Oz to Gallon Converter:
Convert Fluid Ounces to US Gallons
128 fluid ounces equal 1 US gallon. To convert ounces to gallons, divide the number of fluid ounces by 128. This page uses US customary units, so "oz" here means the US fluid ounce (fl oz) and "gallon" means the US liquid gallon. Want to convert gallons back to oz? Check our gallon to oz converter.
How Many Ounces Are in a Gallon?
A US gallon holds 128 fluid ounces. NIST lists the 231 cubic inch US gallon as equal to 4 quarts and 128 fluid ounces.
A US fluid ounce (fl oz) is 1/16 of a US pint, or about 29.57 milliliters. A US gallon (gal) is about 3.785 liters. These are the standard symbols used by NIST.
| Unit | Fluid ounces |
|---|---|
| 1 gallon | 128 fl oz |
| 1/2 gallon | 64 fl oz |
| 1 quart | 32 fl oz |
| 1 pint | 16 fl oz |
| 1 cup | 8 fl oz |
For a quick estimate, count in quarters and eighths. Every 32 fl oz is a quarter gallon and every 16 fl oz is an eighth, so 48 fl oz is three eighths, or 0.375 gallons.
How to Convert Ounces to Gallons
- Check that the amount is in US fluid ounces, not weight ounces.
- Divide by 128.
- Round to match the precision of your starting number.
gallons = fluid ounces ÷ 128
To go the other way, multiply gallons by 128, or convert gallons to fluid ounces with our tool.
fluid ounces = gallons × 128
Examples:
- 15 fl oz: 15 ÷ 128 = 0.1171875 gallons
- 90 fl oz: 90 ÷ 128 = 0.703125 gallons
- 500 fl oz: 500 ÷ 128 = 3.90625 gallons
- 2.5 gallons: 2.5 × 128 = 320 fl oz
Because 128 is a power of 2, every whole number of ounces gives a terminating decimal of at most seven places. When you chain several steps, round only at the end. NIST's guidance is to keep a number of significant digits consistent with the original measurement, so a two-digit amount like 15 fl oz is fine as 0.12 gallons.
Gallons and Ounces Together
A whole number plus a remainder often reads better than a decimal. Divide by 128, keep the whole number as gallons, and treat what is left as ounces.
- 300 fl oz: 300 ÷ 128 gives 2 gallons (256 fl oz) with 44 fl oz left over. That is 2 gallons and 44 fl oz.
- 1 gallon plus 40 fl oz: 40 ÷ 128 = 0.3125, so the total is 1.3125 gallons.
Oz to Gallon Conversion Chart
Common Amounts
| Fluid ounces | Gallons | Fraction of a gallon | Gallon filled |
|---|---|---|---|
| 8 fl oz | 0.0625 | 1/16 | |
| 16 fl oz | 0.125 | 1/8 | |
| 24 fl oz | 0.1875 | 3/16 | |
| 32 fl oz | 0.25 | 1/4 | |
| 48 fl oz | 0.375 | 3/8 | |
| 64 fl oz | 0.5 | 1/2 | |
| 96 fl oz | 0.75 | 3/4 | |
| 128 fl oz | 1 | 1 | |
| 192 fl oz | 1.5 | 1 1/2 | |
| 256 fl oz | 2 | 2 | |
| 384 fl oz | 3 | 3 | |
| 512 fl oz | 4 | 4 | |
| 640 fl oz | 5 | 5 | |
| 1,000 fl oz | 7.8125 | 7 13/16 |
Bars show how much of one US gallon is filled. A full bar with a × label means more than one gallon.
Every Ounce from 1 to 100
The values are exact, not rounded.
| fl oz | Gallons | fl oz | Gallons |
|---|---|---|---|
| 1 | 0.0078125 | 51 | 0.3984375 |
| 2 | 0.015625 | 52 | 0.40625 |
| 3 | 0.0234375 | 53 | 0.4140625 |
| 4 | 0.03125 | 54 | 0.421875 |
| 5 | 0.0390625 | 55 | 0.4296875 |
| 6 | 0.046875 | 56 | 0.4375 |
| 7 | 0.0546875 | 57 | 0.4453125 |
| 8 | 0.0625 | 58 | 0.453125 |
| 9 | 0.0703125 | 59 | 0.4609375 |
| 10 | 0.078125 | 60 | 0.46875 |
| 11 | 0.0859375 | 61 | 0.4765625 |
| 12 | 0.09375 | 62 | 0.484375 |
| 13 | 0.1015625 | 63 | 0.4921875 |
| 14 | 0.109375 | 64 | 0.5 |
| 15 | 0.1171875 | 65 | 0.5078125 |
| 16 | 0.125 | 66 | 0.515625 |
| 17 | 0.1328125 | 67 | 0.5234375 |
| 18 | 0.140625 | 68 | 0.53125 |
| 19 | 0.1484375 | 69 | 0.5390625 |
| 20 | 0.15625 | 70 | 0.546875 |
| 21 | 0.1640625 | 71 | 0.5546875 |
| 22 | 0.171875 | 72 | 0.5625 |
| 23 | 0.1796875 | 73 | 0.5703125 |
| 24 | 0.1875 | 74 | 0.578125 |
| 25 | 0.1953125 | 75 | 0.5859375 |
| 26 | 0.203125 | 76 | 0.59375 |
| 27 | 0.2109375 | 77 | 0.6015625 |
| 28 | 0.21875 | 78 | 0.609375 |
| 29 | 0.2265625 | 79 | 0.6171875 |
| 30 | 0.234375 | 80 | 0.625 |
| 31 | 0.2421875 | 81 | 0.6328125 |
| 32 | 0.25 | 82 | 0.640625 |
| 33 | 0.2578125 | 83 | 0.6484375 |
| 34 | 0.265625 | 84 | 0.65625 |
| 35 | 0.2734375 | 85 | 0.6640625 |
| 36 | 0.28125 | 86 | 0.671875 |
