Rule of 72 Calculator
Estimate how long it takes to double your money at any interest rate using the Rule of 72.
The Power Behind the Rule of 72
The Rule of 72 is one of the most useful mental math tricks in personal finance. Simply divide 72 by your annual interest rate to get the approximate number of years required to double your money. At 6%, your money doubles every 12 years. At 9%, every 8 years. At 12%, every 6 years.
The math behind it: the exact doubling time is ln(2)/ln(1+r), where ln is the natural logarithm. Since ln(2) ≈ 0.693, and 0.693 ≈ 72/100 for practical rates, the shortcut works remarkably well for rates between 5% and 15%.
Apply it to see the dramatic impact of small rate differences. A portfolio earning 6% vs 8% doesn't just earn 2% more — at 6%, it doubles every 12 years; at 8%, every 9 years. Over 36 years: 6% produces 3 doublings (8x), while 8% produces 4 doublings (16x). That small difference compounded produces twice as much wealth.
The Rule of 72 also works in reverse for debt: if you carry a balance at 24% APR and make no payments, your debt doubles in just 3 years. This is why high-interest debt should almost always be prioritized for payoff before investing.