One of the important indices of population concentration is the density of population. It is defined as the number of persons per sq km. The population density of India in 2001 was 324 per sq km.
The density of population was increased in all States and Union Territories between 1991 and 2001. Among major states, West Bengal is still the most thickly populated state with a population density of 903 in 2001. Bihar is now the second highest densely populated state, pushing Kerala to the third place.
Source: India 2010 - A Reference Annual