The Rule of 72: How Fast Will Your Money Double?
In personal finance, speed and compound growth are everything. The Rule of 72 is a famous mental math formula that allows you to calculate instantly how many years it will take to double your money at a given interest rate or annual investment return.
The Rule of 72 Formula
Quick Reference doubling Times
| Annual Rate of Return | Calculation | Years to Double Initial Money |
|---|---|---|
| 3% (High-Yield Savings / Inflation) | 72 ÷ 3 | 24.0 Years |
| 6% (Conservative Portfolio) | 72 ÷ 6 | 12.0 Years |
| 8% (Balanced Stock/Bond Mix) | 72 ÷ 8 | 9.0 Years |
| 10% (Historical S&P 500 Average) | 72 ÷ 10 | 7.2 Years |
| 12% (Aggressive Growth Stocks) | 72 ÷ 12 | 6.0 Years |
Why It Works and How to Use It
Mathematically, solving for doubling time requires natural logarithms: t = ln(2) / ln(1 + r). For reasonable interest rates between 2% and 14%, 72 provides a remarkably accurate approximation that anyone can compute in their head in seconds.
You can also invert the formula to find the interest rate required to double your money in a specific timeframe:
If you want to double your money in 6 years, you need an annual return of 72 ÷ 6 = 12% per year. Test precise compounding scenarios on our Investment Growth Calculator.
Use the Rule of 72 to measure the erosion of inflation! At 3% annual inflation, the purchasing power of your cash gets cut in half every 72 ÷ 3 = 24 years.