So how much fluid does the average, healthy adult living in a temperate climate need? The Institute of Medicine determined that an adequate intake (AI) for men is roughly about 13 cups (3 liters) of total beverages a day. The AI for women is about 9 cups (2.2 liters) of total beverages a day. Everyone has heard the advice, Drink eight 8-ounce glasses of water a day.. That's about 1.9 liters, which isn't that different from the Institute of Medicine recommendations. Although the 8 by 8 rule isn't supported by hard evidence, it remains popular because it's easy to remember.