"Individual water needs depend on the weather, your health, your weight, and your body composition," Cara Pensabene, MD, of EHE Health in New York City, told POPSUGAR. "You also get water from the foods you eat." So, instead of focusing on an arbitrary number, she suggests taking note of how you're feeling and watching for any signs of dehydration, like feeling thirsty or headache-y. You may find you need more or less than eight glasses of water a day.