According to the United States Census Bureau, the CDP has a total area of 5.8 square miles (14.9 km²), of which, 2.5 square miles (6.5 km²) of it is land and 3.2 square miles (8.3 km²) of it is water. The total area is 56.00% water.
There were 2,324 households in Nānākuli, out of which 46.0% had children under the age of 18 living with them, 58.8% were married couples living together, 22.3% had a female householder with no husband present, and 9.8% were non-families. 6.7% of all households were made up of individuals and 2.0% had someone living alone who was 65 years of age or older. The average household size was 4.65 and the average family size was 4.74.
In the CDP the population was spread out with 35.8% under the age of 18, 11.4% from 18 to 24, 27.4% from 25 to 44, 18.5% from 45 to 64, and 6.9% who were 65 years of age or older. The median age was 27 years. For every 100 females there were 99.0 males. For every 100 females age 18 and over, there were 92.8 males.
The median income for a household in Nānākuli was $45,352, and the median income for a family was $45,677. Males had a median income of $30,722 versus $24,731 for females. The per capita income for the CDP was $11,690. 21.3% of the population and 19.2% of families were below the poverty line. Out of the total population, 26.7% of those under the age of 18 and 9.6% of those 65 and older were living below the poverty line.