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Use our free Rule of 72 calculator to discover exactly how many years it will take for your investment portfolio to double. We overlay the famous mental heuristic against the exact logarithmic compound interest formula.
Last updated: February 2026
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Primary Calculation
72 ÷ Interest Rate = Years
Calculates exact compounding friction to output the actual timeline to a 100% gain.
Reverse Calculation
72 ÷ Years = Rate %
If you know you only have 6 years before retirement, this finds the aggressive yield you must hunt for.
The Limit Test
Logarithmic Overlay
High or low interest rates break the Rule of 72. We calculate the exact logarithmic answer behind the scenes to keep you honest.
Visualizing the power of compounding requires looking at standard historical asset class returns to set your expectations correctly.
How does the "Rule of 72" estimation compare to the actual rigorous algebra required to track compound interest?
Target Return Rate
8.0%
Rule of 72 Result
9.00 Yrs
Exact Geometric Math
9.01 Yrs Deviation: 0.01
Notice how at 8%, the rule is miraculously accurate. At 25%, the calculation completely fractures.
The Rule of 72 is taught in elementary finance classes, but the reason it actually works stems from the complex calculus governing exponential growth systems.
Rule of 72 Approximation: t ≈ 72 / rExact Geometric Formula: t = ln(2) / ln(1 + r/100)Exact Continuous Formula: t = ln(2) / r ≈ 0.693 / rBecause humans struggle dividing numbers like 24 into 69.3 inside their heads, the standard became "72", simply because 72 is highly divisible by integers (1, 2, 3, 4, 6, 8, 9, 12).
Now that you know how long it takes to double your money, utilize identical compound mechanics to track your Dividend Yield Pipelines.
Share this math tool with your spouse or business partner to quickly identify bad deals.
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