See what your money becomes over time.
Set your initial investment, monthly contribution, time horizon, and expected return, and watch the numbers move. The black bars are what you put in. The gold bars are what compounding did on top.
One-time amount you start with
Added every month after the initial
How long the money stays invested
Average annual growth, before inflation/taxes/fees
Educational only. Real returns vary, are not guaranteed, and are reduced by fees, taxes, and inflation. Past performance does not predict future results.
After 30 years at 8% a year. You put in $109,000, and compounding added $349,044 on top.
Start with your $1,000 initial investment and add $300 every month for 30 years.
Months = 30 years * 12 = 360
Each month, grow the running balance by one-twelfth of the annual return, then add that month's contribution.
New balance = previous balance * (1 + 8% / 12) + $300
After 30 years, separate what you put in from what compounding added on top.
Final balance = money you put in + growth = $109,000 + $349,044 = $458,044
Constant-return math. Real returns vary year to year and are reduced by fees, taxes, and inflation.
The shape of the curve.
The 8% default is roughly the long-run nominal return of the U.S. stock market, before inflation, taxes, and fees. After inflation, a more conservative figure is closer to 6%. The point of the exercise isn't predicting the exact number, it's seeing the shape of the curve, and how dramatically time horizon changes the answer.
Try this: keep everything constant and change the time horizon from 20 years to 40 years. The final balance roughly quadruples. That's the time value of money in action, and it's why starting earlier matters more than starting larger.
For more, read the compound growth lesson or check the time value of money glossary entry.
Assumptions
- The annual return is constant year to year. Real markets vary year to year; the same long-run average produces a different shape if the early years are bad.
- Contributions happen at the end of each month and earn no interest during the month they were contributed.
- No taxes, no fees, no inflation. The result is a pre-tax, pre-fee, pre-inflation nominal number.
- The default 8% return is the user input, not a prediction. Long-run U.S. stock returns have historically averaged near this figure before inflation; after inflation a more conservative figure is closer to 6%.
Limitations
- Constant-return math hides sequence risk. A real portfolio that earns 10% one year and -10% the next does NOT return to its starting value; it ends below.
- Bond, cash, and mixed portfolios behave differently. This calculator does not model a specific account or asset mix.
- Inflation reduces the purchasing power of the final balance. Subtract about 2 to 3 percentage points from your return assumption for an after-inflation view.
- Taxes on dividends, interest, and realized gains reduce returns in a taxable account. A retirement account changes the timing of tax but not the math here.
- It is not a forecast. The return is the user's input, not a prediction of any specific market's future.
- It is not a recommendation to invest a specific amount or expect a specific outcome.
- It is not after-tax or after-inflation. Both reduce the eventual real-world value of the number shown.
- It is not personalized advice. Talk to a CFP for the household-specific call.