The Rule of 72: How Long to Double Your Money at Different Return Rates
Published 30 June 2026 · Investing
Sanjana in Kolkata saved ₹2 lakh from her first two years of working. She asked her father how long it would take to become ₹4 lakh. He said: "It depends where you put it." Then he showed her a rule that took about 5 seconds to apply to any investment, anywhere, at any rate. The Rule of 72 — and after this, Sanjana could answer that question herself, instantly.
The Rule of 72: How It Works
Years to double your money = 72 ÷ Annual return rate (%)
That is it. If your investment earns 12% per year, it doubles in 72 ÷ 12 = 6 years. At 7%, it doubles in 72 ÷ 7 = 10.3 years. At 4%, 72 ÷ 4 = 18 years. The rule is accurate within 1–2 years for return rates between 5% and 20% — close enough for any planning decision.
Sanjana's ₹2 lakh: in a savings account at 4% → ₹4 lakh in 18 years. In a fixed deposit at 7% → ₹4 lakh in 10.3 years. In an equity mutual fund at 12% → ₹4 lakh in 6 years. That is the same ₹2 lakh, doubling in 6 years vs 18 years — a 12-year difference purely from where it sits.
Doubling Time for Common Indian Investments
| Investment | Nominal Rate | Years to Double | After-Tax Rate (30% slab) | Real Doubling Time |
|---|---|---|---|---|
| Savings account | 4% | 18 years | 2.8% | 25.7 years |
| Fixed deposit | 7% | 10.3 years | 4.9% | 14.7 years |
| PPF | 7.1% | 10.1 years | 7.1% (tax-free) | 10.1 years |
| Nifty 50 SIP (median) | 12% | 6 years | 10.7% (after LTCG) | 6.7 years |
| Small-cap funds | 14% | 5.1 years | 12.6% | 5.7 years |
PPF stands out in this table: its nominal rate is only slightly above FD (7.1% vs 7%), but because PPF interest is completely tax-free, the after-tax return is identical to the nominal rate. For a 30% bracket taxpayer, PPF beats FD on an after-tax basis despite the same headline rate. The real doubling time column makes this clear: PPF takes 10.1 years to double in real value; FD takes 14.7 years.
Real Doubling Time: Adjusting for Inflation
Nominal doubling (your rupees double) is not the same as real doubling (your purchasing power doubles). If inflation is 5% and your investment earns 12%, your real return is 12% − 5% = 7%. Real doubling time: 72 ÷ 7 = 10.3 years — not 6 years.
Sanjana's ₹2 lakh in an equity fund at 12% becomes ₹4 lakh in 6 years. But if inflation was 5%, that ₹4 lakh only buys what ₹2.97 lakh buys today — not ₹4 lakh in real terms. Her purchasing power doubled in about 10.3 years, not 6.
FD is particularly stark: 7% nominal, 5% inflation, 30% tax → after-tax real return ≈ –0.1%. At zero real return, her money never doubles in purchasing power — it stays flat. The FD investor is running to stand still.
Run this for your own numbers
Calculate Your Investment Growth →What Most People Get Wrong
They calculate nominal doubling without inflation and without tax. ₹2 lakh in a 7% FD becomes ₹4 lakh in 10.3 years — that feels good. But at 5% inflation for 10.3 years, ₹4 lakh only buys what ₹2.46 lakh buys today. Real purchasing power doubled in closer to 25.7 years after accounting for 30% tax on FD interest. That is not doubling; that is barely treading water. The Rule of 72 applied to the real after-tax return gives the honest answer — and for most Indian savings vehicles, the honest answer is sobering.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a shortcut to estimate how long it takes to double your money: Years to double = 72 / Annual return rate. At 12% return, your money doubles in 72/12 = 6 years. At 7%, it takes 72/7 = 10.3 years. At 4%, 72/4 = 18 years.
How long does it take to double money in a mutual fund SIP in India?
At a 12% CAGR (historical Nifty 50 median): your invested corpus doubles in approximately 6 years. However, SIP corpus grows differently from a lump sum because you are continuously adding money — the effective doubling is not a clean 6 years for a running SIP, but individual invested amounts double at the 12% rate.
How long does it take to double money in an FD in India?
Current FD rates are approximately 6.5–7.5% p.a. At 7%: Rule of 72 gives 10.3 years to double. At 6.5%: 11.1 years. These are pre-tax figures — after tax at 30% slab, the effective rate is ~4.9%, meaning 14.7 years to double in real after-tax terms.
What investment doubles money fastest in India?
Historically: equity mutual funds (10–14% CAGR over 10+ years) double money in 5.1–7.2 years. This compares to FDs (7.3–11.1 years), PPF (9 years at 7.1% rate), and savings accounts (18+ years at 4%). Small-cap funds can exceed 15% but with high volatility.
Does inflation affect how fast money doubles?
Yes — real doubling time (adjusted for inflation) is: 72 / (nominal return − inflation). At 12% nominal return and 5% inflation, real return = 7%, so real doubling takes 72/7 = 10.3 years, not 6. Your money doubles nominally in 6 years but only in real purchasing power in 10.3 years.
What rate of return are your current investments growing at — and how many years will it take to double your money in real (after-inflation) terms?