The Statue of Liberty and Ellis Island are both popular destinations for visitors to New York City. If you're interested in seeing both during your trip to New York City, seeing them on the same day is highly recommended.
The Statue of Liberty and Ellis Island are located on two separate islands in the New York Harbor. They are served by the same ferry, so visiting them together allows visitors to make the most of their time, though it can make for a long day if you're going to experience both the Statue of Liberty and Ellis Island to their fullest. It will take 5-6 hours to visit both islands and the museums, as well as round-trip transportation from Battery Park.
The ferry leaves Battery Park every 20-40 minutes, but you need to allow time for clearing security (even if you purchase your tickets in advance which is a good idea). You'll need even more time if you want to buy tickets once you arrive at Battery Park.
After about a 10-minute ride, the ferry will stop at Liberty Island first. Whether you want to visit the Statue of Liberty or not, you'll need to disembark. If you want to tour Liberty Island or enter the Statue of Liberty, you can catch a ferry on to Ellis Island whenever you are done. You'll need a ticket for Crown or Pedestal access in order to visit the museum or enter the pedestal of the Statue of Liberty. Crown tickets are extremely limited and cost $3 additional, while Pedestal access tickets are more plentiful and don't cost extra, but still must be reserved in advance.
Another 10-minute ferry ride will bring you to Ellis Island. Here you will want to plan to allow at least an hour to visit the Ellis Island Immigration Museum. Take the free Ranger Led Tour and also allow yourself time to explore the museum on your own.
Once you are done at Ellis Island, you can board the ferry again to return to Battery Park. The ferry leaves Ellis Island every 20 minutes. Be sure to get the ferry bound for Battery Park as there are also ferries going back to Liberty Island for visitors who took the ferry from New Jersey.
Statue Cruises recommends that you board the ferry no later than 1 p.m. if you want to visit both islands.