Gallons to ounces converter, US and UK gallons, kept apart.

To convert gallons to ounces, multiply the gallons by 128. One US gallon is 128 US fluid ounces, so half a gallon is 64 fl oz, a quarter gallon is 32 and 5 gallons is 640. A UK gallon is a bigger unit that holds 160 UK fluid ounces, so if the gallons are British the multiplier is 160 and the ounces are a little smaller. Both are shown below, each with its label.

How to use the converter

1. Type the amount in the US gallons (gal) box. Fractions such as 3/4 work.
2. The big result shows US fluid ounces (fl oz). The cards give cups and liters for the same amount.
3. Two cards start with "As UK gallons". They read your number as UK gallons and show UK fluid ounces and US fluid ounces. The first card says what a British label would show. The second says how much liquid that is in US ounces.
4. Type fluid ounces in the second box to go back to gallons. Use Decimals for a fixed number of places.
5. Click a row in Common values to load it. Copy result, Copy link, Copy table and Print chart keep the answer.

Where 128 comes from

NIST Handbook 44 (2026), Appendix C defines the US gallon as 231 cubic inches, divided into 4 quarts, 8 pints or "128 fluid ounces (exactly)". One US fluid ounce is therefore 231 ÷ 128 cubic inches, which is 29.5735 mL, and the whole gallon is exactly 3.785411784 liters. Page checked 19 Oct 2026.

The UK gallon, on its own line

The same table says the British (Imperial) gallon is "divided into 160 fluid ounces", against 128 for the US gallon, and that it is about 20 percent bigger. NIST SP 811, Appendix B.8 gives the British gallon as exactly 4.54609 liters. A British fluid ounce is then 4.54609 ÷ 160 = 28.4131 mL, slightly smaller than the US ounce of 29.5735 mL. So one UK gallon is 160 UK fl oz, and in US ounces it is 153.72 fl oz. Checked 19 Oct 2026.

A shortcut, and the way back

Since 128 is 2 multiplied by itself seven times, you can halve down from a gallon: 128, 64, 32, 16, 8. Those are a gallon, a half gallon, a quart, a pint and a cup. Going the other way, divide the ounces by 128. For 64 fl oz that is 0.5 gallon. For 100 fl oz it is 0.78125 gallon, which is 3.125 quarts. Type fluid ounces in the second box and the tool does the division, exactly, without rounding the factor first.

Worked examples

• A half gallon of milk. 0.5 × 128 = 64 fl oz, or 8 cups.
• A 5 gallon bucket. 5 × 128 = 640 fl oz, which is 18.93 liters.
• How many 12 fl oz cans in a gallon. 128 ÷ 12 = 10.667, so ten cans and two thirds of an eleventh.
• How many 8 fl oz glasses in a gallon. 128 ÷ 8 = 16, which is the 16 cups in a gallon.
• A 55 gallon drum. 55 × 128 = 7,040 fl oz, or 208.2 liters.
• 2 1/4 gallons of paint thinner. 2.25 × 128 = 288 fl oz.

Gallon fractions in fluid ounces

Every US row is exact. The liter column is rounded to three decimals.

Fluid ounces are not weight ounces

The ounces here measure volume. A fluid ounce of water, a fluid ounce of oil and a fluid ounce of honey fill the same space and have different weights, so a gallon in fluid ounces cannot be turned into pounds without knowing the liquid. If you need weight, weigh it, or look up the density of that liquid. For weight units on their own see Ounces in a pound and Ounces to grams.

Mistakes to avoid

The biggest is mixing the two gallons. A British recipe or label that says 2 gallons means 2 UK gallons, which is 9.092 liters, against 7.571 liters for 2 US gallons. Check where the number came from. The second is using the UK ounce count with the US gallon: 160 UK ounces is not a US gallon. The third is the dry gallon. Handbook 44 lists dry volume as pints, quarts, pecks and bushels and has no dry gallon in that table, so a gallon of liquid is the one this page uses.

Smaller amounts are on Cups to ounces. For liters see Liters to gallons, and for quarts Quarts to gallons. The Unit converter handles every other unit.