Those who paid attention to the news or social media Saturday likely saw coverage of worldwide protests related to the election of President Donald Trump. A political science professor at the University of Connecticut believes this weekend’s protests marked the largest in American history, and he’s provided some publicly available data to back up his claims.

Dr. Jeremy Pressman, UConn’s Director of Middle East Studies and an associate political science professor, used data submitted by attendees at various locations from Saturday’s protests to create baseline estimates of how many people participated in the Women’s March.

As the old cliché goes: necessity is the mother of invention. Pressman didn’t have much of a resource for tracking so he began creating his own. 

“I tweeted to see if anybody knew of a list, and nobody offered anything up,” Pressman told Complex. “So I started keeping track, and people began sending links on Twitter. My friend and colleague Erica Chenoweth, from the University of Denver, contacted me to help.”

According to Pressman’s data, which is available in a publicly accessible Google spreadsheet, low estimates have roughly 3.2 million people participating in Saturday’s march, while high estimates peg the number at approximately 4.2 million.

Similar data obtained by Washington Post transportation reporter Faiz Siddiqui appears to support Pressman’s estimates. According to Siddiqui, Saturday was the second-busiest day in D.C. Metro history, with 1,001,613 trips. Those numbers would put Saturday’s march just behind President Barack Obama’s 2009 Presidential Inauguration in terms of Metro traffic.

Comparing Saturday’s march with other organized protests in terms of sheer numerical volume becomes difficult for a number of reasons—one in particular.

“[The Women’s March on D.C.] definitely had many more people than the March on Washington,” Pressman noted. “But the original March on Washington during the U.S. Civil Rights Movement wasn’t billed as a march that also featured marches in different locales. So it’s apples and oranges comparing the two.”

Pressman added that at least anecdotally, seeing men participate in the marches confirmed the notion of this being a human rights issue with broad appeal. He had a similar reaction to many observers of Saturday’s events after seeing the data firsthand.

“Part of my reaction after seeing the list is just the geographic breadth of places where people marched. I was really struck by that. Everyone knew there were marches in L.A., New York, and Chicago, but they weren’t all huge. People in battleground states, red states, and blue states all turned out in all different kinds of states. It was both rural and urban areas. That dynamic was very striking to me.”

With Trump’s history of insulting women who oppose him—and, of course, the infamous Access Hollywood tape—the marches drew celebrity supporters such as Madonna, Alicia Keys, and Janelle Monae, as well as millions from cities across the nation.

You can view Dr. Pressman’s data here.