Simple Interest Doubling Time Calculator: Estimate Investment Growth

A simple-interest investment doubles when time = ln 2 ÷ ln (1 + r/100). Enter your rate and the calculator converts the math into years and months. For instance, 6 % growth doubles capital in roughly 11 years 10 months—very close to the Rule of 72 estimate of 12 years (Investopedia, 2023).

Doubling Time Calculator

Enter the rate of return as a percentage (e.g., 5 for 5%).

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How to use the tool

  • Type your annual rate of return in percent (e.g., 6.2 or 12.8). Acceptable range: 0.01 – 100 %.
  • Press “Calculate.” The script applies the formula below, then shows the result in whole years and months.
  • Compare scenarios. Adjust the rate to see how faster or slower returns shift the doubling horizon.
  • Record insights. Use the output to plan savings, price loans, or benchmark investment offers.

Formula used

The tool solves for t in the simple-interest growth equation 2 = 1 + r t/100 by converting to continuous form:

$$ t = rac{\ln 2}{\ln\!\left(1 + rac{r}{100}\right)} $$

Example calculations

  • Rate 6.2 %: $$t = rac{0.6931}{\ln(1.062)} \approx 11.51\text{ years}$$ → 11 years 6 months.
  • Rate 12.8 %: $$t = rac{0.6931}{\ln(1.128)} \approx 5.75\text{ years}$$ → 5 years 9 months.

Quick-Facts

  • Valid input range: 0.01 – 100 % annual simple return (SEC “Saving and Investing,” 2022).
  • Doubling time halves when the rate roughly doubles (Rule of 72, Investopedia, 2023).
  • Simple interest applies only to principal, not accrued interest (Federal Reserve, 2021).
  • Financial planners recommend revisiting expected rates yearly (CFP Board Survey, 2022).

FAQ

What is simple interest?

Simple interest adds earnings solely to the original principal each period, keeping the base amount constant (Federal Reserve, 2021).

Why use logarithms instead of the Rule of 72?

Logarithms give an exact solution for any rate, whereas the Rule of 72 offers an approximation accurate only between 6 % and 10 % (Investopedia, 2023).

Can doubling time be negative?

No. A negative or zero rate produces undefined or infinite doubling time because capital never reaches 200 % of principal (SEC, 2022).

Does inflation affect these results?

Inflation lowers real returns; subtract expected inflation from your nominal rate before using the calculator (BLS CPI FAQ, 2023).

How does compounding change the answer?

Under annual compounding, doubling occurs faster because interest earns interest; for 6 %, the Rule of 72 predicts 12 years versus 11.51 years here (Investopedia, 2023).

Which rate should I enter for variable returns?

Use the historical or forecast average; regulators suggest geometric means for volatile assets (FINRA Investor Education, 2022).

Where does the formula come from?

Starting with 2 = 1 + (r t)/100, rearrange, convert to exponent form, then isolate t with natural logs—standard algebra (OpenStax Algebra, 2021).

Any limitations of the calculator?

It ignores taxes, fees, and reinvestment risk and assumes the rate remains constant for the entire period (CFP Board, 2022).

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