People assume event organizers have their best interests at heart, but just like many businesses, organizers often operate on a tight budget and sometimes corners get cut creating disastrous results.
Image recognition algorithms, military satellites, and mobile data networks can all help estimate crowd sizes. But the underlying maths still comes down to a basic formula: density multiplied by area.
A professor of journalism explains why you should be skeptical of any numbers that you see over the next few days.
Lessons from the British 19th century protests over electoral reform about the significance of crowd sizes.