The answer to your question of how many ounces (oz) there are in a gallon of water depends on where in the world you are. You see, the US liquid gallon measures less (3.785 liters) than the UK imperial gallon (4.546 litres). You should probably bear this in mind when thinking about daily water intake because there’s a big difference between the two (you could be drinking 3/4 of a litre more water if you use the UK imperial gallon conversion.
History of Gallons:
The gallon originated as the base of systems for measuring wine and beer in England. The sizes of gallon used in these two systems were different from each other: the first was based on the wine gallon (equal in size to the US gallon), and the second one either the ale gallon or the larger imperial gallon.
In 1824, Britain adopted a close approximation to the ale gallon knownas the Imperial gallon. The Imperial gallon is based on the volume of10 pounds of distilled water weighed in air with the barometerstanding at 30 inches and at a temperature of 62 degrees Fahrenheit(and in 1963 this definition was refined as the space occupied by 10pounds of distilled water of density 0.998859 g/ml weighed in air ofdensity 0.001217 g/ml against weights of density 8.136 g/ml). This works out at 277.41945 cubic inches.
By the end of the 18th century, there were three definitions of the gallon in common use:
The corn gallon, or Winchester gallon, of about 268.8 cubic inches (≈ 4.405 L),
The wine gallon, or Queen Anne’s gallon, which was 231 cubic inches (≈ 3.785 L)
The ale gallon of 282 cubic inches (≈ 4.622 L).
The United States by this time, had already standardised on a closeapproximation to the old wine gallon. It was at one time defined as the volume of a cylinder 6 inches long and 7 inches in diameter, or230.907 cubic inches. Today, however, the gallon is 231 cu. ins exactly. Thus 10 US gallons equals 8.327 Imperial gallons. TheImperial gallon is about a fifth larger than the US gallon. Both the Imperial and United States gallon are equal to 8 pints. However in the US a pint is 16 fluid ounces whereas an Imperial pintis 20 fluid ounces. Thus a U.S. gallon is 128 fl. oz and an Imperialgallon is 160 fl. oz; this means that a US fluid ounce is around 1.8047 cu. ins and an Imperial fl. oz is around 1.7339 cu. ins. The US fluid ounce is actually bigger than the imperial, although the US gallon is smaller.
Types of Gallons:
So finally we have:
| 1 imperial gallon | = 4.54609 liters (exactly) |
|---|---|
| = 8 imperial pints | |
| ≈ 1.20 U.S. wet gallons | |
| ≈ 1.03 U.S. dry gallons | |
| = 10 pounds of pure water at 17° Celsius | |
| 1 U.S. wet gallon | = 231 cubic inches = 3 in × 7 in × 11 in (This is the gallon commonly used in the US) |
| = 128 U.S. fl. oz. | |
| = 8 U.S. wet pints | |
| = 1⁄42 of a U.S. oil barrel | |
| = 3.785411784 litres (exactly) | |
| ≈ 0.83 imperial gallons | |
| ≈ 0.86 U.S. dry gallons | |
| ≈ 8.33 pounds of pure water at room temperature | |
| 1 U.S. dry gallon | = 268.8025 cubic inches (Occasionally used in the US.) |
| = 8 U.S. dry pints | |
| = 1⁄8 of a U.S. bushel | |
| = 4.40488377086 litres (exactly) | |
| ≈ 0.97 imperial gallons | |
| ≈ 1.16 U.S. wet gallons |