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The Rule of 72: How Long Does It Take to Double Your Money?

The Rule of 72 is a simple mental math trick that tells you exactly how long it takes to double your money at a given interest rate. Learn how to use it instantly.

What Is the Rule of 72?


The Rule of 72 is one of the most useful mental math shortcuts in personal finance. It gives you a quick estimate of how many years it takes to double your money at a given annual interest rate.


The formula is simple: Years to double = 72 Γ· Interest Rate


Examples


Interest RateYears to Double-------------------------------2%36 years4%18 years6%12 years8%9 years10%7.2 years12%6 years

How Accurate Is It?


The Rule of 72 is an approximation, but it's surprisingly accurate for rates between 6% and 10%. Let's compare:


  • At 8% annual rate: Rule of 72 says 9 years. Actual calculation: 9.01 years. βœ“
  • At 6% annual rate: Rule of 72 says 12 years. Actual calculation: 11.90 years. βœ“
  • At 12% annual rate: Rule of 72 says 6 years. Actual calculation: 6.12 years. βœ“

    • For higher rates, use Rule of 69.3 for more precision.


    Real-World Applications


    Savings Account at 4.5% APY: 72 Γ· 4.5 = 16 years to double your savings.


    S&P 500 historical average (~10%): 72 Γ· 10 = 7.2 years to double.


    Credit card debt at 20%: 72 Γ· 20 = 3.6 years for your debt to DOUBLE if unpaid.


    Reverse Application: Finding the Required Rate


    You can also use Rule of 72 in reverse: if you want to double your money in 5 years, you need 72 Γ· 5 = 14.4% annual return.


    This helps set realistic expectations β€” doubling money in 5 years requires a very high return, typically only achievable through high-risk investments.


    The Compounding Behind the Rule


    The Rule of 72 works because of the mathematical relationship between compound interest and logarithms. The precise formula is:


    t = ln(2) / ln(1 + r) β‰ˆ 0.693 / r β‰ˆ 69.3 / r


    For rates around 8%, 72 is a more convenient approximation that gives slightly more accurate results due to rounding. This is why 72 β€” easily divisible by 1, 2, 3, 4, 6, 8, 9, 12 β€” became the standard.


    Use the Calculator for Precise Results


    While the Rule of 72 is great for quick mental math, use our free compound interest calculator β†’ for precise, year-by-year projections of your actual investments.

    Try our free compound interest calculator

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