複利計算機

This compound interest calculator shows how a one-off starting amount plus regular monthly contributions can grow over time at a constant annual return. It separates the total you put in from the interest earned, so the effect of compounding is easy to see.

Contributions are added at the start of each month and the balance is compounded monthly, which matches how most savings and ETF plans actually work, and a year-by-year chart shows the balance climbing.

Compound interest is interest on your interest. Each period, the return is calculated on your contributions plus all the gains so far, so the balance grows faster and faster. Over long horizons this compounding, not the contributions, drives most of the final amount.

A rough mental check is the rule of 72: dividing 72 by the annual return gives the years for money to double. At 7 percent, that is about ten years, which is why starting early matters so much.

Returns are assumptions, not guarantees. A broad global stock ETF has historically averaged roughly 6 to 8 percent per year before inflation over long periods, but any given stretch can be very different, including negative.

The projection is before taxes, fees and inflation, so treat the result as a gross estimate and leave a margin for those. Everything is calculated in your browser.