Rule of 72 Calculator

Estimate how many years it takes to double your money at any annual rate of return — with the more accurate 69.3 and 70 variants for comparison.

What does the Rule of 72 tell you?

Divide 72 by your annual rate of return to get the number of years for your money to double. At 8% return, money doubles in about 9 years; at 6% it takes 12; at 12% it takes 6. The shortcut is accurate within a few months for rates between 6% and 10%, and it lets you compare investments without a spreadsheet.

Your Rate of Return

Expected Annual Rate of Return (%)

Starting Amount ($)

Years to Double

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Rule of 72

0

Exact (69.3)

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Rule of 70

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Doublings in 30 yrs

Your Starting Amount Over Time

Year	Balance (Rule of 72)	Doublings

How to use this calculator

Takes about 30 seconds.

Enter the annual rate of return you expect (use the historical S&P 500 average of ~7% real if unsure).
Read the years-to-double estimate using all three constants — 72, 69.3, and 70.
Use the doublings table to see how your money grows over 10, 20, and 30 years.

Key concepts

Where the number comes from. The exact constant for doubling under continuous compounding is ln(2) ≈ 0.693, which scales to 69.3 when expressed as a percentage. Discrete annual compounding shifts the optimal constant slightly higher; 72 was chosen by Renaissance bankers because it divides cleanly into 1, 2, 3, 4, 6, 8, 9, and 12 — most of the rates anyone would quote.

The accuracy band. The Rule of 72 is most accurate between 6% and 10%. At 8% it's exact to within a month. Above 15% or below 3%, switch to the Rule of 69.3 or 70 for a closer estimate — the calculator above shows all three.

Real vs nominal. Always apply the rule to the real (after-inflation) rate of return when projecting purchasing power. A nominal 9% return with 3% inflation gives a real return of about 6% — so money doubles in real terms every 12 years, not 8.

What the rule cannot do. It assumes a constant rate of return, no contributions, and no withdrawals. For a real retirement projection with monthly contributions, use the Investment Growth Calculator; for inflation-adjusted long-term plans, the Compound Interest Calculator handles the full math.

Worked example — 7% return on $10,000

An investor puts $10,000 into a low-cost global index fund yielding a long-run real return of 7%. Using the Rule of 72: 72 ÷ 7 ≈ 10.3 years per doubling. That means $10,000 reaches $20,000 by year 10, $40,000 by year 20, $80,000 by year 31, and $160,000 by year 41. Three doublings over a 30-year career turn a single five-figure deposit into an inflation-adjusted six-figure sum — entirely from compounding, without a single extra dollar contributed.

The 69.3 variant gives a slightly tighter answer of 9.9 years per doubling — a difference of about 5 months over a 30-year horizon. The error compounds: by year 60, the Rule of 72 estimate predicts 5.8 doublings (factor ~58x) versus the exact 6.06 doublings (factor ~67x). For mental math the gap is irrelevant. For a 60-year retirement plan, switch to a proper compounding model.

Flip the question. If you want to double your money in 8 years, you need 72 ÷ 8 = 9% annual return. That's near the historical equity premium ceiling — possible, but not guaranteed. Aiming for 7-year doublings means targeting 10.3% returns, which historically has required leverage, sector concentration, or a willingness to underperform the index for long stretches.

Common mistakes

Using the nominal rate when you mean real. A 10% nominal return with 4% inflation only doubles purchasing power every 12 years, not every 7. Always run the rule on inflation-adjusted returns for retirement and savings projections.
Applying it to rates above 20% or below 1%. The approximation drifts outside the 3-15% band. For a 25% rate, use the Rule of 69.3 (so 69.3 ÷ 25 ≈ 2.8 years). For a 1% rate, use a calculator — the 72-shortcut gives 72 years, but the exact answer is 69.7.
Confusing doubling time with "doubled in real terms." The rule tells you when your account balance doubles in nominal dollars. If inflation runs at 3% during that time, your purchasing power has grown less than 2× — closer to 1.5× depending on the gap between nominal and inflation rates.
Treating it as a forecast, not a sanity check. The rule assumes a flat annual return. Real markets deliver 7% on average via wildly different annual numbers (+30%, -20%, +12%, etc.). Sequence-of-returns risk matters — see the Retirement Drawdown Calculator.

Frequently Asked Questions

What is the Rule of 72?

The Rule of 72 is a mental-math shortcut for estimating how long money takes to double at a given annual rate of return. Divide 72 by the rate as a whole number — at 8% it takes about 72 ÷ 8 = 9 years.

How accurate is the Rule of 72?

Highly accurate between 6% and 10% — within a few months of the exact answer. At very low rates (1-2%) or very high rates (20%+) the approximation drifts. For tighter accuracy at extremes, use 69.3 (the mathematically exact constant) or 70.

Why 72 and not 70 or 69.3?

The exact formula gives 69.3 for continuous compounding. But 72 is divisible by 1, 2, 3, 4, 6, 8, 9, 12 — most rates investors encounter — so it gives a clean integer answer almost every time. Small accuracy loss in exchange for instant mental math.

Can I use the Rule of 72 to find a needed rate?

Yes — reverse it. If you want to double your money in 10 years, you need roughly 72 ÷ 10 = 7.2% annual return. This is the more useful direction for setting savings or investment targets.

Does inflation change the Rule of 72?

Use the real (after-inflation) rate of return, not the nominal rate. At a nominal 8% return with 3% inflation, your real return is roughly 5%, so money doubles in real purchasing power in 72 ÷ 5 ≈ 14 years, not 9.

What is the Rule of 72 used for in practice?

Three common uses: setting savings-rate targets, comparing investment options, and sanity-checking a salesperson's pitch. Anyone promising to double money in 3 years implies 24% annual return — well above any sustainable strategy.

Written and maintained by James Blanckenberg, Founder & Editor, FinCalcHub.

Last reviewed: 20 May 2026 · See editorial policy