According to the United States Census Bureau, the city has a total area of 12.3 km² (4.7 mi²). 11.9 km² (4.6 mi²) of it is land and 0.3 km² (0.1 mi²) of it is water. The total area is 2.75% water.
As of the census of 2000, there are 17,708 people, 6,336 households, and 4,131 families residing in the city. The population density is 1,486.3/km² (3,850.5/mi²). There are 6,702 housing units at an average density of 562.5/km² (1,457.3/mi²). The racial makeup of the city is 81.46% White, 0.43% African American, 0.89% Native American, 2.11% Asian, 0.24% Pacific Islander, 11.39% from other races, and 3.47% from two or more races. 17.31% of the population are Hispanic or Latino of any race.
There are 6,336 households out of which 35.9% have children under the age of 18 living with them, 50.3% are married couples living together, 10.2% have a female householder with no husband present, and 34.8% are non-families. 27.9% of all households are made up of individuals and 14.3% have someone living alone who is 65 years of age or older. The average household size is 2.64 and the average family size is 3.24.
In the city the population is spread out with 27.4% under the age of 18, 13.4% from 18 to 24, 28.7% from 25 to 44, 16.8% from 45 to 64, and 13.6% who are 65 years of age or older. The median age is 31 years. For every 100 females there are 90.2 males. For every 100 females age 18 and over, there are 86.6 males.
The median income for a household in the city is $40,135, and the median income for a family is $47,733. Males have a median income of $36,139 versus $25,703 for females. The per capita income for the city is $16,992. 14.3% of the population and 10.4% of families are below the poverty line. Out of the total people living in poverty, 16.2% are under the age of 18 and 11.4% are 65 or older.
The principal landmark of Forest Grove is Pacific University, a private school of higher education.