Continuous Compounding

A=PertA = P \, e^{r t}

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Continuous compounding is the limit of compounding ever more often — every instant, the balance grows in proportion to itself, giving A = Pe^(rt). $5,000 at 4% per year for 10 years becomes 5000 × e⁰·⁴ ≈ $7,459.12, only a few dollars above monthly compounding: the compounding-frequency race hits a ceiling, and e is that ceiling.

The same law, with r negative, describes radioactive decay and drug elimination. Solving for t gives the exact doubling time t = ln 2 / r — at 4%, ln 2 / 0.04 ≈ 17.3 periods, which is where the banker's Rule of 72 comes from.

Continuous Compounding
A=PertA = P \, e^{r t}
Where
  • AA= Final amount
  • PP= Principal
  • rr= Interest rate per period (decimal)
  • tt= Number of periods
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