Direct answer
A worked example of real interest rates shows how you start with a nominal interest rate, subtract an inflation measure (usually expected inflation), and interpret the result as the return in “purchasing power” terms—real rather than nominal.
To keep it verifiable, the example below uses only fixed, explicitly stated assumptions and no live market data.
Mechanism: definition and the calculation
A nominal interest rate is the interest rate stated in money terms (for example, “X% per year”).
A real interest rate aims to measure the change in purchasing power after accounting for inflation. The simplest common approximation is:
- Real interest rate ≈ Nominal interest rate − Expected inflation rate
Important interpretation notes:
- “Expected inflation” means what inflation rate you assume will occur over the same horizon as the nominal rate.
- If inflation is higher than assumed, the real return in purchasing power terms will be lower than you expected.
Worked example: transparent scenario with every assumption
Assume the following, stated plainly:
- The relevant nominal annual interest rate is 5.0%.
- The relevant expected annual inflation rate over the same horizon is 2.0%.
- We use the simple approximation: real ≈ nominal − expected inflation.
Step-by-step:
- Real interest rate ≈ 5.0% − 2.0% = 3.0%
Now change only one assumption to show sensitivity: 4) Keep the nominal annual interest rate at 5.0%. 5) Use an alternative expected inflation rate of 3.5%.
Step-by-step:
- Real interest rate ≈ 5.0% − 3.5% = 1.5%
What this illustrates:
- The real rate is not fixed by the nominal rate alone. It depends directly on the inflation assumption.
Limitations and failure modes (what can go wrong)
- Expectation vs. reality: You calculate the real rate using expected inflation, but actual inflation may differ. That gap changes realized purchasing-power outcomes.
- Which inflation measure and horizon: Different inflation indices (and different time windows) can produce different “expected inflation” inputs, changing the real-rate result.
- Costs and implementation: The theoretical calculation does not include real-world costs (spreads, fees, taxes, settlement frictions). Those can reduce the purchasing-power outcome relative to the simple arithmetic.
- Approximation limits: The “real ≈ nominal − inflation” relationship is a simplified rule. Depending on the context, more exact formulas may be used; using the approximation can introduce small errors.
Verification and next question
You can independently verify this worked example by checking the arithmetic with the stated assumptions:
- Real ≈ nominal − expected inflation
- 5.0% − 2.0% = 3.0%
A useful next question is: Which inflation measure and horizon matches your nominal rate’s timing? If those inputs are unclear, the “real” calculation can be internally consistent yet still not correspond to the purchasing-power experience you care about.