You need to take into consideration the amount of exercise you do and the climate you live in to know how much water you need each day. If you live in a hot climate, you will need to drink more. If you participate in sports or very physical work, you will lose more to sweat and will need to replace it. You can count any fruit juice you drink and other forms of liquid. Normally you aim for about 8 glasses of water or other liquids. There is water in most foods as well.