If you've done any amount of cooking or baking (especially baking) by following a recipe, then it's pretty likely you've seen ingredients measured out in both ounces and fluid ounces. They share a name, more or less, so they're the same thing, right?
Not so fast — there's a little more to it.
The Difference Between Ounces and Fluid Ounces
While ounces and fluid ounces share a similar name and both represent units of measurement, they are quite different. Each represents a different system of measurement: one measuring weight, the other measuring volume.
Ounces Measure Weight
Ounces are a unit used to measure weight. This measurement is most often used with dry ingredients, like flour or sugar, but can also be used for things like honey or yogurt. When a recipe call 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 proves to be more accurate. The best way to measure ounces is by using a scale.
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, go ahead and pull out your measuring cup; it's the most accurate way to measure this.
Are Ounces and Fluid Ounces Interchangeable?
Any confusion surrounding these two units of measurement is understandable since they share the same name, and recipes sometimes are not clear about which type they mean.
Also — fun fact! — a cup of water happens to equal both 8 fluid ounces (in volume) and 8 ounces (in weight), so you might naturally assume that 1 cup equals 8 ounces of weight universally in recipes.
But no, water is the exception rather than the rule. Since fluid ounces and ounces are measuring such very different things, they are rarely the same for most ingredients. 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).
So no, fluid ounces and ounces should not be used interchangeably. Pay close attention when reading your recipe to make sure you're measuring correctly!