As of 2015, baby formula is not as restricted as other liquids during air travel because it is considered a medically necessary liquid. It is still subject to screening and must be declared at the beginning of the screening process.
It is acceptable to bring as much formula as necessary for feeding the baby during the flight. It does not need to be kept in a zip-top bag, but should be labeled to make the screening process smoother. Any formula that is brought in quantities larger than what the baby needs on the flight should be in a checked bag. Security officers might ask to open the container for further screening.