The Rule of 72 Explained: How Long to Double Your Money

Published Jan 15, 2025 · 5 min read

Want to know how long it takes to double your savings? You don't need a financial advisor or a spreadsheet. You need one number: 72.

What Is the Rule of 72?

The Rule of 72 is a simple mental math shortcut:

At a 6% annual return, your money doubles in about 72 ÷ 6 = 12 years. At 8%, it doubles in roughly 9 years. At 12%, just 6 years.

The formula works in reverse too. If your investment doubled in 10 years, your annual return was approximately 72 ÷ 10 = 7.2%.

Rule of 72 Quick Reference Table

Notice how accurate the rule is — especially in the 4–12% range where most investments fall.

Real-World Examples

Example 1: High-Yield Savings Account

You put $10,000 in a savings account earning 5% APY. Using the Rule of 72: 72 ÷ 5 = 14.4 years to reach $20,000. After another 14.4 years, you’d have $40,000. No additional deposits needed — that’s the power of compound interest.

Example 2: S&P 500 Index Fund

The S&P 500 has historically returned about 10% per year. 72 ÷ 10 = 7.2 years. If you invested $50,000 at age 25, it could grow to $100,000 by 32, $200,000 by 39, $400,000 by 46, and $800,000 by 54 — all without adding a single dollar.

Example 3: Credit Card Debt (the scary side)

The rule works for debt too. A credit card charging 20% APR means your unpaid balance doubles every 72 ÷ 20 = 3.6 years. A $5,000 balance becomes $10,000 in under 4 years if you only make minimum payments.

Why Does 72 Work?

The exact formula uses natural logarithms: years = ln(2) / ln(1 + r), where r is the decimal rate. Since ln(2) ≈ 0.693, the precise "rule" would be the Rule of 69.3. But 72 is more convenient because it's divisible by 2, 3, 4, 6, 8, 9, and 12 — common interest rates. The slight upward adjustment also corrects for the effect of discrete compounding.

When the Rule Breaks Down

The Rule of 72 is an approximation. It loses accuracy at extreme rates:

• Very low rates (below 2%): The Rule of 69.3 is more accurate.
• Very high rates (above 20%): Consider using the Rule of 78 for better estimates.
• Variable returns: Real investments don’t earn a constant rate each year. The rule gives a rough guide, not a guarantee.

For precise calculations with your actual numbers, try our Compound Interest Calculator.

Tripling and Quadrupling Your Money

The concept extends beyond doubling:

• Rule of 114: Years to triple your money = 114 ÷ rate
• Rule of 144: Years to quadruple = 144 ÷ rate (or simply double the Rule of 72 result, since quadrupling = doubling twice)

At 8% annual returns: double in 9 years, triple in about 14 years, quadruple in about 18 years.

Key Takeaways

• The Rule of 72 is a quick, surprisingly accurate mental math tool for estimating investment growth.
• It applies to anything that grows at a compound rate — savings, investments, debt, or even inflation.
• Small differences in return rate have a massive long-term impact (8% vs 6% means doubling 3 years sooner).
• Start early: the more "doublings" you can fit in before retirement, the better off you’ll be.