Estimating Crowd Sizes
The Gist of Things
In general, 10 people can fit in a 25 square feet box. If we divide the number of people by the total area, we find that one person takes up about 2.5 squared feet of space.
This number is the occupant load factor - the amount of space one person is assumed to take up.
This number can change depending on context. Unless an occupant load factor is given, assume it is 2.5 squared feet per person.
What To Do
Using this information, we can do the following to find the crowd size:
Let an area of some space (a room, a sidewalk, etc.) be taken up completely by a group of people.
Find the area of that space
Divide that space by our occupant load factor
Define your answer with units (we're not counting tractors or dogs - we're counting humans.
Below: An excerpt of NFPA 101, giving different occupant load factors depending on context.
The idea, however, is the same - one person takes up x amount of space.