Rule of 72 Calculator 2026 – How Long to Double Your Money

About the Rule of 72

72 ÷ Rate

The complete formula

<1% error

Accuracy at 6–20% annual return

Any asset

Works for equity, FD, PPF, real estate

Mental math

No calculator required once learned

How the Rule of 72 Works

The Rule of 72 is a quick mental-math shortcut to estimate how long it takes for an investment to double at a fixed annual return. Divide 72 by the annual return percentage — the result is the approximate number of years to double your money.

Formula: Years to Double = 72 ÷ Annual Return (%)

For example, at 9% annual return: 72 ÷ 9 = 8 years. This closely matches the exact compound-interest result (8.04 years), making the Rule of 72 reliable for everyday investment comparisons without a spreadsheet.

The number 72 was chosen because it has many divisors (2, 3, 4, 6, 8, 9, 12) that correspond to common return rates, making mental division easy.

Examples

Annual Return	Years to Double (Rule of 72)	Exact Years (Compound)
6%	12.0 years	11.9 years
8%	9.0 years	9.0 years
10%	7.2 years	7.3 years
12%	6.0 years	6.1 years
15%	4.8 years	5.0 years

When to Use This Tool

Compare asset classes: A fixed deposit at 7% doubles in ≈10 years; an equity mutual fund at 12% doubles in ≈6 years. The difference becomes visceral instantly.
Evaluate offers: If someone promises to "double your money in 3 years", that implies a 24% annual return — a red flag for most regulated investments.
Set expectations: If you invest at retirement age, the Rule of 72 shows whether an investment will double within your time horizon.
Inflation planning: Use it in reverse — at 6% inflation, purchasing power halves in 12 years. This highlights why idle cash loses value.

Limitations of the Rule of 72

Approximation only: The rule is accurate within ±1% for returns between 6% and 20%. At very low rates (1–3%) or very high rates (30%+), the error grows.
Assumes constant returns: Real investments fluctuate. The rule works best for fixed-return instruments like FDs and PPF, or as a long-term average for equities.
Ignores taxes and inflation: Post-tax, post-inflation doubling takes much longer. For LTCG at 12.5%, apply the rule to the after-tax return, not the gross return.
No partial years: The result is an approximation — use a proper compound-interest calculator for precise goal-based planning.
Compounding frequency: The rule assumes annual compounding. For daily/monthly compounding (e.g., savings accounts), the actual doubling time will be slightly shorter.

Frequently Asked Questions

What is the Rule of 72?

The Rule of 72 is a simple mental-math formula to estimate how long it takes for money to double at a fixed annual return rate. Divide 72 by the annual return percentage to get the approximate doubling time in years. For example, at 12% annual return: 72 ÷ 12 = 6 years. It works for any compounding investment — mutual funds, FDs, PPF, real estate — and is accurate within about 1% for return rates between 6% and 20%.

How accurate is the Rule of 72?

The Rule of 72 is very accurate for annual return rates between 6% and 20%, where the error is typically less than 1% compared to the exact compound-interest formula. For example, at 8% the exact doubling time is 9.006 years; the Rule of 72 gives exactly 9 years. At very low rates (1–3%) or very high rates (25%+), the error increases. For those extremes, the Rule of 69.3 is more precise, but 72 is preferred for mental math because of its convenient divisors.

What annual return rate should I use for mutual funds?

For Indian equity mutual funds, a long-term CAGR of 10–14% is commonly used for planning purposes, though past returns don't guarantee future performance. Large-cap funds have historically returned 10–12% over 10+ year periods; mid/small-cap funds have returned 12–16%. At 12%, the Rule of 72 gives a doubling time of 6 years. Always use a conservative estimate for goal-based planning. SEBI mandates that past performance not be cited as a guarantee.

Can I use the Rule of 72 to calculate the impact of inflation?

Yes. You can apply the Rule of 72 to inflation to find out how quickly purchasing power halves. At 6% inflation, purchasing power halves in about 12 years (72 ÷ 6). This is a powerful way to understand why holding idle cash or investing in very low-return instruments is costly. In India, CPI inflation has averaged around 5–6% over the past decade, meaning purchasing power roughly halves every 12–14 years.

What is the difference between the Rule of 72, Rule of 69, and Rule of 70?

All three estimate investment doubling time. The Rule of 72 uses 72 as the numerator — it's preferred for mental math because 72 has many divisors (2, 3, 4, 6, 8, 9, 12), making it easy to divide by common return rates. The Rule of 69.3 is the mathematically exact constant for continuous compounding. The Rule of 70 is sometimes used for lower return rates (1–5%) where it gives slightly better accuracy. For typical investment return rates of 6–20%, all three give similar results, but 72 is the most practical.

Does the Rule of 72 work for PPF and fixed deposits?

Yes. For fixed-return instruments like PPF and bank FDs, the Rule of 72 is very reliable. PPF currently earns 7.1% p.a. — applying the Rule of 72 gives a doubling time of about 10.1 years (72 ÷ 7.1). A 5-year FD at 7.5% would double your money in approximately 9.6 years. Because these instruments have predictable, constant rates, the rule's approximation closely matches the exact compound-interest calculation. For tax efficiency, PPF doubles on a tax-free basis (EEE), making its effective doubling time faster than a comparable taxable instrument.

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Disclaimer: Returns are not guaranteed or assured. The calculator's accuracy is not warranted. Before making any investment decisions, please seek advice from your financial advisors.