Compound Interest Calculator
See how your savings grow with compound interest and regular monthly contributions. Includes a year-by-year chart.
Compound interest is the engine behind almost every long-term investment, retirement account, and savings goal — and it is also the most under-appreciated force in personal finance. This calculator shows how a starting balance, regular monthly contributions, and a rate of return grow over time, with a year-by-year chart so you can see the curve bend upward rather than just read a final number.
Use it to answer the questions that actually shape a financial plan: How much will $300 a month become in 30 years? What does starting five years earlier really cost me? How much do I need to invest to reach a million dollars by retirement? Because the tool separates the money you contribute from the growth your money earns, you can see the exact point where your portfolio starts earning more each year than you put into it — the moment compounding takes over the heavy lifting.
Plug in some numbers —
we'll crunch.
How to use
- 1Enter your initial lump-sum investment amount.
- 2Add a monthly contribution if you plan to invest regularly.
- 3Set the expected annual return rate (historically ~7% for diversified stock index funds).
- 4Choose the investment period in years.
- 5Select how often interest compounds — monthly is most common for savings accounts.
How it works
Compounding means you earn returns not only on your original money but also on the returns it has already generated. Each period, your balance is multiplied by (1 + the periodic rate), and any new contribution is added on top. Over many periods this turns into exponential growth: the formula for a lump sum is A = P(1 + r/n)^(nt), and regular contributions add a future-value-of-an-annuity term on top of that.
The practical takeaway is that time matters more than the rate or even the amount. A dollar invested at 25 has roughly 40 years to double and redouble; the same dollar invested at 35 has only 30. The Rule of 72 is a useful shortcut: divide 72 by your annual return to estimate how many years it takes your money to double. At 8%, that is about every nine years — which is why a single extra decade of compounding can more than double your final balance.
Worked examples
Starting early versus starting late
Two people each invest $300/month at an 8% average annual return until age 65.
- Alex starts at 25 and contributes for 40 years ($144,000 total).
- Jordan starts at 35 and contributes for 30 years ($108,000 total).
- Both leave the money untouched and reinvest all gains.
Alex finishes with roughly $1,006,000 and Jordan with about $679,000 — a $327,000 gap from just a $36,000 difference in contributions. The extra decade of compounding, not extra money, did the work.
The power of a higher rate
Invest a $10,000 lump sum for 30 years at 6% versus 9%.
- At 6%, the balance roughly follows the Rule of 72's 12-year doubling.
- At 9%, it doubles about every eight years.
- Compute both to see the spread widen over time.
At 6% the $10,000 grows to about $57,400; at 9% it grows to about $132,700. A three-point difference in return more than doubles the outcome — which is why investment fees that quietly skim 1–2% matter enormously.
Tips & common mistakes
Be realistic about the rate of return. Long-run stock-market averages land around 7–10% before inflation, but any single decade can be far higher or lower. Modeling 6–8% keeps your plan grounded; assuming 12% will leave you short. Remember that these figures are nominal — subtract roughly 2–3% to see your real, inflation-adjusted growth.
Consistency beats timing. Automatic monthly contributions (dollar-cost averaging) remove the temptation to wait for the "right" moment, which almost never arrives. The biggest, most expensive mistake is delay: every year you postpone starting removes one of your most valuable compounding years from the far end, where the curve is steepest.
Finally, watch fees and taxes. A fund charging 1% per year may sound trivial, but over 30 years it can quietly consume a six-figure chunk of your final balance. Where possible, use tax-advantaged accounts so compounding works on the full amount rather than on what is left after taxes each year.
Frequently asked questions
⚠ For educational purposes only. Past returns do not guarantee future results. Consult a financial advisor before investing.
Last reviewed: June 2026
About this calculator
See how your savings grow with compound interest and regular monthly contributions. Includes a year-by-year chart.