One cup of water weighs approximately 8 ounces, which is equal to 225 grams. One cup holds 225 milliliters of water. The volume and weight of 1 cup of water are the same number under the metric system because the density of water is 1 gram per milliliter.
To convert a measurement of volume to a measurement of weight, it is necessary to know the density of the substance being measured. Multiplying the volume of the substance by its density arrives at the weight of the substance in that amount of volume. One cup of another material besides water has a different weight. For example, 1 cup of sugar weighs about 7 ounces, and 1 cup of honey weighs about 12 ounces.