Ounces to cups:
dividing by eight, and knowing when not to

The formula and a quick table

In the US customary system one cup holds 8 fluid ounces. A 24 fl oz measure of milk is therefore 24 ÷ 8 = 3 cups, and a 12 fl oz can of soda is 1.5 cups. Going the other way, multiply the cups by 8.

The catch is the word ounce. It names both a unit of volume (fluid ounce) and a unit of weight (avoirdupois ounce). The 8-to-1 ratio belongs only to the volume ounce, so it applies to liquids you pour and not to flour or sugar you weigh.

Worked example: a bottle label to a recipe

A sauce recipe calls for 1½ cups of stock and you have a carton marked 32 fl oz. How many recipes does one carton cover, and what is left over?

Working in cups is usually easier at the stove because your measuring jug is marked that way; use fluid ounces when the packaging is.

Not every cup is the same size

The US cup is 236.6 mL. But recipes travel, and other countries define the cup differently.

The UK also has its own imperial fluid ounce at 28.41 mL against the US 29.57 mL, so 20 imperial fl oz in a pint is 568 mL, while a US pint of 16 fl oz is 473 mL. Check the origin of a recipe before relying on the division by eight, especially for baking where proportions matter.

Dry ounces are a different question

When a recipe lists ounces of flour, sugar or chocolate, it means weight. How many cups that is depends on how densely the ingredient packs. All-purpose flour is roughly 4.25 oz (about 120 g) per cup, granulated sugar about 7 oz (200 g) per cup, and butter 8 oz (225 g) per cup, though sources round these differently.

• A cup of water weighs about 8.3 oz, close to its 8 fl oz volume, which is why the two ounces are so often confused.
• A cup of honey is much heavier than a cup of oil, so equal volume does not mean equal weight.
• Kitchen scales avoid the problem altogether for baking.

Converting a recipe between metric and US cups

When a recipe jumps between systems, convert one quantity at a time and keep the ingredient's nature in mind. Liquids convert by volume: 1 US cup is 236.6 mL, so 500 mL of milk is 500 ÷ 236.6 = 2.11 cups, close enough to 2 cups plus a tablespoon or two.

Dry goods convert best through grams. A US recipe calling for 2 cups of all-purpose flour is about 240 g using 120 g per cup, whichever country's cups you own. If you own 250 mL metric cups, two metric cups of flour would be about 253 g, a difference of about 5%, which is within normal baking tolerance for biscuits but can matter for a delicate sponge.

Measuring technique and spoon sizes

Technique decides whether a cup of flour is 4 ounces or 5. Spooning flour lightly into the cup and levelling with a knife gives the lower weight; scooping straight from the bag compacts it and can add 20% or more. Liquid measures should be read at eye level on a flat surface, in a jug with a pouring spout, not in a dry measuring cup filled to the brim.

• US tablespoon: 14.8 mL, three teaspoons, half a fluid ounce.
• US teaspoon: 4.93 mL, commonly rounded to 5 mL.
• Metric tablespoon in Australia: 20 mL, about a third larger than the US one.
• Fluid ounce in the kitchen: two tablespoons, so 8 fl oz is 16 tablespoons, or one cup.

When a recipe gives 3 fl oz of lemon juice, that is 6 tablespoons, or ⅜ cup, which is a quicker measure than hunting for the three-ounce line on a jug.

Scaling a recipe without losing track

Cups and fluid ounces are most useful when a recipe has to be doubled, halved or split. Because a cup is 8 fl oz, halving a ¾ cup measure gives 3 fl oz, which is 6 tablespoons or ⅜ cup. Convert to fluid ounces first, do the scaling, then convert back to the measure you have.

A pot of soup serving 4 with 6 cups of stock becomes a 10-serving batch with 15 cups, or 120 fl oz, which is under a US gallon (128 fl oz). Knowing the gallon is 16 cups, 4 quarts or 128 fl oz tells you at once that it fits in a standard stock pot with a little room left. Seasoning and cooking time do not scale in a straight line, so taste before you add the full scaled amount of salt or spices.

Practical rounding at the stove

Most home measuring cups are marked in quarter-cup steps, so odd fluid-ounce amounts are best rounded sensibly. 5 fl oz is 0.625 cup, which is ½ cup plus 2 tablespoons, since a US tablespoon is half a fluid ounce. Keeping a few relationships in mind speeds things up: 1 cup is 16 tablespoons, and 1 fl oz is 2 tablespoons.