The Compound Interest Calculator shows how your money grows over time when interest earns interest. Enter your initial investment, monthly contribution, interest rate and time period to see your total returns and a full year-by-year breakdown.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both your original principal and the interest already earned. Over time this creates exponential growth — famously described by Einstein as the "eighth wonder of the world".
How often does interest compound?
It depends on the account. Common options are daily, monthly, quarterly or annually. More frequent compounding = slightly more growth.
What is a realistic interest rate to use?
Savings accounts: 3–5%. Index funds (historical average): 7–10% per year. Use conservative estimates for planning.
How much should I invest to become a millionaire?
Investing $500/month at 8% annual return for 30 years grows to over $700,000. Start early — time is the biggest factor.