My personal best estimate is around 1M. This is based on living in DC area for 20 years and reviewing available overhead photos. AJ Strata estimates over 1 M.
http://strata-sphere.com/blog/index.php/archives/10545
I agree.
Here’s another way to calculate the parade numbers:
Take the number abreast in the parade and assume a marching speed and a time elapsed and you can calculate how many people pass a given point (the beginning of the parade route) over that time period.
For example, I would put the number abreast, conservatively, at 30, since that would allow about one yard per person, more than enough. Posters here have claimed it took nearly an hour to go the mile up the street at the slowest pace, so that’s one mile per hour or 1760 yards per hour.
I would also assume one yard between rows on average, which again seems conservative given the pictures of the crowd. so that means that 1760 rows of marchers passed per hour.
Another poster claimed the parade lasted 2.5 hours, but assume 2 hours to be conservative. Thus, we have 30 marchers in a row x 1,760 rows per hour x 2 hours = 105,000 people.
I was surprised it was that high, as I think the numbers are quite conservative. In this format, with a calculator, you can play with the assumptions. For example, going with the 2.5 hours, you run the number up 25% to 132,000. Then assuming an average speed of 2 mph, you double that to 264,000 and if you increase the number per “row” to say, 40 (not unreasonable given the pictures I’ve seen) and you’re up another 1/3 to 352,000 (40 x 1760x2 x 2.5).
Then assume nowhere near everyone marched and it’s easy to see how you could get close to a million. One thing appears certain, the 60-70,000 estimates originally publicized are bogus. The parade could be 20 abreast, take 2 hours, go at the snail’s pace of one mile an hour, and it would be 70,000 people just in the parade itself.