Alum Rock is a census-designated place in Santa Clara County, California.
Alum Rock first appeared as a census designated place in the 2000 U.S. census.
The 2020 United States census reported that the CDP had a population of 12,042. The population density was . The racial makeup of the CDP was 16.7% White, 1.4% African American, 3.4% Native American, 19.0% Asian, 0.3% Pacific Islander, 43.3% from other races, and 15.8% from two or more races. Hispanic or Latino of any race were 67.1% of the population.
The census reported that 98.7% of the population lived in households, 0.4% lived in non-institutionalized group quarters, and 1.0% were institutionalized.
There were 2,922 households, out of which 47.1% included children under the age of 18, 53.1% were married-couple households, 7.2% were cohabiting couple households, 24.0% had a female householder with no partner present, and 15.7% had a male householder with no partner present. 14.1% of households were one person, and 5.8% were one person aged 65 or older. The average household size was 4.07. There were 2,332 families (79.8% of all households).
The age distribution was 23.4% under the age of 18, 10.3% aged 18 to 24, 29.1% aged 25 to 44, 25.5% aged 45 to 64, and 11.7% who were 65years of age or older. The median age was 35.8years. For every 100 females, there were 103.8 males.
There were 3,029 housing units at an average density of , of which 2,922 (96.5%) were occupied. Of these, 62.0% were owner-occupied, and 38.0% were occupied by renters.
In 2023, the US Census Bureau estimated that 45.3% of the population were foreign-born. Of all people aged 5 or older, 27.3% spoke only English at home, 54.6% spoke Spanish, 1.9% spoke other Indo-European languages, 16.0% spoke Asian or Pacific Islander languages, and 0.2% spoke other languages. Of those aged 25 or older, 66.8% were high school graduates and 19.8% had a bachelor's degree.
The median household income was $111,976, and the per capita income was $34,255. About 5.1% of families and 8.7% of the population were below the poverty line.
In the California State Legislature, Alum Rock is in , and in .
In the United States House of Representatives, Alum Rock is in .
Most of Alum Rock CDP is in Alum Rock Union Elementary School District. A small portion is in Mount Pleasant Elementary School District. All of it is in East Side Union High School District.