Rule of 72 Calculator
Fast, accurate and free online rule of 72 calculator tool running directly in your browser.
-
1Enter data
Enter content, paste text or load a file from disk. -
2Click the button
The tool will immediately process your data in the browser. -
3Get the result
Copy the finished text or save the file to your device.
return "Result ready in 0.1s";
}
Show full text summary
Scenariusz: Estimated growth of an index fund Rule constant: 72 (klasyczna) Obliczenie: czas podwojenia na podstawie stopy rocznej Stopa roczna: 7.00% Approximate doubling time (rule):10.29 lat Precise doubling time (compound interest):10.24 lat Difference (rule − precise):+0.04 lat (+15 days)
Rate this tool:
Related tools
Other tools you may find usefulRule of 72 Calculator – Quickly Estimate Your Investment's Doubling Time
The Rule of 72 is a financial shortcut for quickly estimating your investment's doubling time: divide 72 by the annual rate of return (in %) = number of years. 8% return → 72÷8 = 9 years. The calculator calculates precisely and provides both variants: time to doubling and required rate.
Rule of 72 formula
Years to doubling = 72 / y (where r = rate of return in %). Examples: 4% → 18 years. 6% → 12 years. 9% → 8 years. 12% → 6 years. 24% → 3 years. Why 72? This approximation is ln(2) × 100 ≈ 69.3. 72 is better because it is divisible by more integers (2,3,4,6,8,9,12).
Variants – Rule 69.3 and 70
Rule 69.3: more accurate for constant rates (72 overestimates by ~1-2% for high rates). 69.3/yr. Rule of 70: used to estimate inflation. E.g. inflation 7% → capital will lose half its value in 70/7 = 10 years. Rule 72: practical because 72 is divisible by more numbers. The calculator uses the exact formula: t = ln(2)/ln(1+r/100).
Applications of Rule 72
Investment comparison: 3% (savings account) → 24 years. 7% (index fund) → ~10 years. Inflation: 4% inflation → purchasing power halved in 18 years. Debt: 18% credit card → debt will double in 4 years. GDP growth: country 6% GDP → economy doubles every 12 years.
Compound Interest - The Power of Compound Interest
The Rule of 72 works thanks to compound interest. PLN 10,000 × 7% per year for 10 years = PLN 19,672 (not PLN 17,000!). Each year: profit accrued from a growing base. The higher the rate and the longer the time → the snowball effect. Warren Buffett: "The first rule of compounding: never interrupt it unnecessarily."
FAQ
Is Rule 72 accurate?
Rule 72 is an approximation. Error: for r=1%: the rule gives 72 years, exactly 69.7 years (~3% difference). For r=10%: 7.2 years vs 7.27 years (1%). For r=20%: 3.6 years vs 3.8 years (5.5%). The higher the rate, the greater the error. The calculator shows the result of the rule and the exact value of ln(2)/ln(1+r).
How to use rule 72 for inflation?
Inflation 2% (NBP target): purchasing power falls by half in 36 years. Inflation 4%: in 18 years. Inflation 7-8% (Poland 2022-2023): in ~9-10 years. Conclusion: money kept "under the mattress" loses its value. Savings account 3% with inflation 4% = you actually lose 1%/year.
What rate of return is realistic in the long term?
S&P 500 (USA, since 1926): ~10% nominal, ~7% real (after inflation). WIG20 (Poland): variable. Bonds: 2-5%. Real estate: 4-8% (varies regionally). Deposits: 1-5%. For long-term retirement savings: 6-8% index fund is a reasonable assumption.
How does Rule 72 apply to credit?
Rule 72 works for both sides: the lender (profit) and the borrower (cost). APR 15% → debt increases to 2× in 72/15 ≈ 4.8 years. Credit card 20% → 3.6 years. Payday loan 100%+ → debt doubles faster than a year. Visualizing interest strength helps with lending decisions.
What is Rule 114 and Rule 144?
Rule 114: how many years until capital triples. Years = 114/y%. Rule 144: to quadruple height. Years = 144/y%. Example 6%: doubling in 12 years (72/6), tripling in 19 years (114/6), 4x in 24 years (144/6). Together: A useful set of quick financial estimates.
Related Tools: Percentage Calculator, Ratio Calculator and Inflation Calculator.