Since the founding of the United States in 1776 the highest year-over-year inflation rate observed was 29.78% in 1778. Inflation is calculated using the yearly change in the consumer price index (CPI), which was first introduced in 1913. CPI data before 1913 is estimated using a variety of methods and sources. Since the introduction of the CPI, the highest inflation rate observed was 19.66% in 1917. Year-over-year inflation is calculated by subtracting the value of the CPI at the beginning of the year, and subtracting the value at the end of year.