A US liquid gallon is exactly 3.785411784 liters.
Gallons to Fluid ounces Converter — gal to fl oz
Convert Gallon (gal) to Fluid ounce (fl oz) using the exact conversion factor (1 gallon = 128 fluid ounce). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Gallons to Fluid ounces converter
Converter
Volume Converter
Convert metric, US customary, imperial, cooking, dry, and industrial volume units.
Context values
Some unit families need one extra value for a physically accurate result.
A US fluid ounce is 1/128 of a US gallon.
Result = input x 0.003785411784 / 2.95735296e-5.
Volume uses the cubic meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Gallons to Fluid ounces
Gallon and Fluid ounce appear side by side at fuel pumps and on beverage labels, since the US measures liquids in gallons while most of the world uses liters. Gallon is typically associated with united States (fuel, beverage, and liquid measurement), while Fluid ounce is typically associated with united States (beverage, nutrition labeling).
Formula
fluid ounces = gallons × 128
This factor comes from each unit's defined relationship to the category's base unit: 1 gallon equals 0.003785411784 base units, and 1 fluid ounce equals 0.0000295735295625 base units, so dividing one by the other gives the direct gallon-to-fluid ounce factor of 128.
Simple example
1 gal × 128 = 128 fl oz
1 gallon = 128 fluid ounces.
Real-world example
10 gal × 128 = 1,280 fl oz
10 gallons = 1,280 fluid ounces.
Conversion table
| Gallon (gal) | Fluid ounce (fl oz) |
|---|---|
| 1 gal | 128 fl oz |
| 5 gal | 640 fl oz |
| 10 gal | 1,280 fl oz |
| 25 gal | 3,200 fl oz |
| 50 gal | 6,400 fl oz |
| 100 gal | 12,800 fl oz |
Reverse conversion: Fluid ounce to Gallon
128 fl oz × 0.0078125 = 1 gal
128 fluid ounces = 1 gallon.
gallons = fluid ounces × 0.0078125
For a page dedicated to this direction, see Fluid ounce to Gallon.
Gallon vs. Fluid ounce
| Aspect | Gallon | Fluid ounce |
|---|---|---|
| Family | US customary unit | US customary unit |
| Typical use | Fuel pump and vehicle fuel-economy measurement in the US | US beverage container sizing (soda, water bottles) |
| Where it's used | United States (fuel, beverage, and liquid measurement) | United States (beverage, nutrition labeling) |
Understanding the Gallon
Gallon (gal) measures volume. The US liquid gallon is defined as exactly 231 cubic inches, equal to 3.785411784 liters. It is classified as a US customary unit.
Where it's used: United States (fuel, beverage, and liquid measurement)
US gallon vs. imperial gallon. The US liquid gallon (3.785411784 L) and the British imperial gallon (4.54609 L) are different units with the same name; always confirm which one a source is using.
Uses of the Gallon today
- Fuel pump and vehicle fuel-economy measurement in the US
- Beverage and liquid product packaging in the US
- Cooking and recipe measurement in US customary units
History of the Gallon
The US gallon descends from the English wine gallon of 231 cubic inches, a standard used for measuring wine in medieval and early modern England, which the American colonies inherited and retained even after England moved to a different imperial gallon in 1824.
England's 1824 Weights and Measures Act replaced the old wine gallon with a new imperial gallon (based on 10 pounds of water), but the US kept the older 231-cubic-inch definition — which is why the US gallon (3.785 L) and the imperial gallon (4.546 L) differ by about 20% today.
Historical origin. Gallon was not the invention of a single named person; it developed through the process described above.
Understanding the Fluid ounce
Fluid ounce (fl oz) measures volume. The US fluid ounce is 1/128 of a US gallon: exactly 29.5735295625 milliliters. It is classified as a US customary unit.
Where it's used: United States (beverage, nutrition labeling)
Uses of the Fluid ounce today
- US beverage container sizing (soda, water bottles)
- US nutrition and medication dosing labels
History of the Fluid ounce
The US fluid ounce follows from the same English wine-gallon system as the rest of the US liquid volume units, defined as a consistent fraction of the gallon.
Historical origin. Fluid ounce was not the invention of a single named person; it developed through the process described above.
Gallon — sources
- NIST Handbook 44, Appendix C — General tables of units of measurement, including the US gallon definition.
Fluid ounce — sources
- NIST Handbook 44, Appendix C — General tables of units including the US fluid ounce definition.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many fluid ounces are in 1 gallon?
1 gallon equals 128 fluid ounces, using the exact defined conversion factor rather than an estimate.
How do I convert gallon to fluid ounce?
Multiply the gallon value by 128. The converter above does this instantly to whatever precision you set.
How do I convert fluid ounce back to gallon?
Use the reverse factor: 1 fluid ounce equals 0.0078125 gallons. You can also use the swap control in the converter above to flip the direction instantly.
What is a gallon?
Gallon (gal) measures volume. The US liquid gallon is defined as exactly 231 cubic inches, equal to 3.785411784 liters. It is classified as a US customary unit.
What is a fluid ounce?
Fluid ounce (fl oz) measures volume. The US fluid ounce is 1/128 of a US gallon: exactly 29.5735295625 milliliters. It is classified as a US customary unit.
Is the gallon to fluid ounce conversion exact?
Yes. The US fluid ounce is 1/128 of a US gallon: exactly 29.5735295625 milliliters. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
What is the difference between us gallon vs. imperial gallon?
The US liquid gallon (3.785411784 L) and the British imperial gallon (4.54609 L) are different units with the same name; always confirm which one a source is using.
Where is the gallon still used today?
United States (fuel, beverage, and liquid measurement)