US gallons to US fluid ounces Converter

When to use US gallons → US fluid ounces

US gallons to US fluid ounces multiplies by 128 — the ratio 3.785411784 ÷ 0.0295735295625 is exactly 128, since the fluid ounce is defined as 1/128 gallon. A half-gallon carton is 64 fl oz; the popular "gallon a day" water challenge is 128 fl oz. Both units belong to the same US customary ladder, so the arithmetic is exact and integer. The imperial system uses 160 fl oz per gallon, a different ladder entirely — never mix the two systems' rungs.

Bartenders and beverage-program managers costing out a full bottle or keg in terms of individual pour sizes use this to figure servings per container. 1 gal yields 128 fl oz, or roughly 85 standard 1.5 fl oz shots; 5 gal (a Cornelius keg) yields 640 fl oz, about 53 pint pours; 15.5 gal (a full keg) yields 1984 fl oz. This exact 128:1 ratio is the reason US recipe and bar-math shortcuts almost always work in eighths and sixteenths rather than metric fractions.

How to convert US gallons to US fluid ounces

1. 1

   Take your value

   Start with your value in US gallons.

2. 2

   Multiply by 128

   Multiply the US gallon value by 128. This is the exact conversion factor from US gallons to US fluid ounces.

3. 3

   Read the result

   The product is your value in US fluid ounces. For example, 5 gal × 128 = 640 fl oz.

Conversion formula

The exact factor from US gallons to US fluid ounces is 128.

US fluid ounces = US gallons × 128

US gallons to US fluid ounces conversion table

Fifty common reference values, hand-picked for skim utility. Use the calculator above for any value not listed.

Real-world reference

A standard US milk jug holds 1 gal. An average car's fuel tank is 12–16 gal. In US fluid ounces, that gives you: 1 gal = 128 fl oz, 10 gal = 1280 fl oz, 100 gal = 12800 fl oz.

Frequently asked questions

How many US fluid ounces are in 1 US gallon?

1 US gallon equals 128 US fluid ounces.

How do I convert US gallons to US fluid ounces?

Multiply the US gallon value by 128. So x gal × 128 = the equivalent in fl oz.

What is 5 US gallons in US fluid ounces?

5 US gallons equals 640 US fluid ounces.

What is 10 US gallons in US fluid ounces?

10 US gallons equals 1280 US fluid ounces.

What is 100 US gallons in US fluid ounces?

100 US gallons equals 12800 US fluid ounces.

How do I convert back from US fluid ounces to US gallons?

Multiply the US fluid ounce value by 0.0078125. For example, 1 fl oz = 0.0078125 gal.

Is the US gallon bigger than the US fluid ounce?

Yes. 1 US gallon is larger than 1 US fluid ounce — specifically 128 times larger.

Are these conversion factors exact?

The factor is derived directly from the SI definitions of US gallon and US fluid ounce; the displayed value is rounded for readability, and the browser computes using IEEE 754 double-precision arithmetic.

About the US gallon and the US fluid ounce

Sources

• NIST Special Publication 811 — Guide for the Use of the International System of Units (SI)
• BIPM — SI Brochure (9th edition)
• Reverse: fl oz to gal
• All volume converters