Ounces in a gallon
One US gallon of water weighs approximately 133.53 ounces (oz), or 8.345 pounds (lb), or 3.788 kilograms (kg).
One imperial gallon of water weighs approximately 160.36 ounces (oz), or 10.022 pounds (lb), or 4.546 kilograms (kg).
If by "ounce", you are referring to fluid ounces, the relationships between the fluid ounce and the imperial and US customary gallons are as follows:
1 US fluid ounce = US gallons
1 imperial fluid ounce = = imperial gallons
There are many different definitions of a gallon. The US has a definition for both a liquid and a dry gallon. Above, we are referring to a liquid, rather than a dry gallon. The imperial gallon is the definition of the gallon used in the United Kingdom, as well as some other Commonwealth countries:
1 US gallon = 3.788 liters
1 imperial gallon = 4.54609 liters
Thus, how much a gallon of water weighs depends on a number of factors, including which definition of a gallon we are using.
Ounces to gallons conversion
The following converter can be used to convert fluid ounces to gallons, or gallons to fluid ounces. It can also determine the weight of water in ounces (the measurement of weight not fluid ounces) relative to the number of either imperial or US customary gallons it contains. Use the drop-down menus to choose what you would like to convert between, and enter the appropriate values.
Gallons and fluid ounces are units of volume in the imperial and US customary systems of measurement. The definition of a gallon, as well as the definition of the number of fluid ounces in a gallon differs in both of these systems. These relationships are mentioned above, and their conversions are based on those relationships.
How much does a gallon of water weigh?
We've answered this question above, but below is an explanation of how we get this answer. 1 liter of water weighs almost exactly 1 kilogram. This is because one of the past definitions of the kilogram was based on the mass of 1 cubic decimeter (which equals 1 liter) of water at the temperature of 3.98° C at sea level. Definitions of the kilogram have since changed, so the relationship is no longer exact, but 1 liter of water still weighs very close to 1 kg.
This is the relationship we use to estimate the weight of 1 gallon of water. Since there are 3.788 liters in a US gallon, 1 US gallon weighs approximately 3.788 kilograms. Likewise, since there are 4.54709 liters in an imperial gallon, 1 imperial gallon weighs 4.54709 kilograms.
It is worth noting that temperature affects the weight of water to a degree. The warmer the water, the less it weighs; the colder the water, the more it weighs. At 3.98° C water is the most dense. This is because colder water is more dense, and thus weighs more than warmer water. However, as an estimate, using a 1:1 relationship between liters and kilograms works well because we would need more extreme temperatures, which are unlikely to occur in everday situations, to significantly affect the weight of the water.