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Free Rule of 72 Calculator — Doubling Time for Any Interest Rate

Enter your annual interest rate to instantly see how long it takes to double, triple, and 10× your investment. Includes a comparison table for rates 1–15%.

Use this free Rule of 72 calculator to estimate investment doubling time using the simple mental math formula: 72 ÷ annual rate = years to double. Accurate to within 1 year for rates 3–15%. No sign-up required.

What Is the Rule of 72?

The Rule of 72 is a simple mental math shortcut to estimate how long it takes to double an investment at a given annual compound interest rate. Divide 72 by the annual rate and you get the approximate doubling time in years.

At 6%, money doubles in 12 years. At 8%, it doubles in 9 years. At 12%, just 6 years. The rule is remarkably accurate — within one year — for rates between 3% and 15%.

The rule also works in reverse: to find the required rate to double in N years, compute 72/N. Want to double your money in 8 years? You need 72/8 = 9% per year. For exact results, use the full compound interest formula via our compound interest calculator.

How to Use This Calculator

  1. 1

    Enter the Annual Interest Rate

    Enter the annual return rate of your investment as a percentage. For index funds use 7–10%, for savings accounts 4–5%, for bonds 3–5%. The formula works for any positive rate.

  2. 2

    Add a Starting Amount (optional)

    If you enter a starting amount, the calculator will show the actual dollar value when your money doubles, triples, and reaches 10×. Without an amount, it only shows the time in years.

The Rule of 72 Formula

Doubling Time (years) = 72 ÷ Annual Rate (%)

Example: At 7% → 72/7 = 10.3 years to double

Tripling Time (years) = 114 ÷ Annual Rate (%)

Example: At 7% → 114/7 = 16.3 years to triple

For the exact mathematical doubling time, use: t = ln(2) / ln(1 + r) where r is the decimal rate. At 7%: t = 0.693 / 0.0677 = 10.24 years. The Rule of 72 gives 10.3 — a difference of about 3 weeks.

Real-World Rule of 72 Examples

Quick benchmarks for common investment scenarios.

Index Fund at 7% (Inflation-Adjusted)

At 7% real return, your money doubles every 10.3 years. A 25-year-old with $10,000 in a Roth IRA at 7% will have $80,000 by 65 just from doubling alone — three doublings (10, 20, 30 years) = 2³ = 8× growth.

HYSA at 5% (Current Savings Account)

At 5%, money doubles every 14.4 years. $10,000 in a HYSA becomes $20,000 in ~14 years — better than cash under a mattress, but slower than an index fund. Use HYSAs for emergency funds; invest for long-term goals.

Credit Card at 20% APR (Debt)

Credit card debt at 20% APR doubles in 3.6 years. A $5,000 balance becomes $10,000 in under 4 years if only minimum payments are made. The Rule of 72 shows why high-interest debt must be eliminated before investing.

Frequently Asked Questions

What is the Rule of 72?
The Rule of 72 is a quick mental math formula: divide 72 by your annual interest rate to estimate the years it takes to double your money. At 6%, money doubles in 72/6 = 12 years. At 9%, it doubles in 8 years. The rule is accurate to within 1 year for rates between 3% and 15%.
How accurate is the Rule of 72?
The Rule of 72 is accurate to within 1 year for interest rates between 3% and 15%. At exactly 7%, the rule gives 10.3 years; the exact answer (using logarithms) is 10.24 years — a difference of under 1 month. For rates below 3% or above 15%, the rule becomes less precise.
What rate doubles money in 10 years?
A 7.2% annual return doubles your money in exactly 10 years (72/10 = 7.2%). The S&P 500's historical inflation-adjusted return of 7% doubles money in about 10.3 years. A 10% nominal return doubles money in 7.2 years.
How long does it take to double $10,000?
At 7% annual return, $10,000 doubles to $20,000 in approximately 10.3 years. At 10%, it doubles to $20,000 in 7.2 years. At 3% (inflation only), doubling takes 24 years — which illustrates why beating inflation matters for building real wealth.
What is the Rule of 114?
The Rule of 114 estimates tripling time: divide 114 by your annual rate. At 7%, money triples in 114/7 = 16.3 years. Combined with Rule of 72 (doubling in 10.3 years), you can estimate the full compounding trajectory without a calculator.
Can the Rule of 72 be used for debt?
Yes — the Rule of 72 works for debt, too. Credit card debt at 18% APR doubles in 72/18 = 4 years. A $5,000 balance at 18% grows to $10,000 in just 4 years if unpaid. This is why minimum-only payments on high-interest debt are so damaging.

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