Convert US gallons to fluid ounces instantly. One gallon is exactly 128 fl oz, so every result is exact. Free, with no sign-up required.
Quick Answer: One US gallon equals exactly 128 fluid ounces. To convert gallon to oz, multiply the number of gallons by 128. Common values: 0.25 gal = 32 fl oz · 0.5 gal (half gallon) = 64 fl oz · 1 gal = 128 fl oz · 2 gal = 256 fl oz · 5 gal = 640 fl oz.
Formula: fl oz = gallons × 128
Need to go from fluid ounces back to gallons? Our oz to gallon converter handles the reverse calculation.
Fluid Ounces = Gallons × 128
Gallons = Fluid Ounces ÷ 128
One US liquid gallon is defined as exactly 128 US fluid ounces, so the gallon to oz conversion is a whole-number relationship with no rounding. To convert gallons to ounces, multiply by 128: 2.5 gal × 128 = 320 fl oz, and 3.2 gal × 128 = 409.6 fl oz. Dividing by 128 reverses the calculation. For the reverse, visit our oz to gallon calculator.
Use this gallon to oz table to find any whole-gallon amount, from a single gallon up to a 100-gallon tank. The cups column uses the US customary cup of 8 fl oz. For values that are not listed, multiply the number of gallons by 128.
| Gallons (gal) | Fluid Ounces (fl oz) | US Cups | Common Reference |
|---|---|---|---|
| 0.125 gal | 16 fl oz | 2 cups | 1 pint |
| 0.25 gal | 32 fl oz | 4 cups | 1 quart |
| 0.333 gal | 42.67 fl oz | 5.3 cups | — |
| 0.5 gal | 64 fl oz | 8 cups | half gallon |
| 0.75 gal | 96 fl oz | 12 cups | 3/4 gallon |
| 1 gal | 128 fl oz | 16 cups | 1 gallon jug |
| 1.5 gal | 192 fl oz | 24 cups | — |
| 2 gal | 256 fl oz | 32 cups | 2 gallons |
| 2.5 gal | 320 fl oz | 40 cups | — |
| 3 gal | 384 fl oz | 48 cups | 3-gallon jug |
| 4 gal | 512 fl oz | 64 cups | — |
| 5 gal | 640 fl oz | 80 cups | water cooler jug |
| 6 gal | 768 fl oz | 96 cups | — |
| 10 gal | 1,280 fl oz | 160 cups | — |
| 20 gal | 2,560 fl oz | 320 cups | 20-gallon aquarium |
| 25 gal | 3,200 fl oz | 400 cups | — |
| 30 gal | 3,840 fl oz | 480 cups | — |
| 55 gal | 7,040 fl oz | 880 cups | standard drum |
| 100 gal | 12,800 fl oz | 1,600 cups | — |
Recipes, labels, and containers often use fractions of a gallon rather than whole numbers. The gallon divides evenly into halves, quarters, and eighths, so these values map directly to common US containers and measuring cups.
A half gallon equals 64 fluid ounces (0.5 × 128 = 64). The answer is the same whether it is written as 1/2 gallon, 0.5 gal, or .5 gallon. Half of a half gallon is a quarter gallon, or 32 fl oz.
A quarter gallon is 32 fl oz, an eighth of a gallon is 16 fl oz, and three quarters of a gallon is 96 fl oz. Smaller fractions follow the same rule: 1/10 gallon is 12.8 fl oz, and 1/32 gallon is 4 fl oz.
| Fraction of Gallon | Gallons | fl oz | US Name |
|---|---|---|---|
| 1/16 gallon | 0.0625 | 8 fl oz | 1 cup |
| 1/8 gallon | 0.125 | 16 fl oz | 1 pint |
| 1/4 gallon | 0.25 | 32 fl oz | 1 quart |
| 1/2 gallon | 0.5 | 64 fl oz | half gallon |
| 3/4 gallon | 0.75 | 96 fl oz | 3/4 gallon |
| 1 gallon | 1.0 | 128 fl oz | 1 gallon |
For decimal amounts, multiply by 128 and keep the decimals in the answer. Mixed amounts work the same way: 1 1/2 gallons is 192 fl oz, and 2 1/2 gallons is 320 fl oz.
| Decimal Gallons | Fraction | Fluid Ounces (fl oz) |
|---|---|---|
| 0.1 gal | 1/10 | 12.8 fl oz |
| 0.2 gal | 1/5 | 25.6 fl oz |
| 0.3 gal | 3/10 | 38.4 fl oz |
| 0.4 gal | 2/5 | 51.2 fl oz |
| 0.5 gal | 1/2 | 64 fl oz |
| 0.6 gal | 3/5 | 76.8 fl oz |
| 0.7 gal | 7/10 | 89.6 fl oz |
| 0.8 gal | 4/5 | 102.4 fl oz |
| 0.9 gal | 9/10 | 115.2 fl oz |
For the full hierarchy of cups, pints, quarts, and gallons all mapped together, see our liquid measurement conversion chart.
A US gallon holds 128 fluid ounces. It breaks down into 4 quarts, 8 pints, 16 cups, or 128 fl oz, so every common container size is a clean multiple of the same unit. The answer is the same for a gallon of water, milk, juice, oil, or gasoline, because fluid ounces measure volume.
