According to Consumer Reports as of September 2014, a 1-foot square paper towel absorbs between 1/2 ounce and 2 1/2 ounces of fluid. The amount of fluid soaked up depends on the brand.
To determine how much water each type of paper towel absorbs, Consumer Reports places paper towels of various brands in a bucket of water and records the amount of water absorbed.
Paper towels absorb fluids, because the sheets consist of cellulose, which is made up of sugar molecules. Liquids cooperate with sugar molecules. They all stick together, and this leads to absorption. In addition, paper towels have a looser weave than napkins or tissues. The extra space within the towel allows liquids to move through the sheet quickly.