The Rule of 72 is one of the most famous rules of thumb in finance. It provides a quick estimate of how long it takes for an investment to double at a given interest rate. This simple method is used by financial advisors, investors, and savers worldwide to better assess long-term investment decisions.
The name comes from the magic number 72, which is used as an approximation value for the calculation. The rule is based on the principle of compound interest – one of the most powerful concepts in wealth building, which Albert Einstein allegedly called "the eighth wonder of the world."
Using this calculator is straightforward:
The calculator instantly shows you how many years it will take for your invested capital to double.
Doubling Time (Years) = 72 / Interest Rate (%)
The mathematical derivation is based on the compound interest formula. For doubling: 2 = (1 + r)^n, where r is the interest rate and n is the number of years. By taking logarithms, we get n = ln(2) / ln(1+r). For small interest rates, this approximates to: n ≈ 0.693 / r = 69.3 / (r×100). The number 72 is preferred because it's divisible by many numbers (2, 3, 4, 6, 8, 9, 12).
Savings Account (2% interest): 72 / 2 = 36 years
Stock ETF (7% return): 72 / 7 = ~10.3 years
Crypto Staking (12% interest): 72 / 12 = 6 years
Real Estate (5% return): 72 / 5 = 14.4 years
The Rule of 72 provides very accurate results for interest rates between 6% and 10%. For lower rates (below 6%), the number 70 is more accurate, while for higher rates (above 12%), you should use 69. For rates around 8%, 72 is nearly perfect.
The classic Rule of 72 assumes annual compounding. With monthly compounding, capital doubles slightly faster because the compound interest effect occurs more frequently. For monthly compounding, you can use 69.3 instead of 72.
Yes! If you want to know what interest rate you need to double your money in a certain time, simply divide 72 by the desired years. Example: Doubling in 10 years requires 72/10 = 7.2% annual return.
No, the Rule of 72 shows nominal doubling. For real purchasing power, you must subtract the inflation rate. With 7% return and 2% inflation, the real return is about 5% – real doubling then takes 72/5 = 14.4 years.
The Rule of 72 illustrates the enormous power of compound interest and helps with important financial decisions: