The Rule of 72 is one of the most useful mental shortcuts in investing. It tells you approximately how long it takes to double your money at a given rate of return.
The Formula
72 / Annual Return = Years to Double
That is it.
Examples
- At 6% annual return: 72 / 6 = 12 years to double
- At 8% annual return: 72 / 8 = 9 years to double
- At 10% annual return: 72 / 10 = 7.2 years to double
- At 12% annual return: 72 / 12 = 6 years to double
Why It Matters
The Rule of 72 makes the abstract concept of compound growth feel concrete.
If someone tells you a fund averages 8% per year, you immediately know your money doubles roughly every 9 years. Starting with $10,000:
- 9 years: $20,000
- 18 years: $40,000
- 27 years: $80,000
- 36 years: $160,000
Seeing those numbers makes the case for starting early much more tangible than talking about "compound interest" in the abstract.
It Works in Reverse Too
You can also use it to estimate how quickly inflation erodes your money.
At 3% inflation: 72 / 3 = 24 years for your money to lose half its purchasing power.
$100 today buys the equivalent of $50 in 24 years if you leave it in a zero-interest account. This is why investing matters.
It Works for Debt Too
At 18% credit card interest: 72 / 18 = 4 years for your debt to double if you make no payments.
A $5,000 balance becomes $10,000 in 4 years. This is why paying off high-interest debt is often the best first investment you can make.
How Accurate Is It?
The Rule of 72 is an approximation. It is most accurate for returns between 6% and 10%. At very high or very low rates, it becomes less precise. But for quick mental math, it is remarkably useful.
The Bottom Line
Next time someone mentions an annual return, divide 72 by that number. You will instantly understand what that return means for your money over time.
The Progressive Trailblazer includes calculators that let you model compound growth with exact numbers. The Rule of 72 is the quick version. Educational only. Not financial advice.