How the math works
Everything this site shows comes from one transparent formula. No AI guesses, no hidden assumptions.
The formula
- Every month your balance grows by the yearly return ÷ 12, then your monthly investment is added.
- Once a year your monthly investment rises by the “raise contributions” percentage (0% by default).
- The goal date is the first month the balance reaches your goal.
- “Today’s money” divides the goal by (1 + inflation) for every year that passes, to show its buying power now.
Default assumptions
| Average yearly return | 7.0% — deliberately below the US stock market’s long-run average of about 10% before inflation, to leave room for fees and bad decades. |
| Inflation | 3.0% a year |
| Contribution raise | 0.0% a year unless you change it |
| Taxes & fees | Not modelled separately — lower the return to include them (1% in fees ≈ 1% lower return). |
| Horizon | Up to 70 years |
What this calculator will not tell you
Real markets do not grow in a straight line: some years fall 20–40%. The average still matters most over decades, which is why the calculator shows several return rates side by side. Anyone promising a guaranteed shortcut to a million — a secret method, a special audio, a trading bot — is selling the shortcut, not the million.