If you're wondering about a dry ounce, the answer is pretty clear cut. One dry ounce adds up to 28.35 grams, or 28.3495231 grams, if you want to go digit crazy. Dry ounces are measurements of weight, generally calculated by placing the material on a scale. Measuring fluid ounces, however, isn't as straightforward.
Fluid ounces (abbreviated as fl. oz.) are all about volume, and volume requires insight into the density of a material for an accurate reading. For example, take two measuring cups. Fill one with jelly beans to the 1-cup line. Pour lemonade into the other cup, to the same 1-cup level. The two substances have different densities . . . and different weights.
Poke around your kitchen to locate common tools for measuring fluid ounces. You're likely to find tablespoons, measuring cups, pint jars, quart bottles, and gallon jugs. Fluid ounces correspond to liters, with 1 fluid ounce equaling 29.5735297 milliliters. A liter of liquid measures out to slightly over 33.8 fluid ounces in the United States. (Oddly enough, the U.S. fluid ounce is bigger than the British fluid ounce.)