MSNBC explainshow to calculate the size of a crowd, whether they are gathering on the National Mall in Washington, D.C., or on a college campus:
After gathering data on numerous demonstrations, Jacobs came up with some rules of thumb that still are used today by those serious about crowd estimation. A loose crowd, one where each person is an arm’s length from the body of his or her nearest neighbors, needs 10 square feet per person. A more tightly packed crowd fills 4.5 square feet per person. A truly scary mob of mosh-pit density would get about 2.5 square feet per person.
The trick, then, is to accurately measure the square feet in the total area occupied by the crowd and divide it by the appropriate figure, depending on assessment of crowd density. Thanks to aerial photos or mapping applications like Google Earth, even outdoor areas can be readily measured these days.