A US gallon contains 128 fluid ounces. That is 4 quarts, 8 pints or 16 cups, and about 3.785 liters. To convert gallons to fluid ounces, multiply by 128: 2 gallons is 256 fl oz, and half a gallon is 64 fl oz.
How many ounces are in a gallon?
NIST's conversion factors give the US gallon as 3.785412 liters and the US fluid ounce as 29.57353 milliliters. Dividing, 3,785.412 ÷ 29.57353 = 128.000. The fluid ounce here is a unit of volume, not weight; see how many ounces in a cup for the difference.
Gallon, quart, pint and cup conversion chart
| Unit | Fluid ounces | Cups | Pints | Quarts | Liters (approx.) |
|---|---|---|---|---|---|
| 1 fluid ounce | 1 | 1/8 | 1/16 | 1/32 | 0.0296 |
| 1 cup | 8 | 1 | 1/2 | 1/4 | 0.2366 |
| 1 pint | 16 | 2 | 1 | 1/2 | 0.4732 |
| 1 quart | 32 | 4 | 2 | 1 | 0.9464 |
| 1/2 gallon | 64 | 8 | 4 | 2 | 1.8927 |
| 1 gallon | 128 | 16 | 8 | 4 | 3.7854 |
A quart is a quarter of a gallon, which is why 4 quarts fill one. Converters exist for each step: gallons to quarts, gallons to cups and gallons to oz.
How many ounces are in common fractions of a gallon?
| Amount | Fluid ounces | Milliliters |
|---|---|---|
| 1 gallon | 128 | 3,785 |
| 3/4 gallon | 96 | 2,839 |
| 1/2 gallon | 64 | 1,893 |
| 1/4 gallon (1 quart) | 32 | 946 |
| 1/8 gallon (1 pint) | 16 | 473 |
| 1/16 gallon (1 cup) | 8 | 237 |
| 2 gallons | 256 | 7,571 |
| 5 gallons | 640 | 18,927 |
Worked examples
Example 1: filling bottles
How many 16.9 fl oz bottles fill a gallon? 128 ÷ 16.9 = 7.57 bottles: 7 full bottles (118.3 fl oz) with 9.7 fl oz left over. For 16 fl oz bottles it is exactly 8.
Example 2: a daily water goal
A goal of 64 fl oz per day is half a gallon, or 8 cups. A goal of one full gallon is 128 fl oz, or 16 cups. The gallons to liters converter gives the metric amount, 3.785 L.
Example 3: diluting a product per gallon
A label that calls for 2 fl oz of concentrate per gallon uses 2 ÷ 128 = 1/64 of the volume, or about 1.56% concentrate. For 5 gallons use 5 × 2 = 10 fl oz.
Example 4: converting to milliliters
2.5 gallons × 128 = 320 fl oz. 320 × 29.57353 = 9,463.5 mL, matching 2.5 × 3.785412 = 9.463 L.
What is the difference between a US gallon and an imperial gallon?
The US gallon is defined as 231 cubic inches: 231 × 2.54³ cm³ equals 3,785.41 mL. UK legislation (Weights and Measures Act 1985) defines the imperial gallon as exactly 4.54609 liters, which is 1.2009 times the US gallon. The imperial gallon is divided into 160 imperial fluid ounces of about 28.41 mL each, against 128 US fluid ounces of 29.57 mL. A gallon in a British recipe or fuel figure is therefore not the same as a gallon on a US label, and converting with the wrong one is a 20% error. The overview in metric vs imperial lists the other near-twin units.
Quick ways to remember
- 128 = 2⁷. Halving a gallon seven times gets to 1 fl oz: 64, 32, 16, 8, 4, 2, 1.
- Gallon → quarts → pints → cups doubles each step after the first: 4, 8, 16.
- Cup = 8 fl oz, pint = 16, quart = 32, gallon = 128: each unit after the cup doubles the one before.