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.

The
current inflation rate
in 2017 is 1.99%^{1}. If this number holds, $1 today will be equivalent to $1.02 next year.

CPI in 2017 / CPI in 1940 * 1940 USD value = 2017 USD value

Then plug in historical CPI values. The U.S. CPI was 14 in the year 1940 and 244.786 in 2017:

244.786 / 14 * $1 =

The "purchasing power" of $1 from 1940 is $17.48 in 2017.

**
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.