A quick mental-math trick to estimate how fast your money doubles
💡 Why This Matters The Rule of 72 is the fastest way to size up an investment or interest rate in your head — no compound interest formula required. It's the same shortcut financial planners use to explain, in seconds, why a small difference in return rate matters so much over time.
72 was chosen over the mathematically exact 69.3 because it divides evenly by so many small numbers — 2, 3, 4, 6, 8, 9, and 12. That makes the Rule of 72 genuinely usable as mental math, which was the entire point.
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The Rule of 72 is a shortcut: divide 72 by your annual interest rate to estimate how many years it takes an investment to double. Run it the other way — divide 72 by a target number of years — and you get the rate you'd need to hit that goal. Both directions are the same simple division, just solving for a different variable.
In "Years to Double" mode, the result tells you roughly how long your money takes to double at a steady compound rate. In "Reverse from Target" mode, the result tells you what average annual return you'd need to double your money by a certain year. Either way, it's a fast gut-check, not a precise financial projection.
The Rule of 72 is most accurate for rates between roughly 6% and 10% — outside that band, the approximation drifts a bit from the true logarithmic calculation. It also assumes a constant compound rate with no withdrawals or additional contributions, which real investments rarely deliver exactly.
Use "Years to Double" to sanity-check how a specific rate of return (savings account, index fund, bond) stacks up over time. Use "Reverse from Target" to figure out what rate you'd realistically need to hit a goal — a useful gut-check before assuming an aggressive return is achievable.
Forward example: At an 8% annual return, your money doubles in about 72 ÷ 8 = 9 years.
Reverse example: To double your money in 6 years, you'd need roughly 72 ÷ 6 = 12% annual return.
It's a close approximation, most accurate for interest rates between roughly 6% and 10%. Outside that range, the estimate drifts slightly from the precise logarithmic calculation, but it's typically within a few months of the exact answer — plenty accurate for quick mental math.
Divide 72 by your target number of years. For example, to double your money in 6 years, you'd need roughly a 12% annual return (72 ÷ 6 = 12%). Use the Reverse tab above to calculate this instantly for any target timeframe.
Yes — it works for any quantity growing at a steady compound rate, including inflation (how fast prices double), population growth, or debt growing at a fixed interest rate. The math is the same regardless of what's compounding.
It's a simplification of the natural logarithm of 2 (ln(2) ≈ 0.693) divided by the interest rate, rearranged into a number that's easy to divide by mentally. 72 was chosen because it has many small whole-number divisors (2, 3, 4, 6, 8, 9, 12), making quick mental math easier than the mathematically "purer" 69.3 would allow.