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Rule of 72 Calculator

Find how long it takes to double your money at any interest rate. Compare savings, investments, debt, and inflation side by side.

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The Rule of 72 is a shortcut: divide 72 by the annual rate to estimate years to double your money.

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Enter a rate to see doubling time

Worked Examples

UAE savings account at 4.5% AED fixed deposit

72 ÷ 4.5 = 16 years to double your money. AED 100,000 invested today becomes AED 200,000 in approximately 16 years. At 5.5% (top UAE fixed deposit rate), doubling time falls to 72 ÷ 5.5 = 13.1 years. The 1% rate difference saves 2.9 years.

Equity index fund at 10% CAGR (historical S&P 500 average)

72 ÷ 10 = 7.2 years to double. Starting at AED 50,000: Year 7.2 = AED 100,000; Year 14.4 = AED 200,000; Year 21.6 = AED 400,000; Year 28.8 = AED 800,000. That is a 16× return over roughly 29 years. Inflation at 3% cuts real purchasing power in half every 24 years (72 ÷ 3).

Credit card debt at 36% APR — debt doubling

72 ÷ 36 = 2 years for debt to double if you make no payments. AED 20,000 of credit card debt becomes AED 40,000 in 2 years, AED 80,000 in 4 years. UAE credit cards typically charge 24-36% APR. Making minimum payments only extends the debt cycle — always pay more than the minimum.

Frequently Asked Questions

4 min read
What is the Rule of 72?▾
The Rule of 72 is a quick mental calculation that estimates how long it takes to double an investment at a fixed annual return. Divide 72 by the annual interest rate (as a percentage): doubling time ≈ 72 ÷ rate. Example: at 6%, money doubles in approximately 72 ÷ 6 = 12 years. The rule works for any consistent annual growth rate — investments, inflation, debt, or economic growth.
How accurate is the Rule of 72?▾
The Rule of 72 is an approximation. For interest rates between 6% and 10%, the error is less than 1%. At lower rates (1-5%), it slightly overestimates doubling time; at higher rates (15%+), it slightly underestimates. For precise calculations, use the exact formula: T = ln(2) / ln(1 + r). At exactly 8%, Rule of 72 gives 9 years; exact calculation gives 9.006 years — essentially identical.
Can I use the Rule of 72 for inflation?▾
Yes — the Rule of 72 works equally well for inflation. At 3% annual inflation: 72 ÷ 3 = 24 years for prices to double (or purchasing power to halve). UAE CPI inflation has averaged around 3-4% historically. This means AED 1,000,000 in today's purchasing power is equivalent to only AED 500,000 in real terms after 18-24 years — a critical factor for long-term financial planning.
What interest rate doubles money in 10 years?▾
Using the Rule of 72: rate = 72 ÷ 10 = 7.2% per year. To double money in exactly 10 years, you need approximately 7.2% annual return. In the UAE context, this requires a balanced portfolio of equities and fixed income — bank deposits alone (currently 4-5.5%) won't achieve this. A blended portfolio of UAE equities and global index funds has historically reached this range.
Does the Rule of 72 work for monthly rates?▾
Yes, but use 6 instead of 72 for monthly rates. If a loan charges 2% per month, doubling time ≈ 6 ÷ 2 = 3 months. For annual rates applied monthly, divide the annual rate by 12 first: 12% annual = 1% monthly, so 6 ÷ 1 = 6 months to double. Always clarify whether a rate is monthly or annual.
How does the Rule of 72 relate to compound interest?▾
The Rule of 72 is derived directly from the compound interest formula. At continuous compounding, doubling time = ln(2) / r ≈ 0.6931 / r. Multiplying by 100 gives roughly 69.3 / rate (%). Einstein's famous 'eighth wonder of the world' quote refers to compound interest — small rate differences create massive long-term wealth differences. At 7%: AED 100,000 → AED 800,000 in 30 years; at 10%: AED 100,000 → AED 1,745,000 in 30 years.
What is the Rule of 115 and Rule of 144?▾
These are variations for different multipliers. Rule of 115: 115 ÷ rate = years to triple your money (3× growth). Rule of 144: 144 ÷ rate = years to quadruple (4× growth). Example at 8%: doubling = 9 years; tripling = 14.4 years; quadrupling = 18 years. Use these rules together to see the full compounding trajectory.
Why is the number 72 used specifically?▾
72 is chosen because it is divisible by many common interest rates (1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36), making mental arithmetic easy. The mathematically exact constant is 69.3 (from ln(2) × 100), but 69.3 is awkward to divide. 72 is close enough to be accurate and convenient. Some textbooks use 70 as a compromise.

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The Rule of 72 is an approximation. For precise calculations, use the exact doubling time formula: T = log(2) / log(1 + r). Investment returns are not guaranteed.

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