Merriam's population was estimated to be 10,773 in the year 2006, a decrease of 230, or -2.1%, over the previous six years.
As of the U.S. Census in 2000, there were 11,008 people, 4,839 households, and 2,903 families residing in the city. The population density was 2,550.9 people per square mile (983.8/km²). There were 5,042 housing units at an average density of 1,168.4/sq mi (450.6/km²). The racial makeup of the city was 89.24% White, 4.02% Black or African American, 0.42% Native American, 2.09% Asian, 0.02% Pacific Islander, 1.95% from other races, and 2.25% from two or more races. Hispanic or Latino of any race were 5.41% of the population. The ancestral makeup was 21.9% German, 12.9% English, 11.5% Irish and 7.7% American.
There were 4,839 households out of which 26.3% had children under the age of 18 living with them, 46.8% were married couples living together, 9.7% had a female householder with no husband present, and 40.0% were non-families. 33.3% of all households were made up of individuals and 7.6% had someone living alone who was 65 years of age or older. The average household size was 2.23 and the average family size was 2.86.
In the city the population was spread out with 21.7% under the age of 18, 8.3% from 18 to 24, 34.0% from 25 to 44, 23.8% from 45 to 64, and 12.2% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 92.6 males. For every 100 females age 18 and over, there were 90.9 males.
The median income for a household in the city was $48,455, and the median income for a family was $54,639. Males had a median income of $37,358 versus $29,815 for females. The per capita income for the city was $23,988. About 4.3% of families and 6.2% of the population were below the poverty line, including 9.7% of those under age 18 and 7.1% of those age 65 or over.