Compound Interest Calculator
Project future value with monthly contributions and a configurable compounding frequency. Includes total interest earned and the Rule of 72 doubling time.
$691,150
$190,000
$501,150
Rule of 72: at 7%, money doubles roughly every 10.3 years.
Compounding is interest earning interest
Simple interest applies the stated rate only to the original principal. Compound interest applies the rate to principal plus all previously accumulated interest. Every compounding period, the base grows. That difference sounds academic until you plot two curves side by side across 30 years and see the compound curve bend upward while the simple curve stays a straight line.
The formal expression is future value equals principal times one plus the periodic rate raised to the number of periods. In practice, the input that matters most is not the formula. It is the length of time you let the exponent do its work.
Frequency matters, but less than time
Moving from annual compounding to monthly compounding at 7 percent lifts the effective yield by roughly 0.23 percent per year. Moving from monthly to daily lifts it by a fraction of a basis point. The frequency lever runs out of room quickly. Compare that to adding five years of runway, which at 7 percent increases the ending balance by roughly 40 percent, and the structural point is obvious.
The Rule of 72
The Rule of 72 is a mental-math shortcut for doubling time. Divide 72 by the annual return, expressed as a whole number, and you get the approximate number of years for money to double. At 8 percent, roughly 9 years. At 6 percent, 12 years. At 12 percent, 6 years. It is not exact, but it is accurate enough inside the range of realistic long-term returns to be useful without a calculator.
Time is the dominant variable
The reason age is such a dominant variable in retirement planning is that time enters the formula as an exponent, not a multiplier. A saver who begins at 25 and stops at 35, then leaves the money alone, typically ends with more than a saver who starts at 35 and contributes every month until 65. The math is not intuitive. It is why every honest retirement conversation begins with the question, when did you start.
What the tool does not do
The projection is nominal and pre-tax. It does not model inflation, tax drag inside a taxable brokerage account, or the sequence-of-returns risk that matters once you begin drawing down. Use it to build intuition. Use a retirement or 401(k) projection to plan a specific number.
Questions people ask about this tool
- Compound interest is interest calculated on the original principal plus all previously accumulated interest. Each compounding period grows on top of the last, which is why time is the most important variable in any long-term projection.
Educational tool. Estimates only. Not tax, legal, or investment advice. Federal figures are labeled in the source and should be reviewed annually. Consult qualified professionals before acting.