World population growth accelerated in the last century. During the twentieth century alone, the number of people increased from 1.5 billion to 6 billion.
In the year 1800 the world’s population had reached 1 billion, with most people living in Asia and Europe. By 1927, 127 years later, the population of the world had doubled, reaching an staggering 2 billion people.
The world’s population reached 3 billion inhabitants by 1960, 4 billion by 1974, 5 billion by 1987, 6 billion by 1999, 7 billion by 2011.
Source: NOVA: Human Numbers Through Time