According to the United States Census Bureau, the city has a total area of 1.3 square miles (3.4 km²), of which, 1.3 square miles (3.3 km²) of it is land and 0.1 square miles (0.1 km²) of it (3.85%) is water. Wapello is on the Iowa River.
There were 836 households out of which 31.7% had children under the age of 18 living with them, 56.7% were married couples living together, 9.4% had a female householder with no husband present, and 31.5% were non-families. 27.2% of all households were made up of individuals and 15.1% had someone living alone who was 65 years of age or older. The average household size was 2.48 and the average family size was 3.02.
In the city the population was spread out with 25.0% under the age of 18, 9.6% from 18 to 24, 26.3% from 25 to 44, 21.0% from 45 to 64, and 18.1% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 93.8 males. For every 100 females age 18 and over, there were 90.4 males.
The median income for a household in the city was $37,556, and the median income for a family was $45,395. Males had a median income of $31,346 versus $21,220 for females. The per capita income for the city was $17,947. About 10.0% of families and 14.3% of the population were below the poverty line, including 25.4% of those under age 18 and 8.3% of those age 65 or over.