Calculate compound interest growth on any savings or investment account. See S&P 500 average returns, monthly contributions, and year-by-year growth — free savings calculator.
Watch your money grow with compound interest
Compound interest is often called the "eighth wonder of the world" — and for good reason. Unlike simple interest, compound growth earns returns on your returns, creating exponential growth over time. The S&P 500 has historically delivered approximately 10% annually before inflation (roughly 7% after inflation), making it the benchmark US investors use when projecting long-term portfolio growth. On $10,000 invested at 7% for 30 years with monthly compounding, you end up with about $76,100 — on a $10,000 input. Add $300/month in contributions and the same scenario produces $340,000, with over $230,000 coming from investment growth rather than your own deposits.
The savings calculator mode is especially powerful for planning specific goals. Want to save $500,000 for retirement in 25 years? At 7% average annual return, you need to start with about $90,000 and contribute nothing, or start from zero and contribute roughly $750/month. Most US investors fall somewhere between — a combination of an existing balance and ongoing monthly contributions that, together, hit the target. Running different scenarios in this calculator takes seconds and can reshape how you think about the trade-off between starting earlier and contributing more.
$10,000 invested at age 25 grows to roughly $217,000 by age 65 at 8% return. The same amount invested at 35 grows to only $100,000. Ten extra years of compounding nearly doubles the outcome — without adding a dollar more.
Divide 72 by your annual interest rate to find how many years it takes to double your money. At 7%, money doubles every 10.3 years. At 10% (S&P 500 long-term average), every 7.2 years — four doublings in 30 years.
Adding just $100/month to a $10,000 base at 7% for 30 years produces over $220,000 — vs $76,000 without contributions. That $36,000 in total contributions (300 months × $100) generates over $144,000 in compound growth on top.
Maximize tax-advantaged accounts first: 401(k) up to employer match, then Roth IRA ($7,500/yr in 2026), then taxable brokerage for anything above those limits. Tax-free or tax-deferred growth dramatically improves long-term outcomes.