What is the real (inflation-adjusted) return on an investment, and how much of the nominal gain is eroded by inflation over time?
This tool is for: Investors evaluating a long-term return on savings, bonds, or a portfolio against expected inflation · Savers comparing a quoted nominal yield against the real purchasing-power growth it delivers · Anyone planning a long-horizon goal (retirement, home purchase, tuition) where today's dollars and future dollars must be compared
- Your real (inflation-adjusted) annual return using the Fisher relation
- The future value of your investment in both nominal and real terms
- How much of the nominal gain is purely compensation for inflation rather than genuine purchasing-power growth
Formulas Used
Fisher Real-Return Relation
Real Return = (1 + Nominal Return) / (1 + Inflation) − 1
Where: Nominal Return = Headline annual return before inflation adjustment (decimal), Inflation = Annual inflation rate over the same period (decimal)
Source: Federal Reserve Bank of St. Louis — FRED educational material on nominal vs real interest rates (Fisher identity)
Future Value (Compounded)
FV = PV × (1 + r)^n
Where: PV = Present value / initial investment (USD), r = Annual return (nominal or real) (decimal), n = Years of compounding (years)
Source: Standard time-value-of-money identity — U.S. Securities and Exchange Commission investor education
Key Insight
Inflation erosion compounds multiplicatively, not additively. Over a 30-year horizon, a 3% inflation rate reduces purchasing power by roughly 59%, not 90% — but it also means that the gap between nominal and real future value widens exponentially, so the cost of using the nominal figure for long-range planning grows the longer the horizon.
Frequently Asked Questions
Why use the Fisher relation instead of just subtracting inflation from the nominal return?
Subtracting inflation (nominal − inflation) is a linear approximation that is accurate only when both rates are small. At higher rates, the approximation understates the erosion. The exact identity is the Fisher relation — (1 + nominal) / (1 + inflation) − 1 — which recognises that inflation compounds alongside the return, not separately from it. For a 10% nominal return and 5% inflation, the approximation gives 5%, the exact Fisher real return is 4.76%. The gap widens further at higher rates.
Is the inflation_rate input supposed to be a forecast or a historical average?
Either can be defensible, but each carries a different meaning. A historical long-run average (U.S. CPI has averaged near 3% over recent decades) produces a planning-grade estimate for long horizons but can be badly wrong for short ones. A forward forecast (from TIPS break-even spreads or professional surveys) better reflects current expectations but is noisy and subject to revision. For personal planning, many practitioners use the long-run historical average as a base case and also run the calculator with a higher and lower inflation rate to see the sensitivity.
How does this calculator treat taxes?
It does not. Both the nominal and real future values are pre-tax. In practice, taxes are typically levied on the nominal return (dividends, interest, and realised capital gains are taxed on their nominal dollar amount, regardless of inflation). The after-tax real return is therefore lower than the pre-tax real return this calculator reports — often meaningfully so over long horizons. To approximate, reduce the nominal return input by your expected effective tax rate on investment income before running the calculation.
About This Calculator
Sources:
- U.S. Bureau of Labor Statistics — Consumer Price Index (CPI) program — Official U.S. inflation measurement used as the reference series for the inflation rate input and for historical CPI context
- Federal Reserve Bank of St. Louis (FRED) — Nominal vs Real Interest Rates explainer — Federal Reserve educational reference for the Fisher relation used in this calculator and for the distinction between nominal and real returns
Limitations:
- Assumes constant nominal return and constant inflation across the full horizon — both vary significantly in practice
- Pre-tax — does not model that taxes are levied on nominal returns, which further lowers the real after-tax return
- Ignores fees, transaction costs, and bid-ask spreads that reduce realised return
- Does not model sequence-of-returns risk — the order in which returns arrive matters for investors taking withdrawals
- Uses a single inflation rate for all categories of spending — real-world inflation varies by category (housing, healthcare, education) and by region
When to consult a professional: Before sizing a retirement, college, or major-purchase plan whose viability depends materially on the real return assumption — a licensed financial planner can stress-test the horizon against historical inflation-return distributions
This calculator estimates real (inflation-adjusted) return and future value under the assumption of constant nominal return and constant inflation over the horizon. Real-world returns and inflation vary year to year, often substantially; this is a planning model, not a forecast, and it does not constitute investment advice. Results are pre-tax and do not model taxes, fees, or the sequence of returns.