How does an estimate of 100,000 to 122,000 participants sound for Saturday’s Womxn’s March in Seattle? It’s not 200,000, but still wildly beyond the organizers’ original expectation of 50,000 protesters. And those numbers are based on some real math.

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Now that we live in a world of alternative facts — at least when it comes to estimating crowd sizes — here is a way you can decide for yourself.

That Womxn’s March on Saturday in Seattle?

The photos and aerial video were dramatic, a river of protesters filling the streets from Judkins Park in the Central District all the way to Seattle Center.

News stories had figures of “at least” 100,000 participants. On Monday, the organizers were claiming 200,000.

Seattle police wisely don’t give crowd estimates.

Such numbers are fraught with controversy, depending on who is promoting what, or where your emotions lie.

Just take a look again at the video of White House press secretary Sean Spicer alleging “deliberately false reporting” of crowd size at President Trump’s inaugural.

Even “The Five” on Fox News, who aired the news conference live, afterward looked astonished at the vituperativeness.

You can talk about “alternative facts,” as did Trump counselor Kellyanne Conway, but it’s tough to argue about side-by-side images of the crowd at the National Mall for Obama’s 2009 inauguration and the significantly smaller crowd for Trump.

But back to Saturday’s march in Seattle, how does an estimate of 100,000 to 122,000 participants sound? It’s not 200,000, but still wildly beyond the organizers’ original expectation of 50,000 protesters.

And those numbers are based on some real math.

“There’s nothing really exotic about the method — it’s elementary-school math,” says Steve Doig, the Knight Chair in Journalism at Arizona State University, who has tracked crowd estimates for more than 20 years.

First, he says, you need the size of the area where the crowd is gathered.

“You can use the old Boy Scout trick that if you’re 6 feet tall, your stride is 3 feet,” says Doig.

Or, these days, use Google Earth Pro.

You can zoom in and click on the protest route as it turns this way and that way. Google will provide you with the overall length.

Then you’ll have to account for the width of the streets. On the march route, the widths varied from 66 feet to 96 feet. So average them out.

“It would be nice to be more precise than that, but this is an estimate,” says Doig. But, he adds, “They are reality-based estimates.”

For better numbers, we got the widths from Seattle Public Utilities’ Geographic Information Systems folks.

Supervisor Brian Rosete explains this is done using “impervious surface data,” and, anyway, you don’t want to get into the boring details, do you?

It turns out that the protest march was on 995,164 square feet of street surface.

In addition, the sidewalks and planting strips along the march added up to 443,366 square feet.

The next figure you use is that each person in the march occupies 10 square feet.

“That’s close enough so that if you reach out and extend your arm, you touch the shoulder of the person in front of you,” says Doig.

Closer than that, he says, “and you’re stomping on each other’s heels.”

So let’s estimate the Saturday crowd:

995,164 divided by 10 rounds out to 100,000 participants.

Some marchers used some parts of the sidewalks on some parts of the protest. Being generous, let’s use half of the 443,366 square feet of sidewalks. That translates to an extra 22,000 people.

Total: 100,000 to 122,000.

“I have no problem in being, I guess, generous in estimates,” says Doig. “You just don’t want to be ridiculously generous. Inflating estimates creates pressure for the next time. It tends to be a public-relations thing, like with Seattle’s Super Bowl parade. Look how great we are.”

Oh, yes.

The downtown Seattle parade honoring the Seahawks after they won the 2014 Super Bowl had crowd estimates of 700,000 to 1 million people.

Doig was among those quoted who put the estimate at 250,000 to 450,000.

The lower estimate wasn’t exactly well-received.

KIRO Radio talk-show host Ron Upshaw explained his reasoning for calling “B.S.” on the analysis, “I was there. There was 700,000 people there.”

Butch Street was another expert quoted in that Seahawks story. Living in Denver, he’s now retired from his job in charge of public-use statistics for the National Park Service.

He understands why those attending an event often feel more people attended it than really did.

“They have no idea. They say, ‘God, it looks like a million people,’ ” says Street.

It’s because of Street that the Park Service no longer gives crowd estimates for events.

In 1995, Louis Farrakhan called for a Million Man March at the National Mall, which is part of the Park Service.

Using aerial photos and a grid system, Street estimated attendance at about 400,000. After the ensuing controversy in which the Nation of Islam claimed they had reached the 1 million man goal, Congress told the Park Service to stop giving crowd estimates.

“He called me,” says Street about Farrakhan. “I told him that it was like the sixth biggest gathering (at the mall) at the time. You should have called it the 400,000 Man March.”

Now Street is enjoying his retirement.

“It’s been a hot potato ever since,” he says, of estimating crowds.