What this compound interest calculator does
Compound interest is the reason a modest monthly saving habit turns into real money, and the reason credit card debt spirals if you let it. This calculator shows both edges of that sword: enter a starting amount, an optional monthly contribution, a rate, and a time period, and it shows you the final balance — split honestly between money you actually put in and interest the money earned on its own.
That split is the part most calculators bury, and it's the part worth seeing. Save $200 a month for ten years at 6% and you'll have contributed $24,000 of your own money on top of a $10,000 start — but the balance will be north of $51,000. The gap is compounding at work: interest earning interest, every month, on an ever-larger pile. The year-by-year table makes the acceleration visible — notice how the interest column grows faster each year while your contribution column climbs at a steady, flat rate.
The calculator simulates your balance month by month rather than plugging numbers into a single formula, which means contributions are handled accurately: each deposit starts compounding when it's actually made, not averaged across the year. Choose daily, monthly, quarterly, or yearly compounding to match your actual account.
How to use it
- Enter your starting amount — what's already saved. Zero is fine if you're starting fresh.
- Add a monthly contribution if you'll keep depositing. This is usually the biggest driver of the final number.
- Enter the annual rate — your account's APY, or an estimated return for investments.
- Pick the compounding frequency your account uses (most savings accounts compound daily or monthly).
- Set the time period and read the result. Try 10 years versus 20 — the second decade earns dramatically more than the first.
Frequently asked questions
What is compound interest, in plain terms?
Interest earned on interest. With simple interest, you earn only on your original deposit, forever. With compound interest, each period's interest joins the balance, and the next period's interest is calculated on that bigger balance. The effect is small at first and enormous over decades — which is why the standard advice about starting early isn't a cliché, it's arithmetic.
How much does compounding frequency actually matter?
Less than people think. $10,000 at 5% for ten years grows to about $16,289 compounded annually and about $16,486 compounded daily — a difference of under $200. The rate and the time horizon dominate. Frequency is worth getting right for accuracy, but it's not the lever that changes your outcome.
What's the compound interest formula?
A = P(1 + r/n)nt — starting amount P, annual rate r as a decimal, n compounding periods per year, t years. Regular contributions complicate the closed formula, so this calculator simulates the account month by month instead, which handles deposits exactly the way a real account would.
What interest rate should I use?
For a savings account or CD, use the APY printed on your account — it already accounts for compounding. For long-term investment estimates, something in the 6–8% range reflects broad US stock index historical averages, but real markets swing hard year to year. A projection at a steady rate is a planning tool, not a forecast.
Why do later years grow so much faster than early ones?
Because the balance doing the earning keeps getting bigger. In year one, 6% of $10,000 is $600. By year ten, you're earning 6% of $50,000+ — over $3,000, without saving a dollar more per month. Scan the interest column in the table above and watch the year-over-year jumps widen. That widening is compound interest.
Is this calculator free? What happens to my numbers?
Free, no account needed. All math runs in your browser; nothing you enter is stored or sent anywhere.
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This calculator is for informational purposes only and is not financial or investment advice. Projected growth assumes a constant rate; real returns vary. Confirm rates and terms with your financial institution.