The US liquid gallon is defined as 231 cubic inches, which is about 3.785 liters. Each step down halves the volume: a quart is a quarter of a gallon, a pint is half a quart, a cup is half a pint, and a US cup holds 8 fl oz. That gives 4 × 2 × 2 × 8 = 128 fluid ounces, and one US fluid ounce is about 29.57 milliliters.
Fluid ounces measure volume, while ounces on a kitchen scale measure weight, and the two are not interchangeable. This gallon to oz conversion always uses fluid ounces. A gallon of water weighs about 8.34 pounds, or roughly 133 ounces by weight, even though it measures 128 fl oz. Heavier or lighter liquids change the weight of a gallon but never its volume.
Yes. A gallon is much larger, and one fluid ounce is only 1/128 of a gallon (0.0078125 gal). That makes size comparisons easy: 16 fl oz is an eighth of a gallon, and 88 fl oz is 0.6875 gallons, which is 40 fl oz short of a full gallon.
One of the most common reasons to use a gallon to oz conversion is counting water bottles toward a daily gallon goal. Here are the exact counts for every common bottle size:
| Bottle Size | Bottles per Gallon | Total oz | Notes |
|---|---|---|---|
| 8 fl oz | 16 bottles | 128 fl oz | small / kids' bottle |
| 12 fl oz | 10.67 bottles | 128 fl oz | soda-can size |
| 16 fl oz | 8 bottles | 128 fl oz | standard bottle |
| 16.9 fl oz | ~7.57 bottles | 128 fl oz | 500 ml standard |
| 20 fl oz | 6.4 bottles | 128 fl oz | vending machine |
| 24 fl oz | 5.33 bottles | 128 fl oz | sport bottle |
| 32 fl oz | 4 bottles | 128 fl oz | Nalgene / 1 quart |
| 40 fl oz | 3.2 bottles | 128 fl oz | large tumbler |
| 64 fl oz | 2 bottles | 128 fl oz | half-gallon jug |
For a full breakdown of water bottle math and hydration tracking, see our dedicated water bottles per gallon guide.
Gallons appear across a wide range of everyday products. Knowing the fl oz equivalent of each standard container size removes guesswork in the kitchen, garage, and garden.
| Container / Product | Gallons | Fluid Ounces | Context |
|---|---|---|---|
| Milk jug (half gallon) | 0.5 gal | 64 fl oz | US grocery standard |
| Milk jug (full gallon) | 1 gal | 128 fl oz | most common US size |
| Juice jug | 1 gal | 128 fl oz | common bulk size |
| Laundry detergent | 1–2 gal | 128–256 fl oz | large economy bottles |
| Water cooler jug | 5 gal | 640 fl oz | office water dispenser |
| Paint can (large) | 1 gal | 128 fl oz | standard wall paint |
| Gasoline can | 1–5 gal | 128–640 fl oz | portable fuel cans |
| Industrial drum | 55 gal | 7,040 fl oz | standard US barrel |
Two-stroke fuel and many concentrates are mixed by ratio, with the oil or concentrate measured in fluid ounces and the fuel or water measured in gallons. Divide 128 by the ratio to get the ounces needed per gallon, then multiply by the number of gallons. At 50:1, a 10-gallon batch needs 10 × 128 ÷ 50 = 25.6 fl oz of oil. Always follow the ratio the manufacturer specifies.
| Mix Ratio | Per 1 Gallon | Per 5 Gallons | Per 10 Gallons |
|---|---|---|---|
| 32:1 | 4 fl oz | 20 fl oz | 40 fl oz |
| 40:1 | 3.2 fl oz | 16 fl oz | 32 fl oz |
| 50:1 | 2.56 fl oz | 12.8 fl oz | 25.6 fl oz |
| 100:1 | 1.28 fl oz | 6.4 fl oz | 12.8 fl oz |
Both the US and UK use the word "gallon" but the volumes are not the same. The UK imperial gallon is larger, and the UK fluid ounce is slightly smaller than the US one, so the ounce counts differ too. The difference affects fuel economy ratings, beverage imports, and any cross-border recipe that specifies gallons.
| Measure | US Gallon | UK Imperial Gallon | Difference |
|---|---|---|---|
| In fluid ounces | 128 US fl oz | 160 UK fl oz | UK counts 32 more (smaller) ounces |
| In US fluid ounces | 128 US fl oz | ~153.72 US fl oz | UK gallon holds ~25.7 US fl oz more |
| In liters | 3.785 L | 4.546 L | UK is ~20% larger |
| In milliliters | 3,785 ml | 4,546 ml | UK holds 761 ml more |
| In quarts | 4 US qt | 4 UK qt | same count, different volume |
There is no separate metric gallon: a gallon is either a US customary unit or a British imperial unit. Older Canadian recipes, equipment specs, and fuel records that mention gallons usually refer to the imperial gallon, which is 160 imperial fl oz.
All calculations on this page use the US liquid gallon. If you need metric conversions, our oz to liter converter handles those precisely.
The US gallon and fluid ounce definitions are maintained by the National Institute of Standards and Technology (NIST) — the official US measurement authority.
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