Bluebulb Projects' The Measure of Things presents:
18,000 is about half the number of Seats in Fenway Park.
In other words, it's 0.485550 times the count of Seats in Fenway Park, and the count of Seats in Fenway Park is 2.0595 times that amount.
(2013 figures; daytime capacity) (a.k.a. The Cathedral of Boston) (Boston, Massachusetts)