An ounce is a unit of measure which indicates how much a container weighs when it's full. In the case of water, a fluid ounce (abbreviated fl oz) is a unit of measure which indicates how much water the container can hold. An ounce is usually abbreviated as "oz." and 1 oz is equal to 28.35 grams, or 437.5 grains. This means that you can fit four fluid ounces of water into one ounce.