Compound Interest Calculator
See how an initial deposit plus regular monthly contributions can grow when interest earns interest on itself.
Your numbers
Your result
Balance by year
Assumes a steady rate of return. Real investments fluctuate year to year.
How compound interest works
Compound interest means you earn interest not just on your original deposit, but also on the interest that has already been added. Over long periods that snowball effect is powerful: in the example above, interest eventually accounts for more of the final balance than the money you contributed.
Future value = P × (1 + r/n)nt + monthly contribution × [((1 + r/n)nt − 1) ÷ (r/n)], where P is your starting amount, r the annual rate, n the compounding periods per year, and t the years.
Two levers matter most: time and consistency. Starting ten years earlier usually beats contributing more later, because compounding has more years to work. Even a modest monthly amount, started early, can outgrow much larger contributions started late.
Frequently asked questions
What rate of return should I use?
For a savings account, use the APY your bank actually pays. For long-term stock market investing, 7–10% per year is a commonly used historical average for a diversified US portfolio — but it is an average, not a promise, and any single year can be far higher or lower (including negative).
Does compounding frequency make a big difference?
A smaller one than most people expect. Daily vs. monthly compounding on $10,000 at 7% over 20 years differs by well under 1% of the final balance. The rate, the time period, and your contributions matter far more.
Are taxes and inflation included?
No. This calculator shows growth before taxes and inflation. To think in “today's dollars,” you can subtract expected inflation (often around 2–3% per year) from your return rate for a rough real-return estimate.