Rule of 72 — How Fast Will Your Money Double?
Example: Starting amount: 10000 $ · Annual return: 8 % · Investment horizon: 30 years
| Final value | $100,627 |
| Rule of 72 — years to double | 9 |
| Exact years to double | 9.01 |
| Number of doublings in your horizon | 3.33 |
Worked example
At 8% annual return, the Rule of 72 says your money doubles every 9 years (72 ÷ 8). The exact math says 9.006 years — the rule is extremely accurate. Over a 30-year horizon, $10,000 undergoes about 3.33 doublings, growing to $100,627. Start at 25 instead of 35 and you add one more full doubling — another $100,000.
Frequently asked questions
How accurate is the Rule of 72?
Very accurate for returns between 6% and 10%, where the error is less than 1%. At 3% the rule says 24 years, the exact answer is 23.4. At 15% the rule says 4.8 years, exact is 4.96. The rule was first documented in the 15th century and is close enough for mental math and quick planning conversations.
Can I use the Rule of 72 for debt?
Yes — the Rule of 72 works just as well for debt doubling. A credit card at 24% APR will double the amount you owe in just 3 years if you make no payments. That framing makes the cost of high-interest debt visceral in a way that interest rate percentages often do not.
What return should I assume?
The S&P 500 has returned roughly 10% annually nominal (before inflation) and about 7% real (after inflation) since the 1920s, per Federal Reserve and academic data. A diversified portfolio including bonds will return less. For mental models: 6% is a conservative baseline for a balanced portfolio; 8–10% for an all-equity long-horizon portfolio.