Real Interest Rate Calculator (Fisher Equation)

Convert a nominal interest rate and inflation rate into the real interest rate using the exact Fisher equation.

Result

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How to use this calculator

Enter the nominal (stated) interest rate.
Enter the inflation rate.
Read the exact real interest rate from the Fisher equation.
Compare it to the simple subtraction approximation and note if it is negative.

About this calculator

The real interest rate is the return on money after stripping out inflation — what your savings actually gain in purchasing power, as opposed to the nominal (stated) rate. The Fisher equation, named after economist Irving Fisher, links the three: one plus the nominal rate equals one plus the real rate times one plus inflation. Solving for the real rate means dividing, not just subtracting: real = (1 + nominal) ÷ (1 + inflation) − 1. The common shortcut of subtracting inflation from the nominal rate is a good approximation only when rates are low; at higher rates the difference grows. This calculator shows both the exact Fisher result and the approximation, plus the error between them, and flags when the real rate is negative — meaning inflation is outpacing your interest and your money is quietly losing value.

How it works — the formula

(1 + nominal) = (1 + real)(1 + inflation)
Real = (1 + nominal) / (1 + inflation) − 1
Approx: Real ≈ nominal − inflation

The exact relationship is multiplicative; dividing isolates the real rate. Subtraction is a first-order approximation.

Source: Investopedia — Fisher Equation

Worked examples

Example 1

Nominal 7%, inflation 3%

Inputs:: nominal=7, inflation=3
Output:: real 3.8835% (approx says 4%)

Example 2

Nominal 2%, inflation 5%

Inputs:: nominal=2, inflation=5
Output:: real −2.857% (negative)

Example 3

Nominal 12%, inflation 8%

Inputs:: nominal=12, inflation=8
Output:: real 3.704% (approx 4%)

Limitations

Uses annual rates; match the compounding period for consistency.
Expected vs realized inflation differ — uses the figure you enter.
Pre-tax; taxes further reduce the real after-tax rate.

Definitional calculation; inflation expectations are uncertain.

Frequently asked

What is the real interest rate?+

It is the nominal interest rate adjusted for inflation — the actual growth in purchasing power your money earns. If you earn 7% but prices rise 3%, your real return is only about 3.88%, not 7%.

What is the Fisher equation?+

It states that (1 + nominal rate) = (1 + real rate) × (1 + inflation rate). Rearranged, the real rate = (1 + nominal) ÷ (1 + inflation) − 1. It precisely relates nominal rates, real rates, and inflation.

Why not just subtract inflation from the nominal rate?+

That approximation (real ≈ nominal − inflation) is close at low rates but increasingly inaccurate as rates rise, because it ignores the cross term. The exact Fisher equation divides, giving a slightly lower real rate than simple subtraction.

What does a negative real interest rate mean?+

It means inflation is higher than the nominal interest you earn, so your money loses purchasing power over time even while the dollar balance grows. Savers in low-rate, high-inflation periods often face negative real rates.

Why does the real rate matter?+

It is what determines whether you are truly getting richer. Investment decisions, loan costs, and savings growth should be judged in real terms, because only real returns increase what your money can actually buy.

Is this the same as the inflation-adjusted return on investments?+

Conceptually yes — it is the same Fisher relationship applied to investment returns. A 10% nominal portfolio return with 4% inflation is a real return of about 5.77%, which is the figure that reflects genuine wealth growth.