💰

Rule of 72 Calculator

Calculate investment doubling time, required return rate, inflation erosion & more

Find Doubling Time

Enter your annual interest or return rate

Annual Interest / Return Rate (%)

Quick:

Enter an interest rate above to calculate doubling time.

Find Required Rate

Enter your target doubling time in years

Target Doubling Time (Years)

Quick:

Enter a target doubling time above to calculate the required rate.

Investment Growth Projector

See how your investment grows year by year with doubling milestones

Principal Amount (₹)

Annual Return Rate (%)

Number of Years

Final Value

—

After N Years

Total Doublings

—

Times Doubled

1st Double

—

Years

2nd Double

—

Years

★

Rows marked with ★ are doubling milestones — your investment has reached 2×, 4×, 8× etc. of the original principal.

Year	Value (₹)	Growth	Milestone

Enter principal, rate and years above to project investment growth.

Inflation Erosion (Reverse Rule of 72)

See how inflation halves your purchasing power over time

Annual Inflation Rate (%)

India's average CPI inflation: ~5–6% (RBI target: 4%)

Enter an inflation rate above to see purchasing power erosion.

Rule of 72 Comparison Table

Rates 1–25% · highlighted sweet spot (6–10%) where accuracy is best

Best Accuracy Rows where error < 1% — the Rule of 72 is most reliable in the 6–10% range.

Rate (%)	Rule of 72 (Years)	Exact Years	Error (%)	Accuracy

Worked Examples

🏦

Bank FD at 6.5%

72 ÷ 6.5 = 11.08 years to double your deposit. A ₹1 lakh FD becomes ₹2 lakh in approximately 11 years.

📈

Mutual Fund SIP at 12%

72 ÷ 12 = 6 years to double your investment. Equity mutual funds have historically delivered 12–15% annual returns over long periods.

📉

Inflation at 6%

72 ÷ 6 = 12 years until purchasing power halves. Your ₹1,00,000 today buys only ₹50,000 worth of goods in 12 years.

What is the Rule of 72?

The Rule of 72 is one of the most powerful and elegant shortcuts in personal finance. By simply dividing 72 by the annual rate of return, you can instantly estimate how many years it will take for your investment to double in value. For example, if your fixed deposit earns 6% per annum, your money doubles in approximately 72 ÷ 6 = 12 years. If your equity mutual fund delivers 12% annually, your investment doubles in just 6 years.

This mental math trick is a favourite among financial advisors, investment professionals, and anyone who wants to make quick, informed decisions without reaching for a calculator. It works for any compounding scenario — savings accounts, fixed deposits, mutual funds, PPF, NPS, real estate appreciation, and even loan costs.

The Origin: Luca Pacioli and 1494

The Rule of 72 has a surprisingly long history. The Italian mathematician and Franciscan friar Luca Pacioli — often called the "Father of Accounting" — mentioned it in his landmark 1494 work Summa de Arithmetica, Geometria, Proportioni et Proportionalità. While Pacioli described the rule, Albert Einstein is often (incorrectly) credited with calling compound interest the "eighth wonder of the world." What is certain is that the rule has been a foundational shortcut in financial mathematics for over 500 years.

How Accurate is the Rule of 72?

The Rule of 72 is most accurate for annual interest rates between 6% and 10%, which conveniently covers the most common return rates for Indian investors — bank FDs, PPF, and diversified equity mutual funds. In this sweet spot, the error vs. the mathematically exact formula is less than 1%.

The exact formula for doubling time uses logarithms: Years = ln(2) ÷ ln(1 + r), where r is the decimal interest rate. At 8%, the Rule of 72 gives 9 years while the exact answer is 9.006 years — an error of just 0.07%. At very low rates (1–2%) or very high rates (above 20%), the rule becomes less precise, and you should use the exact formula or this calculator.

Variations of the Rule of 72

Rule of 69.3: More accurate for continuous compounding. Since ln(2) ≈ 0.6931, dividing 69.3 by the rate gives the exact doubling time under continuous compounding. Used in advanced financial modelling.
Rule of 70: Commonly used in macroeconomics and demographics. Divide 70 by the annual growth rate to estimate how long it takes for a quantity (population, GDP, money supply) to double. Easier to compute mentally when the rate is a round number like 2%, 5%, or 7%.
Rule of 72 (Standard): Preferred for standard annual compound interest because 72 has many integer divisors (1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72), enabling clean mental arithmetic for most common rates.

