September baseball means expanded rosters, playoff races and math for teams in postseason contention. The Philadelphia Phillies are in the postseason and are National League East champions, but they're still seeking to clinch a first-round bye.

The Phillies have five games left in the regular season and already have the division clinched, but it's still magic number season in Philadelphia.

Here's everything fans need to know as the Phillies enter the final stretch of the regular season.

What is a magic number, and how is it calculated?

Baseball uses the term "magic number" to determine how close teams are to punching their tickets to the postseason or winning their divisions.

According to MLB.com, its origin dates back to the 1947 pennant race between the New York Yankees and Boston Red Sox.

The formula to determine a team's magic number is: Games remaining plus one, then subtracting the first-place wins and the second-place team's losses.

What is the Phillies' magic number to make the playoffs?

The Phillies' magic number to make the playoffs for the fourth straight season is zero. The Phils clinched a postseason berth with the San Francisco Giants' loss.

What is the Phillies' magic number to clinch the NL East?

The Phillies' magic number to clinch their second consecutive NL East title is down to zero games. Philadelphia clinched the NL East title with a 6-5 win over the Los Angeles Dodgers on Monday night.

What is the Phillies' magic number to clinch a first-round bye?

Zero. The Phillies have clinched a first-round bye after clobbering the Miami Marlins, 11-1, on Wednesday night. The Phillies will enter the playoffs as no worse than the No. 2 seed with an outside chance at the top seed.

The Phillies can still earn the No. 1 seed, but the Milwaukee Brewers control their own destiny. The Brewers' magic number to clinch the top seed is one game. Milwaukee owns the tiebreaker over Philadelphia, meaning if both teams finish with the same record, the No. 1 seed goes to the Brew Crew.