Donald Trump’s Monday inauguration is likely to revive a familiar question: How many people will attend? The answer, as history shows, is surprisingly difficult to determine — and this year brings unprecedented challenges. The Solidarity Day March (1981) 250,000 (AP) March on Washington for Lesbian, Gay, and Bisexual Equal Rights and Liberation (1993) 1,000,000 (National Mall) Obama’s first inauguration (2009) 1,800,000 (ABC) Trump’s first inauguration (2017) 720,000 (The White House/NBC News) Women’s March on Washington (2017) 470,000 (The New York Times) Due to forecasted sub-zero temperatures in Washington, D.C., the traditional outdoor ceremony will move inside the Capitol Rotunda for the first time, with additional public viewing at Capital One Arena. While this shift eliminates typical weather-related counting obstacles, indoor venues present their own measurement challenges for crowd scientists. The question of attendance numbers is likely to draw particular scrutiny. Throughout his 2024 campaign, Trump has consistently emphasized rally crowd sizes, making this inauguration’s attendance figures a closely watched metric of public support. But what exactly goes into counting a crowd? The science behind these calculations has evolved significantly over the years. The science of counting crowds Modern crowd counting traces its origins to Herbert Jacobs, a University of California, Berkeley journalism professor who developed his groundbreaking method in 1967 while observing student protests from his office window. His solution, now known as the “Jacobs Method,” would revolutionize crowd counting and lay the groundwork for methods still used today. Jacobs found that he could calculate total crowd size by dividing spaces into grids and counting sample areas. He also established key density benchmarks: In loose crowds, each person occupies about 10 square feet (roughly the size of a small dining table) In dense gatherings, 6-7 square feet In extremely tight conditions, space can compress to as little as 3 square feet per person Modern crowd counting has evolved far beyond Jacobs’s calculations. Crowd scientists are now equipped with sophisticated tools—from artificial intelligence to thermal imaging—to analyze aerial photos and track movement patterns. Computer vision algorithms can process countless photos to create detailed models of how crowds distribute themselves across spaces. Yet even with these advances, the core challenge Jacobs identified remains: accurately measuring how densely people pack into spaces. Counting crowds on the National Mall is still as much art as science. These counting challenges become especially significant when the numbers carry political weight. A history of controversy Inauguration crowd numbers have become not just statistics, but measures of political support and influence. The National Park Service’s relationship with crowd counting illustrates just how thorny these estimates can be. For decades, the Park Service was the official judge of crowd sizes on the National Mall. This changed after the 1995 Million Man March, when their estimate of 400,000 attendees sparked intense controversy. This led Congress to specifically bar the agency from counting crowds. “No matter what we said or did, no one ever felt we gave a fair estimate,” U.S. Park Police Maj. J.J. McLaughlin explained in 1996. The agency made a rare exception in 2009, estimating 1.8 million attendees at Obama’s first inauguration. In the absence of an official source, multiple groups have stepped in to fill the void. Politicians, media organizations, and crowd scientists now generate totals using various techniques, frequently arriving at widely different estimates. These totals often spark debates. The issue gained renewed attention in 2017 when the then-Trump administration challenged media reporting on inauguration attendance. On the following day, Trump delivered a speech at the CIA headquarters in Langley, Virginia. “I looked out, the field was — it looked like a million, million and a half people,” he said. Media networks, Trump said, “showed a field where there were practically nobody standing there.” This dispute over crowd size was hardly unique – it highlighted a persistent challenge in crowd science: the gap between what observers perceive and what technical analysis can verify. At the National Mall, the traditional gathering place for presidential inaugurations, these challenges become even more pronounced. The sprawling space, which can accommodate hundreds of thousands of people, poses a perfect storm of crowd counting challenges. The challenges of counting crowds on The Mall One major obstacle is getting a clear overhead view. Federal law restricts building heights in Washington, D.C., while tight security measures create a flight restricted zone around the District, keeping aircraft and drones at bay. The National Mall, stretching two miles from the Capitol to the Lincoln Memorial, poses unique counting challenges. Its vast size, multiple entry points and continuous crowd movement make traditional head counts nearly impossible. Inauguration crowds cluster unevenly across the Mall, concentrating around screens, speakers, and areas with clear views of the Capitol. A section near the Capitol Steps might pack people in tightly, while similar-sized areas further back—such as the Reflecting Pool and the Washington Monument grounds—might hold just a fraction of that number. Winter weather can compound these challenges, with winter gear and umbrellas making individuals harder to distinguish in aerial shots. The cold also changes how crowds arrange themselves – people tend to stand closer together for warmth, making it difficult to accurately measure density. Finally, overcast skies and cloud cover can interfere with satellite imaging, forcing researchers to rely more heavily on ground-level counting methods. Modern crowd scientists employ a combination of methods to tackle these issues. Dr. G. Keith Still, a professor of crowd science at the University of Suffolk who has spent over three decades studying the issue, told NBC that accurate crowd counting involves more than just aerial views. “You don’t necessarily count how many people are on the site,” Still said. “You look at things like transport capacity, because the number of people in the area might be limited by their access. What’s the narrowest point of the system? What’s the maximum flow rate? For things like inaugurations, you can see how the site is laid out. You can measure those areas, compare to previous events, do comparative analysis.” To monitor flow, they can use time-lapse photography and “flow-rate” calculations at checkpoints and Metro stations. To accurately capture crowd density, then can use grid-sampling, as in the Jacobs Method. Most analysts recommend using a combination of approaches to ensure an accurate estimate. As Washington prepares for another rare indoor presidential inauguration, crowd scientists must adapt their methods from the sprawling National Mall to the enclosed spaces of the Capitol Rotunda and the Capital One Arena. While these controlled environments with known capacities eliminate some traditional unknowns, tracking attendance across multiple venues presents its own unique challenges. What’s clear is that regardless of the setting, the task of measuring public participation in this historic moment remains as crucial – and contentious – as ever.