The Rule of 72: Estimate How Fast Your Money Doubles
Divide 72 by the interest rate to estimate the years it takes to double your money. How accurate is it — and how can you use it for loans too?
How long will it take for your money to double? You do not need a calculator to get a good answer. The Rule of 72 is a mental shortcut used by investors and bankers for generations — and once you know it, you will start seeing compounding everywhere.
Key takeaways
- Years to double ≈ 72 ÷ annual rate of return.
- It is very accurate for rates between about 6% and 12%.
- It works for inflation and debt too: at 6% inflation, prices double in about 12 years.
The rule
At 8%, money doubles in about 72 ÷ 8 = 9 years. At 12%, in about 6 years. The rule assumes returns are reinvested — that is, compounded — every year.
How accurate is it?
The exact doubling time is ln(2) ÷ ln(1 + r). Here is how the shortcut compares:
| Annual rate | Rule of 72 | Exact |
|---|---|---|
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 9% | 8.0 years | 8.0 years |
| 12% | 6.0 years | 6.1 years |
| 15% | 4.8 years | 5.0 years |
For everyday planning, the error is negligible.
Practical uses
Comparing investments
An FD at 7% doubles in about 10 years; an investment returning 12% doubles in about 6. Over 30 years, that is roughly 3 doublings versus 5 — an 8-times multiple versus a 32-times multiple. Small differences in return become huge over long periods. See the power of compounding.
Understanding inflation
At 6% inflation, prices double in about 12 years. Your monthly expenses of ₹50,000 today would be about ₹1 lakh in 12 years and ₹2 lakh in 24. That is why retirement plans need to account for inflation — see how inflation eats your savings.
Seeing the cost of debt
Credit card debt at about 42% a year doubles in under 2 years if left unpaid. Even a personal loan at 14% doubles the debt in about 5 years if nothing is repaid. The rule makes the danger of high-interest debt obvious.
Related shortcuts
- Rule of 114: years to triple ≈ 114 ÷ rate.
- Rule of 144: years to quadruple ≈ 144 ÷ rate.
- Reverse use: to double money in 8 years, you need about 72 ÷ 8 = 9% a year.
The Rule of 72 assumes a steady rate. Market-linked investments do not return a fixed rate every year, so treat the result as a rough guide to the long-term average, not a promise.
Get the exact figure
For precise numbers, use the Compound Interest Calculator. Enter any amount, rate and period, and the year-by-year table shows exactly when your money doubles.
This article is for general information only and is not financial, tax or legal advice. Rates, rules and products change — check current terms with your lender or a qualified adviser. Disclaimer.