When the Center City District studied Philly's downtown, they chose to follow where people worked, rather than pulling census data. They found that Philly actually has two downtowns: though Center City certainly dominates in terms of economic activity, lots of Philadelphians are employed in University City too. They also found that Center City has more residents in it than most commercial districts: the only two commercial districts that have more residents are Midtown Manhattan and Downtown Manhattan
The study defined Center City as the area between Race Street and South Street, bounded by the Schuylkill River on the east and the Delaware River on the west. Not only does 11.2% of Philly's population live in Center City or within one mile from Center City, but the area also contains 288,277 jobs. Center City itself contains 203 jobs per acre.
The study also claims that downtown development benefits workers from the entire city, even those who can't afford to live in the downtown area (or the mile that surrounds it.) They say that, in Philly specifically, a quarter of the city's workforce works in Center City. They also say that office development brings lots of jobs, (3,300 jobs for every 500,000 sq ft of new office space), and that residential development brings jobs in too (though not as many.)
· New "downtown" definition shows where Philly ranks [Plan Philly]
· Defining Downtown [Official Site]