Formula to Calculate the Rate of Inflation. The rate of inflation formula helps us to understand how much the price of goods and services in an economy has increased in a year. For example, if the price of goods and services in an economy is now $103 and in the previous year the same was $100, then, the inflation is $3. Let's look at a real-world example. In 2011, the December CPI was 225.672, and the December CPI in 2012 was 229.601. You can calculate the inflation rate from December 2011 to December 2012 by Let’s measure inflation rate. Suppose, in December 2007, the consumer price index was 193.6 and, in December 2008, it was 223.8. Thus, the inflation rate during the last one year was. 223.8- 193.6/ 193.6 x 100 = 15.6. As inflation is a state of rising prices, deflation may be defined as a state of falling prices but not fall in prices.