Tens of Thousands Protest ObamaCare in D.C., Legacy Media Unimpressed
To start with, we have some crowd pictures available, like this one, taken about noon (two hours before the official end of the protest):
That's clearly a picture of the West Lawn of the Capitol; the crowd is pretty well concentrated between the Pennsylvania Avenue and Maryland Avenue extensions, and from the bottom of the steps back to South Capitol Circle.
Since the last time, I've gotten a copy of Google Earth Pro, so it was straightforward to compute the area of that section of the lawn: about 16,500 m2 (or in English that's about 20,000 square yards, or around four acres.)
Using the rule of thumb from the Park Service of about 1.8 people per meter square, that would give us 29,700 people. That picture shows the whole area wasn't full yet, so let's be conservative: say it's 75 percent full, or call it 22,000 people at noon.
Later photos, though, show that section of the lawn much more nearly full, and show the crowd was much more closely packed, at least up front.
It would appear that a decent estimate for the size of the rally by the end was at least 30,000 people; tweeted estimates in excess of 40,000 seem a bit high.
But credit where credit is due: this time, the media estimates are only off by a factor of between 10 and 100.