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Why inflation changes what your portfolio growth actually means
When your investment account shows a 10% gain, that's your nominal return — the raw number before adjusting for inflation. Your real return is what matters: the increase in your actual purchasing power. If inflation was 3%, your real return was approximately 7%. The difference between these two numbers is the single most common source of confusion in retirement planning.
It sounds like a technical distinction, but it has enormous practical consequences. A retirement plan built on nominal returns will systematically overestimate what your money can buy. A plan built on real returns — as Calcifer is — keeps the math honest by measuring everything in today's dollars from start to finish.
The relationship between real and nominal returns is described by the Fisher equation, named after economist Irving Fisher. The simplified approximation works well for low inflation rates. The precise version is needed when inflation is high.
Approximation
real ≈ nominal − inflation
7% ≈ 10% − 3%
Precise (Fisher)
1 + real = (1 + nom) ÷ (1 + inf)
1.068 = 1.10 ÷ 1.03
At the 3% long-run average inflation rate, the approximation is close enough for practical planning. At 8%+ inflation (as in the 1970s), the gap between the two formulas becomes meaningful and the precise version matters more.
Based on roughly 150 years of US market data, the standard reference points for long-run returns are well-established. These are the numbers that underlie most FIRE planning, including Calcifer's defaults.
| Asset class | Nominal | Inflation | Real |
|---|---|---|---|
| US stocks (S&P 500) | ~10% | ~3% | ~7% |
| US bonds (intermediate) | ~4% | ~3% | ~1% |
| Inflation (CPI) | — | ~3% | — |
Long-run historical averages. Individual decades vary significantly. International returns differ from US. Past performance does not guarantee future results.
Why Calcifer defaults to 7% real
The 7% real return assumption reflects 150+ years of US equity market history after accounting for inflation. It's not a guarantee — specific decades can be much worse (the 2000s decade produced near-zero real returns) or much better. It's a long-run planning assumption that has proven reasonable across a wide range of historical scenarios, including the periods studied in the original Trinity Study that produced the 4% rule.
The chart below shows three lines — all measured in today's dollars — for a $500K starting portfolio over 30 years. 10% nominal sounds much better than 7% real, but when you strip out inflation from the nominal return, the gap shrinks dramatically. The relevant number is always the real one.
Nominal returns adjusted for 3% inflation to show real purchasing power
After 30 years: 7% real → $3.8M · 10% nominal (6.8% real) → $3.6M · 7% nominal (3.9% real) → $1.6M in today's purchasing power.
FIRE planning has two sides: your expenses and your portfolio growth. Calcifer models your retirement expenses in today's dollars — your $60K/yr spending goal is $60K in today's purchasing power, not a number that inflates over time. To keep both sides of the equation consistent, the growth rate must also be in today's dollars. That means using real return, not nominal.
The consistency principle
If you model expenses in today's dollars but grow your portfolio at a nominal rate, your model contains a hidden assumption: that your expenses will grow with inflation while your portfolio growth is measured before inflation. This produces wildly optimistic projections — effectively double-counting inflation as a tailwind. Using real returns throughout keeps the math internally consistent.
The practical difference
At 10% nominal and 7% real, a $500K portfolio becomes approximately $8.7M nominal after 30 years — but only $3.8M in today's dollars. That's the number that tells you what you can actually spend. FIRE calculators that use nominal returns need to also apply a separate inflation adjustment to retirement spending each year. The real-return approach builds this in from the start.
Sequence of real returns matters
Even if average real returns are identical, the order in which they occur dramatically affects retirement outcomes. Two retirees with the same 7% average real return over 30 years can have very different results if one experiences large early losses while the other experiences them late. This is sequence-of-returns risk, and it affects real returns exactly as much as nominal ones.
Learn about sequence-of-returns risk →Using a nominal rate with inflation-adjusted expenses
If you enter 10% as your return assumption but model your retirement spending as a fixed dollar amount (not growing with inflation), you've accidentally mixed nominal and real. The model will be far too optimistic — effectively assuming your spending stays flat in nominal terms while your portfolio compounds at full nominal rate.
Confusing account value with purchasing power
$1M growing at 10%/yr for 30 years becomes $17.4M. At 3% inflation, that $17.4M buys only what $7.2M buys today. People who plan around nominal millionaire milestones often underestimate how much inflation erodes the real value of a given dollar target.
Treating the 7% real as a guaranteed floor
The 7% real return is a long-run historical average, not a minimum. The 2000s produced near-zero real returns for a full decade. Japan's market went 30+ years with negative real returns. A robust FIRE plan stress-tests at 4–5% real to understand what happens when the long-run average doesn't apply to your specific retirement window.
The 4% Rule
The study behind safe withdrawal rates and how real vs. nominal returns shaped the original research.
Sequence of Returns Risk
Same average return, completely different outcomes — why the order of returns matters so much.
Why Savings Rate Is Everything
Your savings rate matters more than your return assumption during accumulation. Here's the math.
Monte Carlo Simulation
How running thousands of return sequences reveals your real probability of success.