Growth & Return
Rule of 72 Calculator
Estimate how fast your money doubles, with the exact doubling time too.
Rule of 72 Calculator
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How it works
The rule of 72 is a mental shortcut: divide 72 by the annual rate and you get the approximate years to double. It works because compounding is exponential, so the doubling time is roughly inversely proportional to the rate.
The shortcut is most accurate between 6% and 12%. This calculator also returns the exact doubling time using logarithms, so you can see how close the quick estimate comes.
Formula
Years to double ≈ 72 / r (estimate)
Years to double = ln(2) / ln(1 + r/100) (exact)
Worked example
Example: at 8% per year, 72 / 8 = 9 years by the rule. The exact log formula gives 9.01 years, so the estimate is within a week for this rate.
WORKED EXAMPLE — DEFAULT INPUTS
What to know
The rule also works in reverse for debt: at 18% a credit card balance doubles in about four years if unpaid. That single comparison — 72 divided by the rate — explains why high-interest debt is so destructive and why paying it off early beats most investment returns you can find.
Financial planners sometimes quote the rule of 110 for the stock allocation in a portfolio: hold roughly (110 minus your age) in stocks. It is the same mental-math style applied to a different question, and it shows how far a simple rule can take you when exact math is not needed.
Quick reference
Doubling time at common rates: 4% → 18.0 years (rule 18.0), 6% → 11.9 (rule 12.0), 8% → 9.0 (rule 9.0), 10% → 7.3 (rule 7.2), 12% → 6.1 (rule 6.0). The rule stays within a few months of exact for everyday rates.
FAQ
Why 72 and not 70 or 100?
72 has many divisors, which makes mental math easy, and it is a good fit for rates between 6% and 12%. At higher rates the rule drifts and the exact formula is better.
Does the rule work for inflation?
Yes, in reverse. Divide 72 by the inflation rate to see how many years until your purchasing power halves.
What about a 4% rate?
72 / 4 = 18 years by the rule; the exact answer is 17.67 years. The estimate stays close at low to medium rates.
Can I use it for loans?
You can, but loan costs compound on the balance you carry. The rule describes any exponential growth or decay at a fixed rate.