Tens of thousands of people gathered today in Cairo’s Tahrir Square. Or maybe it was a hundred thousand. Or could’ve been up to two million. Except, by some counts, the Square can’t hold more than 225,000.

Every time a ton of people gather in one place, there are all sorts of pronouncements made about how many folks really showed. The figures rarely agree; Glenn Beck’s rally on the National Mall this summer attracted a million people, according to Beck cohort Rep. Michelle Bachman. CBS News pegged it at more like 87,000.

There is something of a science, however, to sorting out crowd sizes. Here’s how it works.