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

To convert fluid ounces to gallons, divide the ounces by 128. A US gallon holds 128 US fluid ounces, so 64 fl oz is 0.5 gallon, 32 fl oz is 0.25 and 100 fl oz is 0.78125. The UK gallon holds 160 UK fluid ounces, so a British amount divides by 160 instead. The card marked As UK fluid ounces in UK gallons does that sum with the number you typed.

How to use the converter

1. Type the amount in the US fluid ounces (fl oz) box. A decimal such as 2.5 or a fraction such as 3/4 works.
2. The big result shows US gallons (gal). Use Decimals in the side column to fix the number of places.
3. Cards under it give quarts, pints, cups and liters. The last card reads your number as UK ounces and shows UK gallons.
4. Type gallons in the second box, or press ⇄, to see the ounces in a number of gallons.
5. Click a row in Common values to load it. Copy result, Copy link, Copy table and Print chart keep the answer. Start again resets the box.

Where 128 comes from

NIST Handbook 44 (2026), Appendix C defines the US gallon as 231 cubic inches and says it holds "128 fluid ounces (exactly)", split into 4 quarts, 8 pints and 16-ounce pints. One US fluid ounce is then 1 ÷ 128 = 0.007813 gallon exactly, and the gallon is 3.785411784 liters. Checked 24 Oct 2026.

The UK gallon is another size

The same Handbook says the British gallon is divided into 160 fluid ounces, that its pint is 20 ounces, and that it is about 20 percent bigger than the US gallon. NIST SP 811, Appendix B.8 gives it as 4.54609 liters (checked 24 Oct 2026). A UK fluid ounce is then 4.54609 ÷ 160 = 28.4131 mL, a little smaller than the US ounce of 29.5735 mL. So 160 UK fl oz is one UK gallon, but only 153.72 US fl oz.

Worked examples

• 64 oz of water a day. 64 ÷ 128 = 0.5 gallon, or 1.893 liters.
• A 12 fl oz can. 12 ÷ 128 = 0.09375 gallon. Ten cans are 120 fl oz, which is 0.9375 gallon, a bit under one.
• 100 fl oz of paint. 0.78125 gallon, which is 3.125 quarts.
• 300 fl oz of fuel additive. 300 ÷ 128 = 2.34375 gallons.
• 1,000 fl oz. 7.8125 gallons, or 29.57 liters.
• The same number as UK ounces. 300 UK fl oz ÷ 160 = 1.875 UK gallons. Take 300 as US ounces and you get 2.34375 US gallons, so keep the label.

Servings in a gallon

Divide 128 by the serving size in fluid ounces. The last column shows what is left over.

Bottles of water in a gallon

A half-liter bottle holds 16.91 US fl oz. A gallon is 128, so it takes 128 ÷ 16.907 = 124.04 of those bottles to match one. A 1 L bottle holds 33.81 fl oz, so a gallon is 3.785 L. Always use the size printed on the bottle.

Labels use a rounded ounce

The US nutrition-label rule, 21 CFR 101.9(b)(5)(viii), says 1 fl oz means 30 mL for labelling (checked 24 Oct 2026). Take 128 labelled ounces: 128 × 30 = 3,840 mL. A real US gallon is 3,785.412 mL, so the labelled figure is 54.6 mL higher, about 1.4 percent. This tool uses the real ounce, 1/128 of the gallon. If a package counts in label ounces, a very exact sum may differ from its net volume in the last digit.

Common sizes in ounces

Mistakes to avoid

The first is treating a fluid ounce as an ounce of weight. A fluid ounce is a measure of space, so 128 fl oz of honey and 128 fl oz of water are one gallon each but do not weigh the same. The second is dividing by 100 because the gallon "feels" decimal. The third is using 128 for a British label. For the other direction with a longer explanation see Gallons to ounces; for smaller measures see Cups to ounces and Ounces to cups, and for metric Liters to gallons.