Compound Interest Calculator
Starting balance, monthly contribution, rate, years — and you see what compounding does over time: final balance, interest earned, and a year-by-year table.
Final balance –
Monthly compounding, contributions added at the end of each month. Nominal results — taxes and inflation are not included, and assumed rates are not guaranteed.
How compound interest is calculated
The calculator simulates your balance month by month: each month it grows by the annual rate divided by 12, then your contribution is added. That's how most savings and investment accounts actually behave — and it makes the year table exact rather than an approximation.
Enter your plan
What you start with, what you add monthly, an assumed annual rate, and the time horizon.
Read the split
Total contributed vs. interest earned shows how much of the final balance is compounding at work.
Try scenarios
Change one input at a time — especially years and rate — to see which lever moves the result most.
Why time beats amount
$200 a month at an assumed 6% grows to roughly $92,000 in 20 years — but to about $200,000 in 30 years, from only $24,000 more in contributions. The extra decade nearly doubles the outcome because interest starts earning interest on itself. That's the practical lesson of compounding: the years you give an investment matter more than squeezing the contribution. Treat any assumed rate as an illustration, not a promise — real returns vary by investment and over time.
Frequently asked questions
How does compound interest work?
Interest is added to your balance, and from then on that interest earns interest too. Early on the effect is small; over decades it dominates — which is why starting early matters more than starting big.
How is the calculation done here?
Month by month: the balance grows by the annual rate divided by 12, then the monthly contribution is added. This matches how most savings and brokerage accounts compound in practice.
What rate of return should I assume?
That depends entirely on the investment: savings accounts, bonds and stock funds have very different historical returns and risks. Try several rates to see a range of outcomes rather than relying on one number — and remember past returns don't guarantee future ones.
Does the calculator account for taxes and inflation?
No — results are nominal and before taxes. Inflation reduces real purchasing power and taxes on gains vary by country and account type, so treat the output as a growth illustration, not a financial plan.