What changes the ending balance
Compounding means the next period’s starting balance includes last period’s gains (and contributions). Small shifts in return, fees, or years change the ending balance a lot because the base you earn on keeps growing.
Why it matters
Most retirement and college scenarios assume compounding. If you plan with simple interest or ignore contributions, you will misread how achievable a number is.
Core idea
Use monthly or yearly steps: start balance + contributions + return − fees = new balance; repeat. The rate you type is an assumption; actual markets will differ.
Future value grows faster when return applies to a larger balance Fees and taxes flatten the curve
Mechanics you can verify
- Pick starting balance, monthly add, years, and an annual return assumption.
- Run once with dividends reinvested (typical fund default) and once without—see the gap.
- Add 0.5% fee drag and watch the tail of the curve flatten.
Stress-test in a tool
Real numbers
$200 a month for 25 years at 6% annualized (illustrative, not a promise) ends far above the same cash stacked under a mattress. The last decade of that curve does a lot of work—which is why starting earlier matters.
Common mistakes
- Treating a historical average return as next year’s guarantee.
- Forgetting taxes on dividends and gains in taxable accounts.
- Pausing contributions during dips and missing the cheapest shares.
Use the calculator
FAQ
- Is compound growth guaranteed?
No. The math is certain only in a spreadsheet. Markets gap up and down; use ranges, not single-point fantasies.
- Does inflation matter?
Yes. Nominal dollars can grow while buying power stalls. Pair growth tools with inflation context when planning spending power.