What’s the Difference Between Fluid Ounces and Ounces?

updated Jun 23, 2022
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Diptych of measuring bowl with dry goods on left and measuring up with lemon juice being squeezed on right.
Credit: Joe Lingeman/Kitchn

If you’ve done any significant amount of cooking or baking (particularly baking!), then it’s pretty likely you’ve seen some ingredients in the recipe written out in ounces. (Check out these recipes for a tasty layer cake and sugar cookies for two examples).

But when you’re cooking, it’s important keep in mind that there are different types of ounces: fluid ounces and ounces by weight. These two terms are both a form of measurements for foods, though they have completely different functions.

Are Fluid Ounces the Same as Ounces?

To put it simply: No, fluid ounces are not the name same as ounces. Both fluid ounces and ounces represent a unit of measurement, but they are quite different. Fluid ounces, as the name might imply, are specifically meant to measure volume (often of liquid ingredients like water), while ounces measure weight, usually of solid ingredients like all-purpose flour.

Credit: Joe Lingeman

Ounces Measure Weight

Ounces are a unit used to measure weight. This measurement is most often used with dry ingredients, but can also be used for things like honey or yogurt. When a recipe calls for ounces, it’s referring to a specific weight. You may notice baking recipes written in ounces instead of cups — this is because measuring ingredients by weight is more accurate and a better way of getting more equal measurements. The best way to measure ounces is by using a kitchen scale.

Credit: Photo: Joe Lingeman; Food Styling: Jessie Yu Chen

Fluid Ounces Measure Volume

Fluid ounces are a unit used to measure volume, or how much space something takes up. This measurement is used for liquid ingredients, like water or milk. When a recipe calls for fluid ounces, you should use a glass measuring cup with measurements on it — this is the most accurate way to measure a liquid ingredient in baking.

Are Ounces and Fluid Ounces Interchangeable?

If you’re truly looking to prepare a baking recipe in the most accurate way possible, then you should not use fluid ounces and ounces interchangeably. On most occasions, fluid ounces and ounces (weight) are not interchangeable because different ingredients have different weights and take up different amounts of space.

Think of one cup of flour and one cup of tomato sauce; they both occupy the same volume of space (i.e., 8 fluid ounces), but they have very different weights (about 4 ounces for the flour, and about 7.9 ounces for the tomato sauce). One exception to this, however, is water. Water happens to equal both 8 fluid ounces (in volume) and 8 ounces (in weight). All other ingredients should be measured out either by weight or volume. Be sure to pay close attention when reading your recipe to make sure you’re measuring correctly!