It’s common knowledge to gulp down water during your workout to replace lost fluids. But how much do you need to drink? One or two hours before working out, drink 15 to 20 ounces of water. During your workout, drink at least six ounces of water every 15 minutes. Working out in extreme heat or sweating more than usual? Go for eight ounces. This will keep you from dehydration and allow you to perform at your best!