Fisher Real Interest Rate Calculator
A stated, or nominal, interest rate tells you how fast your balance grows in dollars, but not how fast your buying power grows. The Fisher equation strips out inflation to reveal the real rate, the true increase in purchasing power. This calculator takes a nominal rate and an inflation rate, then returns the exact real rate from the Fisher formula alongside the common nominal-minus-inflation approximation so you can compare them. Inflation is a user input you set from an official price index for the relevant period.
Fisher equation formula
Exact real rate = (1 + nominal) / (1 + inflation) - 1
Approximate real rate = nominal - inflation
Difference = approximate - exact
All rates are expressed as decimals inside the formula and shown as percentages. The exact form divides by (1 + inflation); the approximation simply subtracts and grows less accurate as rates rise.
Real versus nominal rates
- The nominal rate is the headline rate before inflation; the real rate is after inflation.
- The exact Fisher formula is named after economist Irving Fisher.
- The approximation is accurate for small rates but diverges as inflation rises.
- A negative real rate means inflation is eroding purchasing power faster than interest accrues.
- Use an official price index, such as the U.S. CPI, for the inflation input.
Real versus nominal rate: frequently asked questions
What is the Fisher equation?
The Fisher equation links nominal interest rates, real interest rates, and inflation. In exact form, 1 + nominal = (1 + real) * (1 + inflation). Solving for the real rate gives real = (1 + nominal) / (1 + inflation) - 1. The familiar approximation is real is about nominal minus inflation.
What is the difference between the nominal and real rate?
The nominal rate is the stated rate of return or interest before accounting for inflation. The real rate is what you actually earn in purchasing power after inflation is removed. If your savings earn 5 percent but prices rise 3 percent, your real return is only about 2 percent.
Why use the exact formula instead of the approximation?
The approximation real equals nominal minus inflation is close when rates are small but understates the gap as rates rise. The exact Fisher formula divides by (1 + inflation), which matters at higher inflation. This calculator shows both so you can see the difference.
What inflation rate should I enter?
Use the inflation rate over the same period as your nominal rate, typically an annual rate. The U.S. Consumer Price Index from the Bureau of Labor Statistics is a common measure. Inflation is a user-editable input because it changes over time and varies by basket.
Can the real rate be negative?
Yes. When inflation exceeds the nominal interest rate, the real rate is negative, meaning your money loses purchasing power even though its nominal balance grows. Negative real rates have occurred during periods of high inflation or very low policy rates.
Official sources
- U.S. Bureau of Labor Statistics: Consumer Price Index.
- Board of Governors of the Federal Reserve System: Inflation and interest rates.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.