The Rule of 72 is the fastest mental math trick in investing. Divide 72 by the annual return to get years to double, or divide 72 by years to get the required return.
Fill in the details and
your result appears here.
Know the rate? Find years to double. Know the target years? Find the required rate.
Rate or years. The result is instant.
Three doublings turn 1x into 8x. See what your investment becomes after three full doublings.
The exact formula is: Years = log(2) / log(1 + r). Rule of 72 is an approximation accurate within 2% for rates between 6% and 20%. It is useful for rapid mental calculations without a calculator.
Works for any doubling scenario — investment returns, inflation, debt, population growth.
A mental math shortcut: divide 72 by the annual return percentage to get approximate years to double an investment. At 12%, money doubles in 6 years. At 6%, it doubles in 12 years. Attributed to Luca Pacioli (1494).
Within 2-3% for rates between 6% and 20%. At exactly 8%, it predicts 9 years — actual is 9.006. At 20%, it predicts 3.6 years — actual is 3.80. For quick estimates, it is highly reliable.
Yes. At 6% inflation, purchasing power halves in 72/6 = 12 years. Your Rs.1 lakh today buys only Rs.50,000 worth of goods in 12 years. This is why beating inflation with investments is essential.
Yes. A credit card at 36% APR: 72/36 = 2 years for your debt to double if unpaid. At 12% personal loan: 72/12 = 6 years. The same rule that grows wealth also shows how fast debt compounds against you.
The Rule of 72 estimates how long it takes to double your money: Years = 72 / annual interest rate. At 8%: 72/8 = 9 years. At 12%: 72/12 = 6 years. It also works in reverse: rate = 72 / years to double.