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Rule of 72 Calculator: How Fast Money Doubles

Calculate the return on any investment: total ROI %, profit, annualized CAGR and the doubling time by the rule of 72 next to the exact answer.

Leave the years empty to get only the total return; fill them in (decimals allowed) to also get the annualized CAGR and doubling time.

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Reading the investment values
Computing the total return
Annualizing the growth rate

Result

This page opens the ROI calculator with the doubling-time comparison in front: enter an initial value, a final value and the years between them, and you get the rule-of-72 estimate printed right next to the exact answer.

The rule of 72 is mental arithmetic: divide 72 by the annual growth rate in percent and you get the approximate number of years until the amount doubles — at 8% per year, about 9 years. It is an approximation of ln 2 / ln(1 + r), and it is genuinely accurate only in the middle of the range: between roughly 5% and 12% the error stays within a few percent, while at 1% or at 30% it drifts noticeably.

That honesty is the point of this page: both numbers are always shown side by side, so you can use the shortcut in conversation and still know exactly how far off it is for your rate.

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Accepted Formats & Rules

  • Total ROI % = (final − initial) / initial, plus the absolute profit or loss.
  • Annualized CAGR = (final/initial)^(1/years) − 1 whenever you enter the holding period, fractional years included.
  • Doubling time two ways: the rule-of-72 estimate (72 / rate) side by side with the exact ln 2 / ln(1 + r).
  • Handles losses honestly: negative ROI and CAGR are shown, and doubling time is marked as not applicable.

Key Benefits

✓ See the total return and the yearly growth rate of the same investment in one shot — the two numbers people most often mix up.
✓ The rule-of-72 estimate is printed next to the exact logarithmic doubling time, so you know how much the shortcut is off.
✓ Plain numbers in, plain numbers out: works with any currency, and nothing leaves your browser.

Use Cases

  • • Check the real yearly return of an investment you have held for several years, not just the total gain.
  • • Compare two investments held for different lengths of time by turning both into annualized CAGR.
  • • Estimate how long your portfolio needs to double at its current growth rate — rule of 72 next to the exact answer.
  • • Verify a fund's advertised annualized return from its start and end values.