How Many Ounces in a Half Gallon?

There are 64 fluid ounces in a US half gallon.

That follows directly from the gallon: a US gallon is 128 fluid ounces, so half of it is 128 ÷ 2 = 64 fl oz.

The full breakdown of a US half gallon:

UK (imperial) half gallon: The imperial gallon is larger — 160 imperial fluid ounces — so an imperial half gallon is 80 imperial fluid ounces, or about 2.27 litres. If a recipe or product is British, Canadian, or Australian, check which system it's using, because the difference is significant.

Handy reference points:

• 1 US gallon = 128 fl oz = 16 cups = 8 pints = 4 quarts = 3.785 L
• ½ gallon = 64 fl oz = 8 cups = 4 pints = 2 quarts ≈ 1.89 L
• 1 quart = 32 fl oz = 4 cups
• 1 pint = 16 fl oz = 2 cups
• 1 cup = 8 fl oz

A useful memory aid is the "Gallon Man": a gallon holds 4 quarts, each quart holds 2 pints, each pint holds 2 cups, and each cup holds 8 fluid ounces.

Fluid ounces vs. ounces by weight — an important distinction. A fluid ounce measures volume; an ounce (oz) measures weight. They only line up for water, and even then only approximately. So a half gallon is 64 fluid ounces of any liquid, but its weight depends on what's in it:

• Water — about 4.17 lbs (66.7 oz).
• Milk — about 4.3 lbs, since milk is slightly denser than water.

Everyday examples of a half gallon: a standard carton of milk or orange juice, a typical tub of ice cream (though many are now sold slightly smaller), and a common size for iced tea and water jugs.

Water intake: the often-cited "8 glasses a day" is 8 × 8 fl oz = 64 fl oz — exactly a half gallon, which is why half-gallon water bottles are so popular.