Compound Interest
Change any value and the rest adjust automatically. Edit the final balance to solve backwards for the initial investment.
| Year | Annual Contribution | Cumulative Contributions | Cumulative Interest | Annual Interest | Balance |
|---|
About this tool
Set a starting amount, a monthly contribution, an expected return and a number of years, and this calculator charts the growth year by year, splitting the final balance into what you put in and what compounding added.
The chart makes the core truth of investing visible: in the early years contributions dominate, and in the late years the compounding does. That crossover is why starting a decade earlier routinely beats contributing twice as much a decade later, and why the most valuable input in the whole calculator is time.
What is the rule of 72?
A mental shortcut for doubling time: divide 72 by your annual return. At 8% a year, money doubles roughly every 9 years; at 6%, every 12.
How much difference does starting early make?
Enormous. At 8% annual returns, a dollar invested at 25 is worth about twice as much at retirement as the same dollar invested at 34, purely because it gets one extra doubling.
What return should I assume for stocks?
US stocks have returned about 10% a year nominally over the past century, roughly 7% after inflation, with individual decades ranging from negative to spectacular. Conservative planners often model 6-7% to leave a margin for error.