Tools / Investing
Compound interest calculator
A starting amount, a monthly contribution, a rate, a horizon. The result updates as you type.
$160,000 contributed · $302,290 growth
Assumes monthly compounding at a constant 7% annual return, contributions at month end, no taxes or fees. Real returns vary; this is arithmetic, not advice.
Year-by-year table
| Year | Contributed | Value | Growth |
|---|---|---|---|
| 1 | $16,000 | $16,919 | $919 |
| 5 | $40,000 | $49,973 | $9,973 |
| 10 | $70,000 | $106,639 | $36,639 |
| 15 | $100,000 | $186,971 | $86,971 |
| 20 | $130,000 | $300,851 | $170,851 |
| 25 | $160,000 | $462,290 | $302,290 |
What this is actually computing
Each month the balance grows by one month of return, then the contribution lands: balance = balance × (1 + r/12) + contribution. Run that 12 times per year for the horizon you chose. That is the whole model. The reason the curve bends upward is that later months apply the return to a bigger base, so the final years of a long plan contribute far more growth than the first years.
Choosing a return assumption
The long-run nominal return of a broad US stock index has historically landed near 10% before inflation and near 7% after it, which is why 7% is the default here. A bond-heavy mix runs lower. Whatever you choose, the number is an assumption about an unknowable future, not a promise; run the calculator at 5% and 9% and treat the gap between the two results as the honest answer.
What it leaves out
Taxes, fees, inflation, and the fact that real markets deliver returns in lumps rather than a smooth 7% every year. Sequence risk matters when you start withdrawing; for that, use the withdrawal rate explorer.
The intuition behind this curve Compound growth: the arithmetic your gut gets wrong 
Where we put probability to work Today's forecasts, with the paper P&L