Explainer

# How Do You Measure a Crowd?

## Disagreements over the Glenn Beck rally in Washington, D.C.

According to CBS News, Glenn Beck’s rally in Washington, D.C. on Saturday attracted about 87,000 people. Beck himself put the number at more than 500,000. Other sources, like NBC News, had it somewhere in between. In an “Explainer” column from last year and reprinted below, Juliet Lapidos described how experts come up with these estimates.

Vast crowds filled the National Mall on Tuesday to watch Barack Obama take the oath of office. Official figures have not yet been released, but there’s widespread speculation that yesterday’s event broke the attendance record set by the 1.2 million people who supposedly showed up at Lyndon B. Johnson’s inauguration. The Associated Press estimated Tuesday’s assembled masses at “more than 1 million,” the Washington Post projected 1.8 million, and CBS reported “between 1.8 million and 2 million.” How do you measure a crowd?

Basic arithmetic. Estimates depend on three variables: the area of the available space, the proportion of the space that’s occupied, and the crowd’s density. While the first measurement is objective, and the second fairly easily determined with aerial photography, the third is a little trickier. It’s customary to assume that in a very crowded place (like a commuter train during peak hours) people occupy 2.5 square feet, whereas in a looser gathering each person takes up more like 5 square feet.

This area-based method dates back to the late 1960s. After rowdy students gathered at Berkeley’s Sproul Hall Plaza in December 1966 as part of the Free Speech Movement, police estimated a crowd of 7,000 to 10,000. Newspapers repeated the range, but readers were skeptical. Then Herbert A. Jacobs, a Berkeley professor, tried to arrive at a more exact figure using an enlarged aerial photograph of the demonstration. He divided the photograph into 1-inch squares and counted heads using a magnifying glass, eventually reaching a total estimate of 2,804. So that he wouldn’t need to repeat this painstaking process, he deduced the average square footage taken up by each student—about 4 square feet at a tightly packed outdoor event—and confirmed this estimate at subsequent rallies.