Investment · February 13, 2026 · 4 min read
The Rule of 72: A Quick Way to Estimate Investment Growth
Before reaching for a precise calculation, the Rule of 72 gives a surprisingly accurate mental shortcut for one of the most common investment questions: how long until this doubles?
How the rule works
Divide 72 by your expected annual rate of return, and the result approximates the number of years it takes your money to double. At a 6% return, that's 72 ÷ 6, or about 12 years. At 9%, it's roughly 8 years.
This mental shortcut is remarkably close to the actual compound interest calculation for typical rates between 6-10%, which is why it remains popular despite modern calculators making exact math instant.
Where it becomes less accurate
The rule's accuracy declines at very low or very high rates, since it's an approximation rather than the exact compound interest formula. For a precise answer at any rate, a Compound Interest Calculator removes the guesswork entirely.
A useful reverse application
The rule also works in reverse: dividing 72 by your target number of years tells you roughly what rate of return you'd need to double your money in that timeframe — useful for quickly sanity-checking whether a specific growth goal is realistic.
Applying it quickly
At a 7% average annual return — a commonly used long-term stock market estimate — the Rule of 72 suggests money doubles roughly every 10.3 years (72 ÷ 7). Over a 30-year investing horizon, that implies almost three full doublings, illustrating why long time horizons matter so much for compound growth.
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Published February 13, 2026. This article is for general informational purposes only — read our disclaimer.