Big Arm is a census-designated place (CDP) in Lake County, Montana, United States. The population was 131 at the 2000 census.
Big Arm is located at 47°48′8″N 114°16′51″W / 47.80222°N 114.28083°W / 47.80222; -114.28083 (47.802296, -114.280952).
According to the United States Census Bureau, the CDP has a total area of 5.4 square miles (14 km), all of it land.
As of the census of 2000, there were 131 people, 56 households, and 39 families residing in the CDP. The population density was 24.0 people per square mile (9.3/km²). There were 124 housing units at an average density of 22.8 per square mile (8.8/km²). The racial makeup of the CDP was 69.47% White and 30.53% Native American.
There were 56 households out of which 23.2% had children under the age of 18 living with them, 53.6% were married couples living together, 8.9% had a female householder with no husband present, and 28.6% were non-families. 21.4% of all households were made up of individuals and 7.1% had someone living alone who was 65 years of age or older. The average household size was 2.34 and the average family size was 2.60.
In the CDP, the population was spread out with 19.8% under the age of 18, 9.2% from 18 to 24, 18.3% from 25 to 44, 29.8% from 45 to 64, and 22.9% who were 65 years of age or older. The median age was 47 years. For every 100 females there were 111.3 males. For every 100 females age 18 and over, there were 110.0 males.
The median income for a household in the CDP was $29,000, and the median income for a family was $38,750. Males had a median income of $16,750 versus $21,875 for females. The per capita income for the CDP was $16,620. There were no families and 8.0% of the population living below the poverty line, including no under eighteens and 30.0% of those over 64.