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Compound interest and the Rule of 72
A 25-year-old who sets aside ten dollars a day in a low-cost retires at 65 with roughly two million dollars. The same person, doing the same thing, starting at 35 instead of 25, retires with roughly seven hundred thousand. The cost of that ten-year delay is more than every dollar the late starter ever saves. Compounding is unforgiving to procrastinators and lavishly generous to anyone who shows up early.
Most beginners assume investing is about picking which stock will go up next. It isn't. The single most consequential investing skill — the one that decides whether you retire wealthy or working — is letting time do its work on a small, steady amount of capital. The mechanism behind it has a sober name and an unsexy reputation: compound interest. Get it working for you and the math is so generous it looks unfair. Get it working against you (credit card debt, cash sitting in a checking account that loses ground to inflation) and the same math is unforgiving. Either way, it never sleeps.
A bank account paying hands you the same dollars every year — your $1,000 earns $50 annually, forever. Compound interest pays you on the original deposit AND on the interest you've already earned. Year one earns $50. Year two earns $52.50, because you started year two with $1,050. Year three earns $55.13. Each year the engine runs a little hotter because it's burning a little more fuel. After thirty years that $1,000 isn't $2,500 — it's $4,322. After fifty years it's $11,467. The growth bends upward instead of running flat.
To feel the bend, picture $10,000 invested in a broad index fund at 10% per year — close to the long-run average of the U.S. stock market. After year one you have $11,000. Earned $1,000. After year two: $12,100. Earned $1,100, slightly more than the year before. After year ten: $25,937. The gain in year ten alone is $2,358 — more than double what you earned in year one, even though you never added another dollar. After year thirty: $174,494. The annual gain in year thirty is roughly $15,800 — sixteen times the year-one number. The original $10,000 is still in there somewhere, but it's the smaller part of what you own. The bigger part is gains earning gains earning gains.
Final wealth is decided by three knobs: how much you contribute, what return you earn, and how long you stay invested. Beginners obsess over the second knob — chasing higher returns is what every brokerage ad is selling. The first knob (saving more) is also intuitive. The third knob — time — gets ignored, because waiting feels passive and unproductive. Quietly, time is the most powerful lever, and it is the only one you can never get back. A dollar invested at 25 compounds for 40 years before retirement. A dollar invested at 35 compounds for 30. That extra decade roughly doubles its final value, before you've added a single extra contribution.
Both investors put in $5,000/year at 10% return. The only difference is when they started. Those ten extra years of compounding created $1.4 million in additional wealth — far more than the $50K extra in contributions.
Drag the sliders to see how daily savings, return rate, and time horizon shape your wealth. Watch how the orange area (investment gains) dwarfs the blue area (your contributions) once enough years go by.
Real markets don't return the same percentage every year. The S&P 500's long-run nominal return averages around 10%, but in any single year it has been as low as -38% (2008) and as high as +37% (1995). The compounding math still works on the long-run average — that's what measures — but you need a horizon long enough to absorb the bumps and the discipline not to sell during the bad years. The rate also has to be ; we'll handle inflation in the next lesson.
The difference between 7% and 10% may look small. Over decades, it is the difference between comfortable and extraordinary.
| Return Rate | 10 Years | 20 Years | 30 Years | 40 Years |
|---|---|---|---|---|
| 3% (savings) | $13,439 | $18,061 | $24,273 | $32,620 |
| 7% (bonds) | $19,672 | $38,697 | $76,123 | $149,745 |
| 10% (stocks) | $25,937 | $67,275 | $174,494 | $452,593 |
| 12% | $31,058 | $96,463 | $299,599 | $930,510 |
When Warren Buffett took control of Berkshire Hathaway in 1965, the stock traded around $19 a share. Fifty-nine years later, a single Class A share traded above $500,000. Berkshire compounded at roughly 19.8% per year over that period; the S&P 500 (with dividends reinvested) compounded at roughly 10.2%. Half the rate, doubled the period, and the wealth gap is not 2x. It is closer to 140x. A $1,000 investment in Berkshire in 1965 became roughly $43.8 million by the end of 2023. The same $1,000 in the S&P 500 became roughly $313,000. Both are extraordinary outcomes by ordinary standards. The gap between them is what a few extra percentage points of compounded return does over a long enough horizon. Source: Berkshire Hathaway 2023 chairman's letter, performance-vs-S&P-500 table.
The Compound Calculator above shows the math abstractly. Once you log in, two screens make it concrete for your own holdings. The Portfolio Hub projects your current portfolio's value at 5, 10, 20, and 30-year horizons using your actual blend of stocks. Portfolio Lab lets you sketch alternative portfolios — say, S&P 500 vs. all-cash vs. a single stock — and watch how compounded outcomes diverge over decades. Every stock page also has a long-horizon total-return chart that shows what reinvesting dividends and gains would have done over the past 5, 10, or 20 years for that company specifically. Same arithmetic; your numbers.
Two common ways. The first is reaching for higher returns by buying speculative stocks, leverage, or option lottery tickets. A steady 10% beats most attempts at 30% because the high-return strategies that fail (and most do) compound the loss instead of the gain. The second, quieter way is interruption — selling in a panic during a bad year, pulling money out for a car, switching brokerages and triggering taxes, or chasing whatever asset is hot. Charlie Munger's standing rule applies: never interrupt compounding unnecessarily. A third trap is silent: a 1% annual over 40 years quietly costs you about a third of your final wealth. Pick low-cost index funds, sit still, and let the second-order effect do the work.
Time is the friend of the wonderful business, the enemy of the mediocre. It may seem like a simple proposition. It is also what allows the patient investor to compound at the rate of the businesses they own.