Infants need around 1-1/2 ounces of water per pound per day. So, each day a 20-pound baby needs around 30 ounces of fluids, usually in the form of milk. Breastfed babies don’t need any extra water, but a small amount of water is necessary for formula-fed infants. In fact, some pediatricians feel that offering excessive water to infants under one year of age may actually be nutritionally unwise because excess water may fill up baby and replace needed calories.
Adults need half as much water as infants – between 1/2 to 3/4 ounces of water per pound per day, depending on the amount of exercise, heat loss, illness, etc. A 120-pound woman should drink at least eight 8-ounce glasses of fluid per day.