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Compound Interest Calculator
See how your savings can grow over time. Enter a starting amount, a regular monthly contribution, an interest rate and a time span — and watch compound interest do the work.
How compound interest builds wealth
Compound interest is interest earned on your interest. Each period, the interest you've already earned gets added to your balance, so the next period earns even more. Over years, that snowball effect can turn modest, regular saving into a substantial sum — which is why it's often called the most powerful force in personal finance.
This calculator compounds monthly and adds your regular monthly contribution, then shows your projected balance, how much of it you contributed, and how much is pure interest.
What the inputs mean
- Starting amount — what you begin with today.
- Monthly contribution — what you add each month.
- Annual interest rate — your expected yearly return (e.g. a savings account or index fund).
- Years — how long your money stays invested.
This is an estimate for education only, assuming a constant return. Real returns vary. It is not financial advice.
Frequently asked questions
How is compound interest calculated?
Interest is added to your balance each period and then itself earns interest. This tool compounds monthly and includes your regular contributions.
Does starting early really matter that much?
Yes — hugely. Because growth compounds, money invested earlier has far more time to multiply. Even small amounts started young can outgrow larger amounts started later.
Are my numbers private?
Completely. Everything is calculated in your browser and nothing is uploaded or stored.