Gallons to Fluid Ounces
An instant gallons-to-fluid ounces converter, with the formula and a reference table below.
| Gallons | Fluid Ounces |
|---|---|
| 1 gal | 128 fl oz |
| 2 gal | 256 fl oz |
| 3 gal | 384 fl oz |
| 5 gal | 640 fl oz |
| 10 gal | 1280 fl oz |
| 25 gal | 3200 fl oz |
| 50 gal | 6400 fl oz |
| 100 gal | 12800 fl oz |
How to use
- Enter a value in the Gallons field.
- The Fluid Ounces figure appears immediately — no button to press.
- Typing in the Fluid Ounces field converts back to Gallons the same way.
- Use the reference table for the values that come up most often.
Gallons to Fluid Ounces formula
Converting gallons to fluid ounces is a single multiplication by 128.
fl oz = gal × 128
Put another way, one gallon is exactly 128 fluid ounces.
Why this factor: the gallon and the fluid ounce are both defined against the liter, so dividing one definition by the other gives 128 directly. Both definitions are set out in full below.
To go the other way, divide by 128 — or use the Fluid Ounces to Gallons converter, which is the same calculation reversed.
Worked example
Take 50 gal and multiply by the factor 128: the result is 6400 fl oz.
50 gal × 128 = 6400 fl oz
The converter above does this for any value you type, in either direction.
What is a gallon?
A US liquid gallon is the standard large-volume unit in the United States.
Definition: Defined as exactly 231 cubic inches, which works out to exactly 3.785411784 liters.
Where it is used: Fuel sales, milk and beverage containers, and paint in the US.
Worth knowing: The imperial gallon used in the UK and Canada is about 20% larger, at 4.54609 L — so 'miles per gallon' figures are not comparable between the two.
What is a fluid ounce?
A US fluid ounce is a small liquid volume, one eighth of a cup.
Definition: Exactly one 128th of a US liquid gallon, so exactly 29.5735295625 milliliters.
Where it is used: Beverage cans and bottles, cosmetics, and small liquid quantities in the US.
Worth knowing: Measures volume, not weight — a fluid ounce of water weighs about 1.04 avoirdupois ounces. The imperial fluid ounce is 28.41 mL.
Gallons vs Fluid Ounces
The gallon and the fluid ounce belong to the same family of units, so converting between them is usually a matter of shifting a decimal point or applying a small whole-number factor.
Frequently asked questions
How many fluid ounces are in a gallon?
One gallon equals 128 fluid ounces.
What is the formula to convert gallons to fluid ounces?
Multiply the number of gallons by 128. To reverse it, divide by 128 instead.
How do I convert fluid ounces back to gallons?
Divide by 128, which is the same as multiplying by 0.0078125.
How many fluid ounces are in 10 gallons?
10 gal works out to 1280 fl oz.
Is there anything to watch out for with gallons?
The imperial gallon used in the UK and Canada is about 20% larger, at 4.54609 L — so 'miles per gallon' figures are not comparable between the two.
How is the gallon defined?
Defined as exactly 231 cubic inches, which works out to exactly 3.785411784 liters.