The rate of inflation formula shown is not to be confused with the purchasing power of goods relative to income. An example, albeit an extreme example, would be an individual who recently discovers that their income will increase to $1,000,000 from $20,000 per year--a 5,000% increase. Rate of Inflation formula = (CPI x+1 – CPI x) / CPI x. Or, Rate of Inflation = ($1110 – $1000) / $1000 = $110 / $1000 = 11%. In a normal scenario, the inflation rate is around 2-3%. Normally, the inflation rate doesn’t reach 11% at all. Inflation rate calculator solving for real interest rate given nominal interest rate and inflation AJ Design ☰ Math Geometry Physics Force Fluid Mechanics Finance Loan Calculator. Inflation Rate Equations Calculator Finance - Real Interest Rates - Formulas. Solving for real interest rate. Inputs: nominal interest rate (n) inflation rate (i)