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Oakland housing activists Moms 4 Housing, who made national news by taking over a vacant West Oakland house in November, frequently cite a statistic alleging that there are four empty homes in Oakland for every homeless person — a figure alluded to in the group's name.

But is this true?

The answer to that question: It's complicated. Estimating both housing and homeless populations can be a tricky chore; depending on the method, San Francisco's 2019 homeless count ranges from just over 8,000 to nearly 17,600 persons.

And while the U.S. Census tallies both the housing totals and the occupancy rates for American cities every year, these estimates have a margin of error in the thousands.

Be that as it may, the best available public figures, variable though they may be, do support the four-to-one claim. In fact, the entire Bay Area has far more empty houses than people without homes in 2019.

Using the latest point-in-time counts from earlier this year and census estimates from 2018 (the most recent year for which data is available), here's where the Bay Area's most populous areas sit.
  • San Francisco: The city's official point-in-time homeless count for 2019 is homeless count 8,011, despite various conflicts suggesting that it should be higher. The previous year, the city had 38,651 empty homes (margin of error: 4,395).
  • Alameda County: Oakland's homeless count came out to 4,071 in 2019, while home vacancies total 15,571 (plus or minus 2,415). Note that this is a ratio of about 3.8 to one; the four to one figure cited by Moms 4 Housing came from a 2018 Mother Jones article. Fremont has 608 homeless persons to a comparably whopping 4,275 vacancies (plus or minus 1,459), and Hayward a ratio of 487 to 3,033 (plus or minus 1,051).
  • East Bay: Concord has the highest homeless count in Contra Costa County, estimated at 350 this year, with a vacancy rate of 1,413 — still enough to cover the homeless after a margin of error of 816. Vallejo is the next most populous East Bay city. Solano County doesn't delineate its homelessness estimates per individual city; for the entire county the count is 1,150, and number of vacant homes countywide is 6,254 (margin of error: 1,527).
  • North Bay: Santa Rosa has 1,803 homeless persons in the most recent count, versus 3,157 vacant homes (plus or minus 1,277). Notice the comparably very low vacancy rate — partly a product of the 2017 fire season, but still enough to cover the homeless count this year.
  • South Bay: San Jose has the second highest estimated homeless population in the Bay Area after San Francisco, with 6,097 persons. It also has the second most home vacancies, estimated at 13,958 (margin of error: 2,907). Neighboring Santa Clara's homeless count is just 326, but it has potentially ten times the number of homes available — 3,457 (plus or minus 1,343). Sunnyvale has nearly twice as many homeless at 624, but a comparable number of homes empty: 3,131 (plus or minus 1,164).
The ten most populous Bay Area cities listed above have a combined point-in-time homeless total of 63,527, and, margins of error notwithstanding, a census-estimated 92,800-plus homes vacant, a ratio of about three units for every two persons.

The census' annual American Communities Survey defines a home as vacant if there is either no occupant or a temporary occupant — temporary meaning "people who will be there for two months or less."

Vacancy rates are estimated by in-person visits to properties or by interviews via mail, email, or phone. The upcoming 2020 census will employ more exact methods and should result in housing and vacancy estimates with less room for error.