Determine the square footage of the venue. Acquire a blueprint or measure the area yourself for an indoor event. If the event is outdoors, acquire an aerial photograph of the outdoor area. Divide the photographed area into a grid. Measure the square footage of each grid square and add the squares to get the area of the outdoor venue.
Apply a reasonable value for the density of people in each region, grid square or square foot. In a loose crowd, where people are at an arm's length of each other, assign a value of 10 square feet of space per person. In a tighter crowd, assign a value of 5 square feet. In a packed crowd, assign a value of 2.5 square feet.
Divide the square footage of the venue by the square footage each person occupies. For example, the exposition center in Madison Square Garden is 36,000 square feet. At a busy expo, the crowd might be tight. Divide 36,000 by 5 and a good estimate for the crowd is 7,200.