How Many Ounces in a Gallon of Water? Ultimate Guide

How Many Ounces in a Gallon

How Many Ounces in a Gallon of Water? The answer to that question is very simple but to answer this one must know about this unit of “ounces” that it actually is. In which system is this unit used and what is the standard unit for measuring the volume of water! In which country in the world is this unit used?

The answer to the above question must be considered first before entering into the ounce and gallon comparison debate. Let’s discuss it all one by one.

Literally an ounce is used to express a small amount of something. According to the common system of measurement originating in England, ounces are basically units used to express different amounts such as mass, weight or volume.

The usual US and British government OUNCE is basically used as a unit of weight approximately equal to 28.3 g which is eleven of a pound of avoirdupois.

Mainly this unit is used in the United States to measure the weight of food portions, postal items, the density of cloth and other paper.

Currently a few ounces are used, mentioned below:

  • Troy ounces (used to measure the mass of precious metals)
  • Strength ounce (based on the usual 1 ounce under standard gravity)
  • Ounces of liquid (volume measure)


A U.S. gallon measures 3.785 liters and in ounces, one gallon of U.S. water equals 128 ounces of liquid.

And finally half of one gallon equals 64 ounces of liquid.

And here’s the final answer to the question we just started.
In today’s world, two different measurement systems are being followed:

  • United States measurement system
  • British Imperial measurement system


Measurement according to US fluid is:

  • 1 gallon according to US fluid is equal to 3.785 liters. • As we know 1 liter is equal to 33.814 ounces (US fluid). • 1 fluid gallon will be calculated as 3.785×33.814 which is equal to 128 ounces. • So, 1 US gallon is equal to the 128 ounces according to US fluid gallon. • With that ½ US gallon will be equal to the 64 ounces.


Measurement according to UK fluid is:

  • 1 gallon according to the UK fluid is equal to 4.54609 liters. • 1 liter is equal to the 31.1951 ounces in the UK fluid. • So,1 fluid gallon will be calculated as 4.54609×31.1951 which is equal to 160 ounces. • 1 UK gallon fluid is equal to the 160 ounces. • If we talk about ½ UK fluid gallon then it will be equal to 65 ounces.

Fluid ounce:

It is the measurement used to measure the US and measurements system related to imperial measurements system. In the calculation it is about 30 millimeters in the metric volume.

How Many Ounces in a Gallon of Water?

Gallons are a measure of volume used in the United States and several other countries. It equals 3.785411784 liters or 231 cubic inches. Many other countries use liters as well.

So answer this question how many ounces in a gallon of water? is 128 ounces. If there is a conversion required to pound into gallons then the number should be multiplied by 128 ounces of liquid.

128 ounces of liquid produces one gallon but that is found in U.S. liquid ounces. The system followed in the British imperial metric makes 160.36 ounces of liquid in one gallon of water.

US metric system and UK metric system

Until 1971 the imperial measurement system was used in the United States and Canada. Then the metric and SI systems state a new version for the measurement adopted by and overall in the whole work. Yet the United States still follows an older metric system where they measure in pounds rather than in kilograms. Although Canada has also declared a new system as standard but it is unacceptable. The reason is that America and Canada are bound to each other to do trade as well. Therefore, Canada remembers American metrics and most sellers quote measurements according to the American system or may mention both metrics on the packaging.

How many cups in a gallon?

  • There are 16 cups in a gallon of water as per US fluid ounce.
  • There 19.215 US cups in an imperial gallon
  • One US cup contains 8 ounces of water.

How Many Ounces in a Gallon; Table of gallon to ounce conversion

I’ve included two table of conversions to help you see how many ounces in a gallon – one using US and one using UK measurements.

Gallon USA Ounces Liters
1 128 3.785
2 256 7.571
3 384 11.356
4 512 15.142
5 640 18.927
6 768 22.712
7 896 26.498
8 1024 30.283
9 1152 34.069
10 1280 37.854
11 1408 41.64
12 1536 45.425
13 1664 49.21
14 1792 52.996
15 1920 56.781


Gallon UK Ounces Liters
1 160 4.546
2 320 9.092
3 480 13.638
4 640 18.184
5 800 22.73
6 960 27.277
7 1120 31.823
8 1280 36.369
9 1440 40.915
10 1600 45.461
11 1760 50.007
12 1920 54.553
13 2080 59.099
14 2240 63.645
15 2400 68.191



An ounce is a unit of weight used primarily in the U.S. and is considered to be the unit of volume of an ounce of liquid used to write a measurement. 128 ounces of liquid is found in one gallon of water. A healthy human daily water intake must consist of about 91-126 ounces.

Frequently Asked Questions

How many inches in a gallon?

1 gallon contains 231 cubic inches.

Gallon metric equivalent

1 gallon is equal to 0.00378541 cubic meters.

The weight of a gallon of water

A US gallon of water weighs 8.34 lbs or 3.78 kg

How many pounds is 1 gallon?

1 Gallon = 8.34 Lbs.

What is an imperial gallon?

British gallons used for liquid and liquid measurements are equivalent to 1.2 US gallons or 4.54 liters.

How many millimeters in a gallon?

One gallon is equal to 3785.41 millimeter.

How many liters in a gallon?

1 gallon is equal to 3.78541 liters

How many cups in a gallon?

1 gallon contains 16 cups.

How many grams in a gallon?

1 gallon = 3.785.41 gram of weight


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