According to the Bureau of Labor Statistics, the dollar experienced an average inflation rate of 3.79% per year. Prices in 2017 are 1648.0% higher than prices in 1940.
In other words, $1 in the year 1940 is equivalent to $17.48 in 2017, a difference of $16.48 over 77 years.
Inflation Data Source: The Bureau of Labor Statistics' Consumer Price Index (CPI), established in 1913. Inflation data from 1665 to 1912 is sourced from a historical study conducted by political science professor Robert Sahr at Oregon State University.