Real Interest Rate Calculator (Fisher Equation)
Calculate the real interest rate from the nominal rate and inflation using the exact Fisher equation.
See how inflation erodes purchasing power over time.
Nominal vs. Real Interest Rates When a bank advertises a 6% savings account, that is the nominal rate — the rate before accounting for inflation. The real interest rate is what you actually earn in terms of purchasing power. If inflation is 4%, a 6% nominal rate only gives you 2% more buying power — that is your real return. If inflation equals or exceeds the nominal rate, your money loses purchasing power even as the balance grows.
The Approximate Formula The simple approximation (Nominal − Inflation) works well when rates are low: Real Rate ≈ Nominal Rate − Inflation Rate Example: 7% nominal − 3% inflation = 4% real rate. This approximation becomes less accurate as rates climb above 10%.
The Exact Fisher Equation Irving Fisher (1867–1947), an American economist, derived the precise relationship in 1930: (1 + Real Rate) = (1 + Nominal Rate) / (1 + Inflation Rate) Rearranged: Real Rate = (1 + Nominal) / (1 + Inflation) − 1
Example with 10% nominal and 6% inflation: Approximate: 10% − 6% = 4.0% Exact Fisher: (1.10 / 1.06) − 1 = 3.774% The difference grows as rates increase — always use the exact formula for precision.
What a Negative Real Rate Means A negative real interest rate means inflation is outpacing your nominal return. Your account balance grows nominally, but you can buy LESS with it each year. This is common during high-inflation periods when central banks keep rates low. Savers are effectively subsidizing borrowers during periods of negative real rates. In 2022, many savings accounts paid 0.5% nominal while inflation ran at 8–9%, creating real rates of −8% or worse.
Purchasing Power Over Time If you invest $1,000 at a 5% nominal rate with 3% inflation for 10 years: Nominal future value: $1,000 × 1.05^10 = $1,629 Real future value (today dollars): $1,629 / 1.03^10 = $1,213 The real gain is only $213, not $629 — inflation consumed $416 of apparent gain.
Why It Matters for Investment Decisions The real interest rate is the true benchmark for investment quality. Bonds, CDs, and savings accounts must offer a positive real rate to actually grow wealth. Equities historically deliver 6–7% real returns over long periods, which is why they outperform fixed income. For retirees, the real rate determines whether their savings will sustain their lifestyle over decades.