Depending on where you are in the globe, the answer to your question about how many fluid ounces (oz) are in a gallon of water varies. The liquid gallon in the United States is smaller (3.785 litres) than the imperial gallon in the United Kingdom (4.546 litres). Similarly, the fluid ounce in the United States is 29.5735 millilitres. The fluid ounce in the United Kingdom (imperial) is 28.4131mL.
- 1 Gallon (US, Fluid) = 128 Ounces (US, Fluid)
- 1 Gallon (UK, Fluid) = 160 Ounces (UK, Fluid)