Practical Applications for Indian Investors

Fixed Deposits (FD): At 7% per annum, your FD doubles in approximately 10.3 years.
Public Provident Fund (PPF): At the current 7.1% rate, PPF doubles in approximately 10.1 years.
Equity Mutual Funds: At an assumed 12% annual return, investments double every 6 years.
Inflation erosion: At 6% annual CPI inflation, your purchasing power halves in 12 years — highlighting why keeping money idle in a savings account (earning 3–4%) is a losing strategy in real terms.
Loan cost: A personal loan at 18% interest rate — the interest burden doubles in just 4 years if left unpaid, illustrating the dangerous power of compound interest on debt.

Rule of 72 vs Rule of 69.3 vs Rule of 70

Rule	Formula	Best For	Accuracy
Rule of 72	72 ÷ Rate%	Annual compound interest (most common)	Best at 6–10% rates
Rule of 70	70 ÷ Rate%	Demographics, GDP growth, round numbers	Good at 2–5% rates
Rule of 69.3	69.3 ÷ Rate%	Continuous compounding	Exact for continuous
Exact Formula	ln(2) ÷ ln(1+r)	All rates, precise calculation	100% accurate

Frequently Asked Questions

What is the Rule of 72?

The Rule of 72 is a simple mental math shortcut used in finance to estimate how many years it takes for an investment to double at a fixed annual interest rate. You divide 72 by the annual rate of return. For example, at 8% per year, your money doubles in approximately 72 ÷ 8 = 9 years. It is attributed to the Italian mathematician Luca Pacioli, who mentioned it in his 1494 book Summa de Arithmetica, and has been a staple of financial planning ever since.

How accurate is the Rule of 72?

The Rule of 72 is most accurate for interest rates between 6% and 10%, where the error is typically less than 1%. At very low rates (1–2%) or very high rates (above 20%), the approximation becomes less precise. The exact doubling time is calculated using the formula: Years = ln(2) ÷ ln(1 + r), where r is the decimal interest rate. The number 72 is used because it is divisible by many common interest rates (2, 3, 4, 6, 8, 9, 12, 18), making mental calculations easy and practical.

Can the Rule of 72 be used for inflation?

Yes. The Rule of 72 can be applied in reverse to understand how inflation erodes purchasing power. Divide 72 by the annual inflation rate to find how many years it takes for the purchasing power of your money to halve. At India's average inflation of 6%, your ₹1,00,000 will have the purchasing power of only ₹50,000 in approximately 72 ÷ 6 = 12 years. This is why financial planners emphasise investing rather than keeping money in low-interest savings accounts.

What is the Rule of 69 vs Rule of 72?

The Rule of 69.3 (often approximated as Rule of 69) is more accurate for continuous compounding, because the exact formula for continuous compounding is ln(2) ≈ 0.6931. The Rule of 70 is commonly used for demographic calculations like population growth and GDP doubling time. The Rule of 72 is preferred for standard annual compound interest because 72 has more integer divisors, making it easier to perform mental calculations without a calculator. For practical investing purposes at normal return rates, all three rules give very similar results.

How do I double my money in 6 years?

Using the Rule of 72, to double your money in 6 years you need an annual return rate of 72 ÷ 6 = 12%. In India, this is achievable through equity mutual funds, which have historically delivered 12–15% annual returns over long periods. Mid-cap and small-cap funds have the potential to achieve this, though they carry higher risk. Large-cap diversified equity mutual funds typically average around 10–12% annually over 10+ year periods. No bank FD or PPF currently offers 12%, so equity investment is the practical path to 6-year doubling.

Does the Rule of 72 work for monthly compounding?

The Rule of 72 is designed for annual compounding. For monthly compounding, you should convert the nominal rate to the effective annual rate (EAR) first: EAR = (1 + r/12)^12 − 1, where r is the nominal annual rate. Apply this effective rate to the Rule of 72 for a more accurate result. For most practical purposes at common interest rates (6–12%), the difference is small — monthly compounding shortens the doubling time slightly compared to annual compounding, but the Rule of 72 with the nominal rate still gives a reasonable ballpark estimate.

What return rate is needed to double money in India?

Using the Rule of 72, the required return rate for different doubling timeframes: 5 years = 14.4%, 7 years = 10.3%, 10 years = 7.2%, 12 years = 6%. In India: bank FDs offer 6.5–7.5% (doubling in about 10–11 years), PPF offers 7.1% (doubling in about 10 years), and equity mutual funds have delivered historical returns of 12–15% (doubling in about 5–6 years). The appropriate instrument depends on your risk tolerance, tax bracket, and investment horizon.

Related Financial Calculators

📊

Compound Interest Calculator

Compute exact growth with compounding

📅

SIP Calculator

Plan mutual fund SIP investments

📉

Inflation Calculator

Track purchasing power over time

🎯

Savings Goal Calculator

Figure out how much to save monthly

🏖️

Retirement Savings Calculator

Plan your retirement corpus