The answer to this question really depends on your location since the measurement units are different in different parts of the world.

If we take the International System of Units or SI Units into account then we can determine the answer to How many ounces in a gallon. SI units are nearly universal and accepted worldwide.

In the US, a gallon is 4 quarts, which makes it equal to 128 fluid ounces or 3.785 liters. A quart is an American unit of measure that equals 1/4 of a gallon.

Before we jump into the measurements let’s see what is a gallon?

When we talk about gallons the first thing that comes to our mind is a water Gallon. But you will be surprised to know that Gallon is a unit measurement first used by the United States of America to measure gasoline.

According to that

1 US Gallon is 3.78 Litres (same in Latin America and the Caribbean)

1 Imperial Gallon is 4.5 Litres( Mostly used in the UK and Canada)

How many Ounces in a Gallon of Water?

So, now we know the volume of a gallon of water in terms of the liter, let’s measure it in terms of Ounces.

Before that, we must know what is an Ounce?

What is an Ounce?

An ounce (Oz) is a Unit of measurement used to measure the mass, weight, and volume of any physical substance whether it’s in a liquid or solid-state. For example, we can use ounces to measure Gold, measure force, and measure any liquid.

For more clarifications:

Unit of Measurement	Units	Example
Mass	34.7 µm per Ounce of Mass	The thickness of Copper in a Printed Circuit Board. You can measure the mass of copper in Ounces
Weight	28.34 Grams per Ounce in Weight	The ounce is used to measure the weight of Grains and fabrics.
Volume	29.6 ml per ounce in Volume	The Ounce is used to measure the volume of any liquid like water, gasoline, and more.

A Fluid Ounce is equal to 29.6 ML (milliliter) in US metric System.

A Fluid Ounce is equal to 28.54 ML (milliliter) in the Imperial metric System(the UK and Canada).

Now we know the volume of a Gallon and the units of an Ounces, let’s calculate How many Ounces are there in a gallon of water?

A Gallon is equal to 3.78 liters and 1 Ounce is equal to 29.6 ml. So a gallon contains 3.78 L/29.6 ml= 3780 ml/29.6 ml=127.7 ≃ 128 Ounces(Oz).

So A gallon is 128 Ounces(oz).

Let’s Look at the US Gallon Conversion Table to different Measurement Units- The US System

Volume	Measurement
1 US Gallon	128 US Ounces(oz)
1 US Gallon	3.78 Litres
1 US Gallon	3785 Milliliters
1 US Gallon	256 US Tablespoons
1 US Gallon	768 US Teaspoons
1 US Gallon	0.0037 Cubic Meter
1 US Gallon	4 Quart
1 US Gallon	0.83 Imperial Gallon
1 US Gallon	3.33 Imperial Quart
1 US Gallon	6.66 Imperial Pint
1 US Gallon	13.32 Imperial Cup
1 US Gallon	133 Imperial Ounce
1 US Gallon	213.16 Imperial Table Spoon
1 US Gallon	639.49 imperial Teaspoon
1 US Gallon	0.13 Cubic Foot
1 US Gallon	231 Cubic inch

Let’s Look at the Imperial Gallon Conversion Table to other Measurement Units- The Uk and Canada System

Volume	Measurement
1 Imperial Gallon	153 US Ounces(oz)
1 Imperial Gallon	4.54 Litres
1 Imperial Gallon	4546 Milliliters
1 Imperial Gallon	307.44 US Tablespoons
1 Imperial Gallon	922.31 US Teaspoons
1 Imperial Gallon	0.0045 Cubic Meter
1 Imperial Gallon	4 Quart
1 Imperial Gallon	1.20 US Gallon
1 Imperial Gallon	4 Imperial Quart
1 Imperial Gallon	8 Imperial Pint
1 Imperial Gallon	16 Imperial Cup
1 Imperial Gallon	160 Imperial Ounce
1 Imperial Gallon	256 Imperial Table Spoon
1 Imperial Gallon	768 imperial Teaspoon
1 Imperial Gallon	0.16 Cubic Foot
1 Imperial Gallon	277.42 Cubic inch

Ounces to Gallon Conversion in US System and UK System

	The US System		The UK System
Gallon	Ounces	Gallon	Ounces
1	128	1	160
2	256	2	320
3	384	3	480
4	512	4	640
5	640	5	800
6	768	6	960
7	896	7	1120
8	1024	8	1280
9	1152	9	1440
10	1280	10	1600