The Rule of 72: How to Calculate When Your Money Doubles in 5 Seconds

There is one piece of mental math every investor should know.
It is called the Rule of 72.
It tells you, in five seconds, how long your money will take to double.
No calculator. No spreadsheet. Just division.

The rule, in one sentence

72 ÷ your annual return = years until your money doubles.

That is it. The entire rule.

Examples that make it click

• Earn 10% per year (S&P 500 average): 72 ÷ 10 = 7.2 years to double.
• Earn 8% per year: 72 ÷ 8 = 9 years to double.
• Earn 4% per year (high-yield savings): 72 ÷ 4 = 18 years to double.
• Earn 2% per year (typical bank): 72 ÷ 2 = 36 years to double.

The Rule of 72 makes one thing brutally clear

Most people compare investments by 1-year returns. The Rule of 72 forces you to think in doublings.

Same money. Same time. The 10% account doubles 5.5 times. The 4% account doubles 2.2 times. That is a 9x difference from a return rate that looks "only" 6 percentage points apart.

What the rule reveals about savings accounts

A typical savings account pays around 0.5% per year. 72 ÷ 0.5 = 144 years to double. Your great-grandchildren might see the result.

This is not financial advice. It is math. Money sitting in a regular savings account is not "safe" — it is silently losing value to inflation while taking 144 years to double. An index fund at 10% doubles in 7. The risk is not in investing. The risk is in not.

How to use the rule today

1. Take your current savings — say, $5,000.
2. Pick a realistic return rate — 10% for an index fund, 4% for a bond fund, 0.5% for savings.
3. Divide 72 by that rate — that is when your money doubles.
4. Double again, and again — that is your real timeline.

$5,000 in an index fund at 10% becomes $10,000 in 7 years, $20,000 in 14, $40,000 in 21, $80,000 in 28. Without adding a single dollar.

Run your actual starting amount and your timeline through the simulator below — see your doublings happen in real numbers, year by year.