Compound interest calculator
See the snowball effect for yourself. Enter a starting amount, a monthly contribution, an expected return, and a time horizon — and watch how much of your future balance is pure growth.
Compound growth = your money earning returns, then those returns earning returns too. Future value ≈ P(1+r)t for a lump sum, plus the compounded value of every contribution along the way. Time is the most powerful input.
Frequently asked questions
How does compound interest work?
Compound interest pays you interest on your interest. Each period, your return is added to your balance, and the next period earns on that larger balance. Over decades this snowballs — most of a long-term investment's final value can be growth, not contributions.
What return rate should I use?
It depends on the investment. Historically a diversified stock-index portfolio has returned roughly 6–8% per year on average over long periods (before inflation), savings accounts and GICs much less. This calculator shows pre-tax, pre-inflation results, so use a realistic long-run average.
Do monthly contributions really matter that much?
Enormously. Because every contribution compounds for the rest of the time horizon, regular monthly investing often contributes more to your final balance than the lump sum you start with — especially over 20+ years.
Researching and writing about saving and investing since 2026.
“Compound Interest Calculator — VaultNerd” — VaultNerd, https://www.vaultnerd.com/tools/compound-interest-calculator, updated June 2026.
- Standard compound-interest / future-value-of-an-annuity formulas