The Rule of 72 Explained: Quick Doubling Math for Indian Investors
How long will it take for your fixed deposit to double? Or that mutual fund SIP corpus? You don't need a spreadsheet — the Rule of 72 gives you a shockingly accurate mental shortcut. Divide 72 by the annual rate of return, and you get the approximate number of years to double your money. It's a tool every investor should carry in their head, from understanding PPF maturity to visualising the magic of equity compounding. This article shows the simple math, a ready‑reckoner table for common Indian investment rates, and a calculator to play with your own scenarios.
Why 72? The Magic Behind the Number
The rule is derived from the natural logarithm of 2 (≈0.693) and the fact that 72 is divisible by many small integers (6, 8, 9, 12), making mental math easy. The actual formula is ln(2) / ln(1 + r/100), but 72/r approximates it extremely well for rates between 6% and 10%. For very low rates, 69.3 is more accurate; for high rates, the approximation drifts. But for Indian investment returns — which typically range from 5% (debt) to 15% (equity) — the Rule of 72 works beautifully.
- Formula: Years to double = 72 ÷ Annual interest rate (%).
- At 8%, money doubles in 72 ÷ 8 = 9 years (exact: ~9.01 years).
- At 12%, doubling takes 72 ÷ 12 = 6 years (exact: ~6.12 years).
Step-by-step: Use the Rule of 72
- Open the Rule of 72 Calculator tool.
- Enter your expected annual return (e.g., 10% for a balanced fund).
- The tool displays the approximate doubling time using 72, and also the exact time for comparison.
- You can also input a target amount to see how many doublings you need from your starting capital.
Rule of 72 Table for Common Indian Investment Rates
| Investment | Approx. Return (%) | Doubling Time (Rule of 72) |
|---|---|---|
| Savings Account | 3–4% | 18–24 years |
| Bank FD (1–3 yr) | 6.5–7.5% | ~10–11 years |
| PPF (current rate) | 7.1% | ~10.1 years |
| NPS (mixed allocation) | 9–10% | ~7–8 years |
| Equity Mutual Funds (long term) | 10–12% | 6–7 years |
| Direct Equity (aggressive) | 12–15% | ~5–6 years |
The Rule of 72 assumes annual compounding. For instruments with quarterly compounding (like some FDs), the exact time might be slightly shorter. Use our FD Calculator to see the precise maturity amount with quarterly compounding.
Frequently Asked Questions
Does the Rule of 72 work for SIPs?
No. It's designed for a lump sum. For SIPs, use the XIRR Calculator to track returns on periodic investments.
Can I use the rule for inflation?
Yes, to estimate how long it takes for prices to double. At 6% inflation, the purchasing power of money halves in 72 ÷ 6 = 12 years.
What about taxes? Does that affect doubling?
The rule uses pre‑tax returns. For post‑tax, adjust the rate downward — for example, an FD at 7% after 30% tax is only 4.9%, so doubling takes 72 ÷ 4.9 ≈ 14.7 years.
Is the Rule of 72 mathematically exact?
It's an approximation. The exact formula is T = log(2) / log(1 + r). The rule is within 0.1–0.3 years for rates between 6% and 12%.
Is it free and private?
Yes — the tool runs entirely in your browser, free, with no sign‑up and nothing uploaded to a server.
Try the Rule of 72 Calculator