Lehi is part of the Provo–Orem, Utah Metropolitan Statistical Area.
As of the census of 2000, there were 19,028 people (although, city projects 46,909 people in 2008), 5,125 households, and 4,602 families residing in the city. The population density was 936.2 people per square mile (361.6/km²). There were 5,280 housing units at an average density of 259.8/sq mi (100.3/km²). The racial makeup of the city was 95.68% White, 0.25% African American, 0.58% Native American, 0.47% Asian, 0.43% Pacific Islander, 1.33% from other races, and 1.26% from two or more races. Hispanic or Latino of any race were 2.99% of the population.
There were 5,125 households out of which 61.4% had children under the age of 18 living with them, 80.0% were married couples living together, 7.2% had a female householder with no husband present, and 10.2% were non-families. 8.9% of all households were made up of individuals and 3.5% had someone living alone who was 65 years of age or older. The average household size was 3.70 and the average family size was 3.94.
In the city the population was spread out with 41.0% under the age of 18, 11.6% from 18 to 24, 31.1% from 25 to 44, 11.1% from 45 to 64, and 5.2% who were 65 years of age or older. The median age was 24 years. For every 100 females there were 100.4 males. For every 100 females age 18 and over, there were 96.6 males.
The median income for a household in the city was $53,028, and the median income for a family was $55,664. Males had a median income of $40,739 versus $25,931 for females. The per capita income for the city was $16,074. About 5.0% of families and 5.5% of the population were below the poverty line, including 7.2% of those under age 18 and 3.6% of those age 65 or over.