According to the United States Census Bureau, the city has a total area of 15.7 km² (6.1 mi²). 15.7 km² (6.1 mi²) of it is land and 0.1 km² (0.04 mi²) of it is water. The total area is 0.33% water.
As of the census of 2000, there are 21,131 people, 7,897 households, and 5,606 families residing in the city. The population density is 1,348.5/km² (3,490.7/mi²). There are 8,146 housing units at an average density of 519.9/km² (1,345.7/mi²). The racial makeup of the city is 87.77% White, 4.33% African American, 0.09% Native American, 4.45% Asian, 0.01% Pacific Islander, 1.70% from other races, and 1.65% from two or more races. 10.17% of the population are Hispanic or Latino of any race.
There are 7,897 households out of which 35.7% have children under the age of 18 living with them, 61.1% are married couples living together, 7.8% have a female householder with no husband present, and 29.0% are non-families. 23.9% of all households are made up of individuals and 10.2% have someone living alone who is 65 years of age or older. The average household size is 2.67 and the average family size is 3.18.
In the city the population is spread out with 27.0% under the age of 18, 4.4% from 18 to 24, 33.0% from 25 to 44, 22.5% from 45 to 64, and 13.1% who are 65 years of age or older. The median age is 37 years. For every 100 females there are 93.8 males. For every 100 females age 18 and over, there are 89.1 males.
The median income for a household in the city is $92,964, and the median income for a family is $117,053. Males have a median income of $85,625 versus $46,811 for females. The per capita income for the city is $62,598. 4.2% of the population and 2.5% of families are below the poverty line. Out of the total people living in poverty, 4.1% are under the age of 18 and 4.3% are 65 or older.
Summit is the birthplace of actress, Meryl Streep.