Estimate how long to double investment using the Rule of 72
The Rule of 72 is a shortcut: divide 72 by the annual interest rate to find how many years it takes to double your money. At 8%, money doubles in 72/8 = 9 years. At 12%, it doubles in 72/12 = 6 years. That's the entire rule — no spreadsheet needed.
It works because compound growth follows an exponential curve, and for rates between 6–12%, dividing 72 gives you a very close approximation to the exact doubling time. At rates above 20% or below 4%, the approximation drifts more, but for most personal finance scenarios, it's accurate enough.
The rule originated from the Rule of 72 in algebra, where the natural log approximation (ln 2 ≈ 0.693) gives you the exact continuous-compounding version. For practical annual compounding, 72 gives a slightly better fit than 69.3.
The Rule of 72 gets all the attention, but there are siblings for tripling and quadrupling.
| Goal | Rule Number | Formula | Example at 12% |
|---|---|---|---|
| Double your money (2×) | Rule of 72 | 72 ÷ rate | 72 ÷ 12 = 6 years |
| Triple your money (3×) | Rule of 114 | 114 ÷ rate | 114 ÷ 12 = 9.5 years |
| Quadruple your money (4×) | Rule of 144 | 144 ÷ rate | 144 ÷ 12 = 12 years |
The Rule of 72 cuts through marketing noise fast. A savings account at 3.5% doubles in 72/3.5 = 20.6 years. An ELSS fund at 14% doubles in 72/14 = 5.1 years. The gap — 20 years vs 5 years — makes the choice clear for anyone with a long horizon.
| Instrument | Approx. Rate | Doubling Time (72 ÷ rate) | Tripling Time (114 ÷ rate) |
|---|---|---|---|
| Savings Account | 3.5% | 20.6 years | 32.6 years |
| Post Office RD | 6.7% | 10.7 years | 17.0 years |
| PPF | 7.1% | 10.1 years | 16.1 years |
| Fixed Deposit (SBI) | 6.8% | 10.6 years | 16.8 years |
| Nifty 50 (historical avg) | 13% | 5.5 years | 8.8 years |
| Small/Mid Cap MF (historical) | 15% | 4.8 years | 7.6 years |
The rule works in reverse too. Apply it to inflation to see how fast purchasing power halves. At 6% inflation, your money's purchasing power halves in 72/6 = 12 years. That ₹1 crore you've saved will feel like ₹50 lakh in 12 years.
This is a powerful argument to make to someone sitting on large cash balances earning nothing. The cost of not investing is not zero — it's the inflation erosion rate. At 6% inflation, ₹10 lakh sitting in a zero-interest account loses the equivalent of ₹60,000 per year in real purchasing power.
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rupiya.io is for research and education only. Calculations are estimates based on publicly available data. Not investment advice.