Cities that conduct homeless counts receive federal funding from the U.S. Department of Housing and Urban Development.
According to San Francisco's Human Services Agency, the city receives about $18 million a year to support its homeless services.
In January 2009, San Francisco recorded a homeless population of 6,514. In 2007, the number was 6,377 and in 2005, 6,248.
In 2002, the homeless population was tallied at 8,640.
The count, which takes place from 8 p.m. to midnight, will have hundreds of volunteers travel 150 counting routes throughout the city by foot and by car, according to the Human Services Agency.
Counters will note on tally sheets all homeless persons they see.
A census will also be taken of those living in shelters and other city-sponsored homeless programs.
Mayor Ed Lee will address the volunteers at 7:45 p.m. at the Department of Public Health.