How Many Ounces (Oz or Fl Oz) Are in a Gallon?
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Conversion Formulas
When somebody asks how many ounces are in a gallon, note that a gallon is a unit of volume, hence the ounces are also 'fluid ounces'  very often labeled as 'fl oz'. Also, there are US Gallon, US Fluid Ounce, Imperial Gallon, and Imperial Fluid Ounce. Their relations are:
1 US Fluid Ounce (fl oz) = 1/128 US Liquid Gallon = 29.5735 milliliters
1 US Liquid Gallon = 128 US Fluid Ounces (fl oz) = 3.78541 liters
1 Imp Fluid Ounce = 1/160 Imp Liquid Gallon = 28.4131 milliliters
1 Imp Liquid Gallon = 160 Imp Fluid Ounces = 4.54609 liters
In order to convert volume given in gallons into volume given in ounces, use these formulas:
V_{(US fl oz)} = 128 * V_{(US gallon)}
V_{(Imp fl oz)} = 160 * V_{(Imp gallon)}
How Many Ounces Are in a Half Gallon of Water?
If You have a half gallons (US/Imp) of water, and want to know how many ounces is that, math is very simple:
V(US fl oz) = 128 * V(US gallon) = 128 * 0.5 = 64 US fl oz = 1892.7 ml = 1.8927 liters
V(Imp fl oz) = 160 * V(Imp gallon) = 160 * 0.5 = 80 Imp fl oz = 2273.0 ml = 2.273 liters
Similarly, if You have 2 gallons (US/Imp) that would be:
V_{(US fl oz)} = 128 * V_{(US gallon)} = 128 * 2 = 256 US fl oz = 7570.816 ml = 7.570816 liters
V_{(Imp fl oz)} = 160 * V_{(Imp gallon)} = 160 * 2 = 320 Imp fl oz = 9092.192 ml = 9.092192 liters
Ounces to Gallons and Gallons to Ounces Conversion Calculator
In order to convert ounces to gallons and gallons to ounces, feel free to use this conversion calculator  write the value that You have and click 'Calculate' to convert it:
US Fluid Ounces to US Gallons 
US Fluid Ounces: US Gallons: 
US Gallons to US Fluid Ounces 
US Gallons: US Fluid Ounces: 
Imperial Fluid Ounces to Imperial Gallons 
Imperial Fluid Ounces: Imperial Gallons: 
Imperial Gallons to Imperial Fluid Ounces 
Imperial Gallons: Imperial Fluid Ounces: 
Ounces to Gallons Conversion Charts
Here are some ounces to gallons quick conversion charts to aid You with units' conversions:

 


For more values, please, use the conversion calculators.