128 | 160 |
Why is there a difference? A Historical Perspective
The reasons for these different measures comes from the historical differences in defining basic units that exist with two different systems.
In fact, both systems were formed from older English measuring devices, but after the American Revolution the US started defining and establishing it’s own measuring standards, while the British continued to develop and assert there measuring standards until the end of the 19th century, which ultimately led to the ever so slightly different standards we have today.
Practical Application of Knowing How Many Ounces Are in A Gallon
Knowing how many ounces are in a gallon could just be a fun to know fact, but it also has practical applications in your everyday life.
- Cooking and Baking – Recipes often define ingredients in ounces and gallons and baking accurately, considering you may adapt a recipe from a different region from the US or the UK is another reason to understand how ounces are converted to gallons. If a recipe with gallons calls for a gallon of milk, the metric system usage makes it different from the gallon explained in another recipe that uses ounces based on imperial gallons.
- Daily Water Intake: Many health professionals suggest you should drink a number of ounces of water every day. If your goal is a gallon of water, knowing a gallon is 128 ounces means you have a better understanding of volumes and can monitor your consumption daily (for example, you would have to drink a 128 ounces of 8 ounces, so that means 16 glasses of 8 ounces would equal one gallon is what you would need to consume.
- Shopping for Beverages: If you are going to the store to purchase larger quantities of any types of liquid like milk, juice, or soda, it is a very good idea to know the total amount in ounces to be able to compare each price that will give you value for your dollar and still be worthwhile to bring it home.
- Gardening and household chores: Many products with instructions on mixing fertilizers to cleaning products are provided in gallons or ounces, so knowing how to convert these products can be valuable!
- Travel or have products that were purchased from a country that doesn’t use ounces or gallons: Being able to convert means you will understand quantities or avoid misrepresentation of quantities.
How to Convert ounces and gallons
How to convert gallons to ounces (in US Customary)
- Multiply the gallons by 128
- Formula: Fluid Ounces = Gallons × 128
- Example: 2 gallons × 128 = 256 fluid ounces
How to convert ounces to gallons (in US Customary):
- Take the number of fluid ounces and divide it by 128.
- Formula: Gallons = Fluid Ounces/ 128
- Example: 64 fluid ounces /128 = 0.5 gallons (or half a gallon)
For conversions in the Imperial System (less common in the US, but good to know):
How to convert gallons to ounces (Imperial):
Multiply the gallons by 160
- Formula: Fluid Ounces = Gallons × 160
- Example: 2 imperial gallons × 160 = 320 Imperial fluid ounces
How to convert ounces to gallons (Imperial):
Take the number of fluid ounces and divide it by 160.
- Formula: Gallons = Fluid Ounces ÷ 160
- Example: 80 Imperial fluid ounces ÷ 160 = 0.5 Imperial gallons
Common Questions and Quick Answers
How many 8-ounce glasses are in a gallon? In the US, there are sixteen 8-ounce glasses in a gallon (128 / 8 = 16).
Is 64 oz equal to 1 gallon? No, 64 oz is equal to one half gallon in the US (128 oz / 2 = 64 oz).
How many 16.9 oz water bottles make a gallon? There are about 7.57 standard 16.9 oz water bottles in a US gallon (128 oz / 16.9 oz ≈ 7.57). You would need 8 standard bottles to exceed 1 gallon.
Is there a dry gallon? Yes, there used to be a dry gallon (used for dry good like grains), but the U.S. dry gallon is a rare entity nowadays. The U.S. dry gallon is around 4.40 liters and is not synced with the liquid gallon. Most people think of “ounces in gallon” as liquid ounces.
Conclusion
The real question of “How many ounces are in a gallon?” has one answer for users in the United States: 128 fluid ounces.
This simple conversion translates to countless daily tasks from cooking an meal inspried by a Rachael Ray recepie to managing your health with a beverage choice.
The Imperial system has a much larger gallon with 160 fluid ounces, but most daily uses in America will require you to use the US Customary systems gallon.
By understanding the differences and how to convert between measurements, you will strengthen your ability to make measurements accurately, whatever the task!