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The mental math of wealth. Audit how long it takes for an investment to double in size based on a fixed annual rate of return using the classic Rule of 72.
Step-by-step breakdown of the underlying equations.
The Rule of 72 is a reasonably accurate shortcut for the complex compound interest formula. At a 7% return, your money doubles every 10.3 years. If you increase the rate by just 2%, you could cut your doubling time significantly.
The Rule of 72 Calculator provides an instant estimate of how long it takes for your investment to double at a given rate of return. This simple yet powerful heuristic has been used by investors for centuries to quickly assess the growth potential of different investment opportunities.
Simply divide 72 by your expected annual return percentage, and you'll know approximately how many years until your money doubles. It's a mental math shortcut that replaces complex compound interest calculations.
Scenario: John has $50,000 to invest in an index fund with historical returns of 7% annually.
Key insight: The same $50,000 at 10% would double every 7.2 years—reaching $400,000 in just 21.6 years instead of 30.
Pro tip: Use this to compare investment options quickly. A 1-2% difference in returns may seem small, but over decades it dramatically affects your doubling time and final wealth.
The Rule of 72 is a simple formula to estimate how long it takes for an investment to double at a fixed annual rate of return. Divide 72 by the interest rate to get the approximate doubling time in years. For example, at 8% annual return, money doubles in approximately 9 years (72 ÷ 8 = 9).
The Rule of 72 is most accurate for interest rates between 6% and 10%. At 8%, it's nearly exact. For rates outside this range, the Rule of 69 or Rule of 70 may be slightly more accurate. For typical investment returns, the Rule of 72 provides a quick and useful approximation.
Yes, you can use the Rule of 72 to estimate how long it takes for prices to double due to inflation. At 3% inflation, prices double in about 24 years (72 ÷ 3). This helps visualize the erosion of purchasing power over time.
The Rule of 72 is a mental shortcut that approximates the compound interest formula. The exact formula is t = ln(2)/ln(1+r), where t is time and r is the rate. The Rule of 72 gives nearly identical results for common rates but is much easier to calculate without a calculator.
The Rule of 72 helps visualize long-term growth. If you're 30 with $100,000 invested at 7% returns, your money doubles roughly every 10 years: $200K at 40, $400K at 50, $800K at 60. This demonstrates why starting early and earning higher returns dramatically impacts retirement savings.