Units of Measure / How Many OZ in a Gallon

How Many OZ in a Gallon

Gallons1 and ounces2 are non-metric units of measuring liquid substances. Nowadays they are in use mostly in the USA and Australia. It is rather easy to convert gallons to ounces and vice versa. The question to the answer, “How many OZ are in a gallon?” is that one gallon has 128 ounces.

But this answer is correct only if you mean US liquid gallons and US fluid ounces. And there are some other definitions of these units, so you should know the most important of them in order not to get confused.

Gallons and Ounces

First of all, you should understand that now we are talking about fluid ounces which are units of volume. The US fluid ounce equals to about 29.6 milliliters, and the imperial fluid ounce equals to about 28.4 milliliters.

Currently, there are two main definitions of the gallon: the imperial gallon (approximately 4.54 liters) and the US liquid gallon (approximately 3.78 liters). There is also the US dry gallon (approximately 4.4 liters), but it is a rare unit.

And the number of ounces in a gallon is different in the imperial and the US systems:

It is not difficult to remember. But what should you do, if you need to know how many imperial ounces there are in some number of US gallons? Such calculations will be too complex for most people. You can consult our table that we have composed to help you. If necessary, use a calculator.

But converting units from different systems is not so easy. We have composed the special table that will help you to do it, though you may need a calculator to perform this task.

One US liquid gallon
One imperial gallon
US fluid ounces
Imperial fluid ounces

So, if you want to calculate how many US fluid ounces are in five imperial gallons, you must multiply 153.72 (154 for rough estimation) by 5.


  1. The gallon is a non-metric liquid measurement unit accepted in many countries. The US liquid gallon (about 3.785 liters) and the imperial gallon (about 4.54 liters) are the most common definitions of this unit. The US gallon consists of 128 US fluid ounces, and the imperial gallon consists of 160 imperial fluid ounces.
  2. The Ounce (abbr. oz) is a widely-spread non-metric unit of mass or volume (fluid ounce), which is used in various systems and has several definitions. The US fluid ounce equals to about 29.6 milliliters, and the imperial fluid ounce equals to about 28.4 milliliters.
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