How do you know how much water is enough to drink in a day? Take your body weight in pounds. Divide that number by 2. That is how many ounces of water you should at least drink per day. I recommend filling a container (or 2) with the right amount for you and fill your glass/bottle from that. That way you won't lose track of how much you've actually consumed. When the container is empty, you've met your goal. If you drink alcohol, coffee or tea, you need to drink more water than what you calculated from your body weight. Being just a little bit deficient can result in signs of dehydration such as fatigue, headache, lightheadedness, difficulty concentrating and muscle aches and pains.