How Long Does It Take to Double Your Money?

The Rule of 72

Divide 72 by your annual return rate. The result is the number of years it takes your money to double.

• 1% return → 72 years to double
• 3% return → 24 years
• 5% return → ~14 years
• 6% return → 12 years
• 8% return → 9 years
• 10% return → 7.2 years
• 12% return → 6 years

The rule is approximate but accurate within a fraction of a year for most returns up to 20%.

Why it works

It's a clever approximation of the exact compound interest formula: years = ln(2) / ln(1 + r). The actual constant is 69.3, but 72 is used because it divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12).

Real-world examples

Savings account at 4%

$10,000 takes 18 years to become $20,000. By comparison, a high-yield account at 5% does it in 14.4 years.

Index fund at 8% historical average

$10,000 doubles in 9 years → $20K. Doubles again in another 9 years → $40K. After 36 years (4 doublings) → $160,000. That's the power of compound growth on a single deposit.

Aggressive portfolio at 10%

Money doubles every 7.2 years. $10K becomes $640K in 42 years, just from doubling 6 times.

The flip side: doubling costs

The Rule of 72 also applies to debts and inflation:

• Credit card at 24%: balance doubles in 3 years if unpaid
• Inflation at 3%: prices double in 24 years
• Inflation at 6%: prices double in 12 years (1970s-style)

Tripling and quadrupling

Two related rules:

• Rule of 114: years to triple. At 8%, money triples in 14.25 years.
• Rule of 144: years to quadruple. At 8%, money quadruples in 18 years.

How to use it for retirement planning

If you're 25 with $20K saved at 8%, you can expect approximately:

• Age 34: $40K (1 doubling)
• Age 43: $80K (2 doublings)
• Age 52: $160K (3 doublings)
• Age 61: $320K (4 doublings)
• Age 70: $640K (5 doublings)

And that's without adding a single dollar more. With monthly contributions, the result is far higher.

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