Compound growth, in real numbers — a calculator
Type a starting balance, a monthly contribution, and a hypothetical return. See what compounding does over three decades.
Nothing you type leaves the browser. No accounts, no sign-in, no saved history — a calculator that does not know who you are.
End-of-month contributions, compounded monthly.
After 10 years
After 20 years
After 30 years
Notice what did not change: the monthly deposit. The differences between the rows are almost entirely made of time. A higher assumed return makes the table steeper; a lower one, flatter. A real portfolio would deliver neither smoothly — it would produce good years and bad years that only average out over long stretches.
Why this is true
Compounding is growth earning its own growth. Year one, your money earns a return. Year two, the return earns a return too. Early on this is nearly invisible. Given decades, it becomes most of the money.
The calculator uses monthly compounding of a hypothetical annual return, with contributions deposited at the end of each month. The return is an assumption you are choosing, not a prediction — real returns vary year to year and can be negative. Inflation, meanwhile, quietly shrinks what those future dollars buy, so the real-world result is more modest than the nominal table.
Two things worth noticing when you scan the rows. The contribution never changes; only time does. And by year 30 the growth column has grown much larger than the contributions column — the account is mostly made of returns on returns by then, not deposits. That is the honest core of compounding, and also its honest limitation: it favours the years you cannot get back, which means starting earlier matters more than starting bigger.
On the Premier tier, Plumb reads your connected brokerage, 401(k), and IRA balances live — so the same arithmetic runs on your actual account totals rather than an example.
Plumb is financial education, not financial, investment, tax, or legal advice.