Chaput, according to the Catholic News Service, indicated yesterday that Francis could be in town for three days, which is longer than previous papal visits to the World Meeting of Families.
The trip will be Francis' first visit to the United States. He is expected to give an outdoor Mass, possibly on the Ben Franklin Parkway, that could draw more than 1 million people.
Vatican spokesman Federico Lombardi has said Francis would consider invitations he has received to make other stops in the United States, like addressing a joint session of Congress or the United Nations General Assembly in New York.
Pope John Paul II visited Philadelphia in 1979 and held Mass on the Parkway. Many contemporary reports said the crowd was well over 1 million people.
But Everett Gillison, Nutter's chief of staff, said yesterday that spatial modeling analyses, based on images of the event, indicate it was more likely in the range of 600,000 and 800,000.
The size of the crowd in Philly may depend on whether Francis holds another event in the U.S. that is open to the public or if he sticks to closed-door events, like a U.N. address. If a Parkway appearance is the only chance for the public to see the enormously popular pontiff, more admirers are expected to come to Philly.
A nonprofit, set up to organize the World Meeting of Families and led by Philadelphia political and business heavyweights, is raising the enormous amount of money needed to host the pope. A target budget has not yet been set.
Read more: The last papal visit to Philadelphia: John Paul II in 1979.
On Twitter: @SeanWalshDN