Inflation rates are calculated by the percentage difference between the Consumer Price Index, or CPI, of two dates. The inflation rate for the annual period between October 2014 and October 2015 is calculated by subtracting October 2014's CPI from October 2015's and dividing the difference by October 2014's CPI.
Multiplying the resulting quotient by 100 and adding a percentage symbol at the end gives the percentage rate of inflation. The Bureau of Labor Statistics publishes CPI data monthly. The CPI is the average price that urban consumers pay for a selection of goods and services, which is then compared to a reference base period.