[supercat]

You can then apply a a divider of about 3 or 4 (square feet) per person, based on the perceived crowd density, to determine a good rough estimate of the crowd size.

Density estimates can be tricky, though, and crowd densities are often not uniform. Area estimations can be a good starting point, though, certainly.

[Wil H]

Density estimates can be tricky, though, and crowd densities are often not uniform

I agree, but there are some minimums that cannot be breached reasonably. The average person is about two feet across the shoulders and requires about 10 -14 inches front to back and so requires a minimum of two square feet, even in a dense crowd.

If the Media are claiming 70,000 showed up in Portland, and the area is definitively measurable as under 50,000 square feet, you know they are lying.