In general, your baby shouldn’t drink water until he's about 6 months old. Until then, he gets all the hydration he needs from breast milk or formula, even in hot weather. Once your baby is 6 months old, it's okay to give him sips of water when he's thirsty. Don't overdo it, though, or you might give him a tummy ache or make him too full to eat well. After his first birthday, when your baby's eating solids and drinking whole milk, you can let him drink as much water as he likes.