| 37 | 0.2890625 | 87 | 0.6796875 |
| 38 | 0.296875 | 88 | 0.6875 |
| 39 | 0.3046875 | 89 | 0.6953125 |
| 40 | 0.3125 | 90 | 0.703125 |
| 41 | 0.3203125 | 91 | 0.7109375 |
| 42 | 0.328125 | 92 | 0.71875 |
| 43 | 0.3359375 | 93 | 0.7265625 |
| 44 | 0.34375 | 94 | 0.734375 |
| 45 | 0.3515625 | 95 | 0.7421875 |
| 46 | 0.359375 | 96 | 0.75 |
| 47 | 0.3671875 | 97 | 0.7578125 |
| 48 | 0.375 | 98 | 0.765625 |
| 49 | 0.3828125 | 99 | 0.7734375 |
| 50 | 0.390625 | 100 | 0.78125 |
Gallons to Ounces Chart
| Gallons | Fluid ounces |
|---|---|
| 1/16 gallon | 8 fl oz |
| 1/8 gallon | 16 fl oz |
| 1/4 gallon | 32 fl oz |
| 1/3 gallon | about 42.67 fl oz |
| 1/2 gallon | 64 fl oz |
| 3/4 gallon | 96 fl oz |
| 1 gallon | 128 fl oz |
| 1.5 gallons | 192 fl oz |
| 2 gallons | 256 fl oz |
| 5 gallons | 640 fl oz |
| 8 gallons | 1,024 fl oz |
Fluid Ounces vs Weight Ounces
The word "ounce" names two different units. The fluid ounce measures volume, about 29.57 milliliters in the US system. The ounce used for weight is the avoirdupois ounce, about 28.35 grams. NIST lists them as separate units, and FDA's food labeling rules also treat a fluid ounce and an ounce in weight separately.
This matters because the 128 ratio only applies to fluid ounces. A weight in ounces cannot be turned into gallons without knowing the density of the substance. A label that lists ounces of a powder or solid is usually describing weight, and this converter does not apply.
Ounces per Gallon and Mixing Ratios
"Ounces per gallon" can appear on product labels for mixing and dilution. When the unit is fluid ounces, it is a volume ratio, and each fluid ounce is 1/128 of a gallon.
| Mix ratio | Fraction of one gallon |
|---|---|
| 1 fl oz per gallon | 0.78125% |
| 2 fl oz per gallon | 1.5625% |
| 4 fl oz per gallon | 3.125% |
To scale a ratio, multiply by the number of gallons. A label calling for 2 fl oz per gallon needs 10 fl oz for 5 gallons. Whether the ratio applies to a gallon of water or a gallon of finished mix depends on the label, so follow the product instructions. Some labels use weight ounces instead, which this converter does not handle.
US Gallons vs Imperial Gallons
The imperial gallon is a separate unit. UK law defines it as 4.54609 cubic decimeters, and the Canadian Weights and Measures Act defines the same size. The imperial fluid ounce is also a different size, set at 1/160 of an imperial gallon.
| US | Imperial | |
|---|---|---|
| Fluid ounces per gallon | 128 | 160 |
| Gallon in liters | 3.785 | 4.546 |
| Fluid ounce in milliliters | about 29.57 | about 28.41 |
The imperial gallon is about 20 percent larger than the US gallon, while each imperial fluid ounce is about 4 percent smaller than a US one. Dividing by 128 gives the wrong answer for imperial measures. To convert imperial fluid ounces to imperial gallons, divide by 160.
The 30 Milliliter Label Rule
Nutrition labels in the US treat 1 fluid ounce as 30 milliliters. That is a labeling rule in FDA's regulation 21 CFR 101.9(b)(5)(viii), not the physical definition. For conversions, use about 29.57 milliliters per fluid ounce.
The exact figures are 29.5735295625 milliliters per fluid ounce and 3.785411784 liters per gallon. They follow from the 231 cubic inch gallon and the inch being exactly 2.54 centimeters.
Mistakes to Avoid
- Dividing by 16 instead of 128. 16 fluid ounces make a pint, so dividing by 16 gives pints.
- Mixing up fluid and weight ounces. The 128 ratio applies only to volume.
- Using the wrong gallon. Check that your container or recipe uses US measures. Imperial measures need 160.
- Rounding too early. Keep the full decimal until the final step.
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Sources
- NIST Handbook 44, current edition: US definition of the gallon and fluid ounce
- NIST Guide to the SI, Appendix B: conversion factors and rounding rules
- FDA guidance on metric equivalents of household measures: the 30 milliliter labeling rule
- Weights and Measures Act (Canada), Schedule II: imperial gallon and fluid ounce
- Weights and Measures Act 1985 (UK), Schedule 1: imperial gallon and fluid ounce