Philadelphia lost almost 25,000 residents in a year, according to new census data looking at a full year of the pandemic released Thursday.

The drop in total population between July 2020 and July 2021 is the largest one-year decline since 1975 and follows almost a decade of population growth for Philadelphia, which topped 1.6 million residents in 2020. The losses were driven primarily by the residents who moved out of the city, which exceeded the number of people moving into Philly.

In the U.S. Census Bureau’s 12-month snapshot, Philly saw the highest disparity since 2001 between people moving in and those moving out. That difference led to a net loss of more than 28,000 residents, doubling what census numbers showed for the year prior.

There are a few possible factors driving the Philly departures. But Kevin Gillen, senior research fellow at Drexel University’s Lindy Institute for Urban Innovation, found that a desire to flee crowded urban centers was a powerful motivator as he studied migration patterns in the first year of the pandemic using Postal Service data.

Gillen’s team found young people were moving back with parents, more affluent residents were taking off to second homes, and city dwellers were ditching cosmopolitan life in exchange for green space.

“During the quarantine with everything shut down, there was nowhere to go and you didn’t want to be around other people,” he said. “So having a backyard now has had a real premium.”

And while Philadelphia saw a drop in its population, the departures did not translate to population booms for most of its collar counties, which saw modest gains.

One of the most dramatic findings in the new data was nearly three-quarters of U.S. counties recorded more deaths than births.

In Philadelphia, deaths reached their highest point since at least 2001. Births for the year marked record lows for the same period, but overall, the city reported slightly more births than deaths.

Philadelphia’s low birth numbers can’t be attributed to the pandemic alone, said Gillen, pointing to cultural shifts among millennials who are having children later in life, if at all, as they deal with rising housing prices and high levels of debt.

“If you’re deep in debt, and you’re still living in an apartment, and you’re not pairing up until you’re well into your 30s — all those are factors that work against having kids or as many kids as typical households would in previous generations,” said Gillen.

While a loss of 25,000 residents is significant, experts caution against putting too much emphasis on one data point collected in an unprecedented year.

“The domestic migration out of Philadelphia has been going on for quite some time,” said Ben Gruswitz, who uses this type of data for socioeconomic and land use analytics at the Delaware Valley Regional Planning Commission.

Gruswitz said without another year of data it’s unclear if the movement out of Philadelphia was permanent for some families. Even if those families don’t move back, Gruswitz believes births could reach pre-pandemic levels as life resumes. And immigration, which took a significant hit during former President Donald Trump’s administration and the pandemic, will likely increase over the rest of President Joe Biden’s term.

Gruswitz also cautions that the methods producing this latest batch of data are not as authoritative as the decennial count.

“It is just an estimate,” he said. “And it turned out that over the course of the last decade, they were pretty off from what actually seemed to happen.”

A city spokesperson was similarly cautious, saying Mayor Jim Kenney’s administration was still going over the numbers and couldn’t offer any more insights into what has caused the “change in population estimate.”

Compared to other metro areas — a central city and its nearby counties — Philadelphia as a region fared better in domestic migration. In the city and its surrounding suburbs, 15,000 more residents moved out than in, a far less steep decline than places like New York, Chicago, and Los Angeles. The Dallas and Phoenix metro areas, meanwhile, continued to see substantial population growth.

But unlike the city, deaths in the region overall exceeded births — a first since at least 2001 — by more than 3,000 people. Of the nation’s 10 largest metro areas, Philadelphia and Miami regions were the only ones to report more births than deaths.

Where we got these numbers
This analysis is based on the Census Bureau’s Population Estimates Program, which issues annual estimates for how the population of the country has changed since the previous year. Those estimates are made at the county level, so we've confined our analysis to county-level data. That makes it difficult to precisely compare the city of Philadelphia, which is its own county, to other big cities, which often aren't.
To compute the change in births, deaths, and migration from one year to the next, the Census Bureau relies on a number of official data sources, such as the National Center for Health Statistics, the IRS, and Medicare and Social Security records. Unlike the recent 2020 Census, which aimed to count every resident, these figures are estimates but do not come with a margin of error. The Census Bureau often adjusts estimates from prior years as new data is collected. For the historical data, population figures were available dating back to 1970, while available migration, birth, and death data only goes back to 2000.