Formula
Future Cost = Today's Cost × (1 + Inflation Rate)ⁿ
Formula
Real Return ≈ Nominal Return − Inflation Rate
Note
The formula above — real return = nominal minus inflation — is the Fisher approximation. It is accurate enough when both rates are low (under ~5%), but it understates the real return when rates are high.
The precise Fisher equation is: (1 + r_real) × (1 + i) = (1 + r_nom), which rearranges to:
r_real = (1 + r_nom) / (1 + i) − 1
Worked example: Nominal return = 10%, inflation = 8%. - Approximation: 10% − 8% = 2.00% real - Precise Fisher: (1.10 / 1.08) − 1 = 1.85% real
The gap is small at low rates but grows material during high-inflation periods. At 10% nominal and 8% inflation, the approximation overstates your real return by ~0.15 percentage points — meaningful if you are comparing real returns across assets or evaluating inflation-linked bonds (TIPS). For everyday personal-finance decisions, the approximation is sufficient. When precision matters (e.g., calculating the real return on a TIPS vs. a nominal bond), use the exact form.
Source: Irving Fisher, The Theory of Interest (1930).
Compare
| Strategy | Nominal Return | Inflation (3%) | Real Return | Purchasing Power in 20 Years |
|---|---|---|---|---|
| Under the mattress | 0% | 3% | -3% | $10,000 → $5,537 in real terms |
| Big-bank savings (0.01%) | 0.01% | 3% | -2.99% | Virtually the same as the mattress |
| HYSA (4.8%) | 4.8% | 3% | +1.8% | $10,000 → $14,289 in real terms |
| S&P 500 Index (10% nominal) | 10% | 3% | +7% | $10,000 → $38,697 in real terms |
Key point
Money Illusion: Humans naturally think in nominal (face value) terms, not real (purchasing power) terms. Seeing $11,000 in a savings account after earning $1,000 in interest feels like a gain — even if inflation rose 6% and your $11,000 buys less than your original $10,000 did. Sophisticated investors think in real returns. When evaluating any investment, always subtract inflation.
Key point
Cash has a role: Emergency fund, near-term expenses (house down payment within 2 years), capital waiting to be deployed. The problem is treating cash as a long-term store of wealth when your investment time horizon is decades. Over 30+ years, inflation turns patient cash into a losing position.
Key point
The Value Investor's Shield: Stocks of highly profitable businesses with pricing power are a natural defense against inflation. When costs rise, a strong brand or essential service can raise its prices — passing inflation to consumers and protecting your real return. A business that cannot raise prices in an inflationary environment is slowly being eaten alive by its own cost structure. Pricing power is one of the most important qualities a value investor looks for.
Try it
Check-in
Check-in
Sit with the ideas.
Your savings account pays 2.0% APY. Inflation is running at 4.5%. What is your real (inflation-adjusted) return, and what is happening to your purchasing power?