Investment Returns Guide: Why 'Average Return' Is Misleading and What to Calculate Instead
The S&P 500's 'average annual return' is commonly cited as 10%. But if you invested $10,000 and got +50% in year 1 and −33% in year 2, your average return was +8.5% per year — and you broke even. The arithmetic average conceals the devastating effect of volatility. CAGR (compound annual growth rate) tells you what actually happened to your money. Understanding the difference is worth knowing before building any financial plan.
Key Takeaways
- CAGR, not arithmetic average, tells you what your portfolio actually earned per year — always prefer it for multi-year analysis
- S&P 500 CAGR (1990–2024) is approximately 10.7% nominal, ~7.5% inflation-adjusted — frequently cited "10%" is an arithmetic average
- Real return = nominal return minus inflation — a 6% return during 4% inflation is only 2% of actual purchasing power gained
- Sequence of returns risk means identical CAGR produces wildly different outcomes depending on when the bad years hit
- IRR handles irregular contributions — use it when you're evaluating investments with cash flows at different times
CAGR Formula and When to Use It
Compound Annual Growth Rate answers: "if my investment grew steadily each year, what single rate would produce the same end result?" The formula: CAGR = (End Value / Start Value)^(1/Years) − 1.
CAGR is best for comparing the historical performance of investments over the same time period, or evaluating whether an investment met its stated return targets. It doesn't tell you about the volatility along the way — two investments with identical CAGRs can have completely different risk profiles. For that, you need standard deviation of returns.
CAGR Examples:
// $10,000 → $25,000 over 10 years
CAGR = (25000/10000)^(1/10) − 1 = 9.6% per year
// $50,000 → $80,000 over 5 years
CAGR = (80000/50000)^(1/5) − 1 = 9.9% per year
// Annualized return converting total return
Total return 65% over 5 years → CAGR = 1.65^(0.2) − 1 = 10.6%
S&P 500 Historical Returns: The Real Numbers
| Period | Nominal CAGR | Inflation-Adjusted CAGR | Note |
|---|---|---|---|
| 1928–2024 (all available data) | ~9.8% | ~6.5% | Includes Great Depression, WWII |
| 1970–2024 | ~10.9% | ~6.8% | Includes 70s stagflation, tech boom/bust |
| 1990–2024 | ~10.7% | ~7.5% | Modern Fed era, dot-com, GFC, COVID |
| 2000–2009 (Lost Decade) | −0.9% | ~−3.4% | Dot-com + financial crisis combined |
| 2010–2024 | ~13.5% | ~10.8% | QE era, low interest rates |
| Arithmetic average (often cited) | ~12% | — | NOT what you earned; misleads |
Sequence of Returns Risk
If you're accumulating (making regular contributions), early bad years are actually better — you buy more shares at lower prices. If you're withdrawing (in retirement), early bad years are catastrophic — you sell shares at the bottom to fund living expenses, permanently reducing your portfolio's recovery potential.
Two retirees with identical portfolios and identical average returns can have completely different outcomes depending on which years the bear market hit. A −30% crash in year 1 of retirement followed by 10%/year for 20 years depletes a portfolio far faster than the same 20 years of gains followed by the same crash. This is why the "safe withdrawal rate" research (the 4% rule) is specific to retirement sequencing risk.
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