According to the United States Census Bureau, the town has a total area of 138.6 km² (53.5 mi²). 137.9 km² (53.2 mi²) of it is land and 0.7 km² (0.3 mi²) of it is water. The total area is 0.50% water.
As of the census of 2000, there are 116,510 people, 45,076 households, and 29,885 families residing in the town. The population density is 844.8/km² (2,188.1/mi²). There are 46,803 housing units at an average density of 339.4/km² (879.0/mi²). The racial makeup of the town is 89.28% White, 3.90% African American, 0.13% Native American, 5.22% Asian, 0.02% Pacific Islander, 0.37% from other races, and 1.09% from two or more races. 1.36% of the population are Hispanic or Latino of any race.
There are 45,076 households out of which 29.7% have children under the age of 18 living with them, 55.6% are married couples living together, 8.3% have a female householder with no husband present, and 33.7% are non-families. 28.3% of all households are made up of individuals and 13.4% have someone living alone who is 65 years of age or older. The average household size is 2.42 and the average family size is 3.01.
In the town the population is spread out with 22.2% under the age of 18, 10.8% from 18 to 24, 25.3% from 25 to 44, 24.0% from 45 to 64, and 17.7% who are 65 years of age or older. The median age is 40 years. For every 100 females there are 89.3 males. For every 100 females age 18 and over, there are 85.2 males.
The median income for a household in the town is $55,427, and the median income for a family is $68,951. Males have a median income of $51,667 versus $32,030 for females. The per capita income for the town is $27,647. 6.4% of the population and 4.2% of families are below the poverty line. Out of the total people living in poverty, 6.6% are under the age of 18 and 5.4% are 65 or older.
There are three institutions of higher education located in Amherst. One of the two campuses of the State University at Buffalo (including most of the non-medical programs), Daemen College (a private 4-year college), and one of the three campuses of Erie Community College are located there.