Bluebulb Projects' The Measure of Things presents:
21,000 is about three-fifths the number of Seats in Fenway Park.
In other words, it's 0.566480 times the count of Seats in Fenway Park, and the count of Seats in Fenway Park is 1.76530 times that amount.
(2013 figures; daytime capacity) (a.k.a. The Cathedral of Boston) (Boston, Massachusetts)
Fenway Park, the home of the Boston Red Sox baseball team, can accommodate 37,071 spectators when configured for daytime seating. Fenway Park holds the record for most consecutive sellout games, with a streak of 794 sellouts that ended in 2013. This figure is a dramatic turnaround from the attendance in 1965, which was at times as low as just 500 people.