What is the Rule of 72?

It is a simple formula: divide 72 by the annual interest rate to estimate the number of years required to double an investment. At 6% growth, money doubles in roughly 12 years.

How accurate is the Rule of 72?

It is very accurate for rates between 2% and 15%. At very high or very low rates the estimate diverges slightly from the exact formula.

Does this work for debt too?

Yes. If you owe money at 18% interest and make no payments, the Rule of 72 says your debt doubles in about 4 years (72 / 18 = 4).

Rule of 72 Calculator

The Rule of 72 is a quick mental-math shortcut for estimating investment doubling time. Divide 72 by your annual rate of return to see approximately how many years until your money doubles.

The Rule of 72 is one of the most useful mental-math shortcuts in personal finance. To estimate how long it takes for an investment (or debt) to double, divide 72 by the annual percentage rate. A portfolio earning 8% doubles in about 9 years. A credit card at 24% APR doubles the balance in about 3 years. No calculator required.

This calculator runs the math more precisely and also shows the exact doubling time using the logarithm-based formula. For typical rates between 2% and 15%, the 72 approximation is accurate to within a fraction of a year — close enough for every practical purpose.

The Rule of 72 makes compound growth tangible. Investment commercials quote percentages; the doubling-time framing makes it concrete: "Will my money double during my career?" "How quickly will this credit card balance get out of hand if I only pay minimums?" Both are answerable in seconds.

Inputs

Annual Rate of Return (%)

Starting Amount ($)

Results

Rule of 72 Estimate

9.0 years

Exact Doubling Time

9.0 years

Doubled Amount

$20,000

Time to 4x

18.0 years

Growth Milestones

Milestone	Time
2x ($20,000)	9.0 yrs
4x ($40,000)	18.0 yrs
8x ($80,000)	27.0 yrs
16x ($160,000)	36.0 yrs

Last updated: April 16, 2026Reviewed by the CalcMountain editorial team

Formula

Rule of 72 approximation: Doubling time (years) ≈ 72 / Annual rate (as a percentage) Exact formula (continuous-equivalent compounding): Doubling time = ln(2) / ln(1 + r) ≈ 0.693 / r (for small r) Where r is the rate as a decimal. 72 is used because ln(2) / r ≈ 0.693/r ≈ 70/r — but 72 has more whole-number factors (1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72), making mental division easier. Example: 8% annual return Rule of 72: 72/8 = 9 years Exact: ln(2) / ln(1.08) ≈ 9.01 years At 1% the Rule of 72 says 72 years (exact: 69.7); at 24% it says 3 years (exact: 3.22). At rates within 2–15%, the rule is within ~0.3 years of exact.

How to use this calculator

Enter the annual rate of return (or interest rate for debt).
The calculator returns both the Rule of 72 estimate and the exact doubling time.
For mental math while reading or watching a video, divide 72 by any quoted rate to get a rough doubling time.
For debts (especially credit cards), use this to see how fast unchecked balances explode.
For multi-doubling estimates: 4 doubles = 16× the starting amount, 6 doubles = 64×. At 7% return, that's about 30 years to 8× and 60 years to 64×.

Worked examples

Stock market over a career

S&P 500 historical real return (after inflation): about 7%. 72 / 7 ≈ 10 years to double. A 30-year career produces roughly 3 doublings: $100K becomes $200K, then $400K, then $800K — purely from market growth, before contributions. Add monthly contributions and you compound into something much bigger.

Credit card trap

Average U.S. credit card APR: ~22%. 72 / 22 ≈ 3.3 years to double. A $5,000 balance left unpaid (zero payments) becomes $10,000 in just over 3 years. Reality is slightly different because minimum payments offset some interest, but the math explains why credit card debt explodes so fast.

Inflation's bite

Long-run U.S. inflation averages ~3%. 72 / 3 = 24 years for prices to double. Today's $1 only buys about $0.50 worth of stuff in 24 years. Retirement plans assuming a $5K/month income in 25 years really mean about $2,500/month in today's buying power.

When to use this calculator

Use the Rule of 72 any time you want a quick gut check of compound growth without opening a calculator. It's perfect for: - Comparing investment options ("This bond pays 4%, my stocks average 7% — that's the difference between 18 and 10 years to double") - Understanding inflation ("3% inflation halves my purchasing power in 24 years") - Sanity-checking debt projections ("22% APR means my balance doubles every 3 years if I do nothing") - Explaining compound growth to anyone who finds percentages abstract

For precise planning, use the compound interest calculator. The Rule of 72 is best for quick reasoning — and is accurate enough to make most decisions correctly.

Common mistakes to avoid

Confusing nominal and real rates. A 7% nominal return with 3% inflation is really 4% real, so the doubling of purchasing power takes 18 years, not 10.
Forgetting that the rule assumes constant compounding. Highly volatile returns can take longer to actually double in practice.
Treating doubling as guaranteed. Markets average a return; any single decade can underperform.
Using the rule at extreme rates. Below 2% or above 20%, the approximation diverges noticeably (use the exact formula).

Frequently Asked Questions

Sources & further reading

Compound interest and the Rule of 72 — U.S. Securities and Exchange Commission
Compound interest learning — FINRA

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