Ounces means two different things

A fluid ounce measures volume. An ounce measures weight. They share a name and almost nothing else, and recipes rarely say which one they mean.

For volume the answer never moves. One US cup is 8 fluid ounces, always, whatever is in it. That is a definition, not a measurement.

For weight the answer moves with every ingredient. A cup of cocoa powder weighs 3.03 ounces. A cup of honey weighs 11.99 ounces. Same cup, nearly four times the weight.

Where the 8 ounce rule comes from, and where it breaks

A US cup of water weighs about 8.36 ounces, close enough to 8 that the rule got repeated until people applied it to everything.

Flour is where it hurts. A cup of all-purpose flour weighs 4.23 ounces, so treating a cup as 8 ounces overstates the real weight by 89 percent. Turn that around and it bites harder. A recipe asking for 8 ounces of flour needs 1.89 cups, so measuring one cup leaves you nearly a full cup short.

One cup of each ingredient, by weight

Every figure below is the ingredient density in grams per cup divided by 28.349523125, the exact number of grams in an avoirdupois ounce. The densities are the same vetted set the cups to grams converter uses.

Ingredient	1 cup, oz	1/2 cup, oz	1/4 cup, oz	1 cup, g
Shredded Coconut	3	1.5	0.75	85
Cocoa Powder	3.03	1.52	0.76	86
Rolled Oats	3.17	1.59	0.79	90
Almonds (sliced)	3.25	1.62	0.81	92
Almond Flour	3.39	1.69	0.85	96
Cake Flour	4.02	2.01	1.01	114
All-Purpose Flour	4.23	2.12	1.06	120
Whole Wheat Flour	4.23	2.12	1.06	120
Powdered Sugar	4.23	2.12	1.06	120
Coconut Flour	4.23	2.12	1.06	120
Walnuts (chopped)	4.23	2.12	1.06	120
Bread Flour	4.48	2.24	1.12	127
Cornstarch	4.52	2.26	1.13	128
Cornmeal	5.29	2.65	1.32	150
Raisins	5.82	2.91	1.46	165
Semolina	5.89	2.95	1.47	167
Chocolate Chips	6	3	1.5	170
White Rice (uncooked)	6.53	3.26	1.63	185
Brown Rice (uncooked)	6.7	3.35	1.68	190
Granulated Sugar	7.05	3.53	1.76	200
Olive Oil	7.62	3.81	1.9	216
Vegetable Oil	7.69	3.84	1.92	218
Brown Sugar (packed)	7.76	3.88	1.94	220
Butter	8.01	4	2	227
Sour Cream	8.11	4.06	2.03	230
Baking Powder	8.11	4.06	2.03	230
Cream Cheese	8.18	4.09	2.05	232
Water	8.36	4.18	2.09	237
Heavy Cream	8.4	4.2	2.1	238
Milk	8.61	4.3	2.15	244
Yogurt	8.64	4.32	2.16	245
Peanut Butter	9.1	4.55	2.28	258
Table Salt	10.16	5.08	2.54	288
Maple Syrup	11.01	5.5	2.75	312
Honey	11.99	6	3	340

Sorted lightest to heaviest. The spread runs from 3 ounces to 11.99 ounces a cup, which is the whole reason a single conversion number cannot work.

Common amounts in fluid ounces and weight

Fluid ounces come straight from the cup fraction. The weight columns change with the ingredient.

Amount	Fluid oz	Flour, oz	Sugar, oz	Butter, oz
1/4 cup	2	1.06	1.76	2
1/3 cup	2.67	1.41	2.35	2.67
1/2 cup	4	2.12	3.53	4
2/3 cup	5.33	2.82	4.7	5.34
3/4 cup	6	3.17	5.29	6.01
1 cup	8	4.23	7.05	8.01
2 cups	16	8.47	14.11	16.01

Which cup the recipe meant

Cup is not one volume. Four are in regular use, and the gap between the smallest and largest is about 20 percent.

Standard	Millilitres	Where you meet it
US customary cup	236.5882	American recipes and measuring cups
US legal cup	240.0	Nutrition labels
Metric cup	250.0	Australia and New Zealand
Imperial cup	284.1306	Older British recipes

The tool above uses the US customary cup, 236.5882 millilitres, because that is what American recipes are written in. If your measuring cup came from Australia it holds 250 millilitres, about 5.7 percent more.

The imperial row hides a nice inversion. An imperial fluid ounce is 28.4130625 millilitres, which is smaller than the US fluid ounce at 29.5735. Yet an imperial cup is about 2 percent larger than a US cup, because a British cup is ten fluid ounces rather than eight.

One more thing worth knowing. The imperial cup is not defined in law. The UK Weights and Measures Act 1985 defines the gallon and the pint, and Canada’s Act defines the imperial gallon, fluid ounce and pint. Neither lists a cup. The figure above is half an imperial pint, a cookbook convention with no statute behind it.

Questions cooks actually ask

Is a cup 8 ounces?: By volume yes, a US cup is exactly 8 fluid ounces. By weight almost never. Only ingredients near the density of water land close, and even water comes in at 8.36 ounces a cup.
How many ounces is a cup of flour?: About 4.23 ounces for all-purpose flour, spooned and levelled. Bread flour is a little heavier at 4.48 ounces, cake flour lighter at 4.02.
My recipe says 8 ounces of chocolate chips. How many cups?: About 1.33 cups. Chocolate chips run 6 ounces a cup, so 8 ounces is a bit over a cup and a third.
Why do American recipes use cups at all?: Speed. A cup scoop is faster than a scale, and for forgiving recipes the error does not matter. For bread and pastry it does, which is why professional recipes weigh.
Should I just buy a scale?: If you bake, yes. A cup of flour can vary by a large margin depending on how it is packed. A gram is a gram no matter who measures it.

Where these numbers come from

The ounce is defined as exactly 28.349523125 grams and the US customary cup as 236.5882365 millilitres, which makes one fluid ounce exactly 29.5735295625 millilitres. Those are definitions, so they carry no measurement error.

The per ingredient weights come from the vetted density set this site maintains and publishes in full on the cups to grams page, where the sourcing and method for each figure are set out. Every weight here is that density divided by the exact grams per ounce, computed in code rather than typed, so the table and the tool cannot disagree.

Flour is the one worth a caveat, and the sources genuinely disagree. This site uses 120 g a cup. USDA FoodData Central lists white all-purpose flour at 125 g a cup, which is {fmt(125/G_PER_OZ)} ounces. Food Standards Australia New Zealand publishes a plain flour density that works out nearer 128 g. The spread is real and comes from how the cup was filled, so weigh it when the recipe is fussy.

Related converters

These use the same density set.

Cups to Ounces.

Cups to ounces converter