A fluid ounce measures volume, or how much space something takes up. An ounce, sometimes called a weight ounce or avoirdupois ounce, measures mass. They are not the same, even though both are called ounces. A cup of water is 8 fluid ounces, but a cup of flour is not 8 ounces on a scale.
Quick conversion
| Measure | Equals |
|---|---|
| 1 U.S. fluid ounce | 29.57 milliliters (volume) |
| 1 ounce by weight | 28.35 grams |
| 1 U.S. cup of water | 8 fluid ounces, about 237 g, or about 8.3 ounces by weight |
| 1 U.S. cup all-purpose flour | about 120 g, or 4¼ ounces by weight |
| 1 U.S. cup granulated sugar | 198 g, or 7 ounces by weight |
NIST gives the fluid ounce as a volume unit and the ounce as a mass unit of 28 grams in kitchen rounding (exactly 28.35 g). Water is close, which is why the old saying “a pint’s a pound” almost works: 8 fluid ounces of water weigh a little more than 8 ounces. Flour, sugar and oil do not follow that rule. Weights above for flour and sugar come from the King Arthur Baking ingredient weight chart. See also ounces in a cup and the kitchen measurement conversion chart.
A cup of flour is not 8 ounces
A measuring cup filled with flour and a measuring cup filled with water occupy the same volume, but the flour is much lighter — about 4¼ ounces, not 8. If a recipe lists ounces of a solid, use a kitchen scale, not a liquid cup. Related: grams in an ounce and how to measure flour correctly.
Package labels add to the confusion because they often print “oz” for both solids and liquids. A 12-ounce can of soda is 12 fluid ounces of volume. A 12-ounce steak is 12 ounces by weight. When a baking recipe says “4 ounces of chocolate,” it almost always means weight, and a cup measure will not get you there unless the recipe also gives a cup amount for that ingredient.
Frequently asked questions
Are fluid ounces the same as ounces?
No. A fluid ounce is volume. An ounce is weight. They only come close for water, and even then a cup of water weighs about 8.3 ounces, not exactly 8.
Does a cup of flour weigh 8 ounces?
No. A cup of all-purpose flour weighs about 120 grams, or 4¼ ounces, when it is spooned and leveled. The 8-ounce figure is for 8 fluid ounces of volume, not the weight of flour.
Why is water close to 1 ounce per fluid ounce?
Water’s density is about 1 gram per milliliter. A U.S. fluid ounce is 29.57 mL, so a fluid ounce of water weighs about 29.6 grams, or about 1.04 ounces.
Helpful kitchen tools
- OXO Good Grips 11-Pound Digital Kitchen Food Scale — Weighs in grams and ounces, with a tare (zero) button.
- OXO Good Grips Glass Measuring Cup Set, 2-Cup and 4-Cup — A 2-cup pint cup and a 4-cup (1-quart) cup with U.S. and metric marks.
- OXO Good Grips Stainless Steel Measuring Cups and Spoons, 8-Piece Set — Dry cups (1/4, 1/3, 1/2 and 1 cup) and spoons with etched marks.
- Kitchen Conversion Chart Magnet (AKS Magnets) — An 11-by-8.5-inch fridge magnet for volume and weight conversions.
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