Compound Interest Calculator
Enter a starting amount, a monthly contribution, an interest rate, and a number of years to see your future balance and a year-by-year growth schedule.
Calculate your savings growth
| Future balance | |
| Total contributions | |
| Total interest earned |
How this is calculated
This simulates your balance month by month: each month it adds a month's worth of interest (the annual rate divided by 12) on the current balance, then adds your monthly contribution, and repeats for the full term. That's mathematically equivalent to monthly compounding with a contribution added at the end of every month, sometimes called an "ordinary annuity" — it's the standard assumption most savings calculators use, though some banks compound daily or add contributions at the start of the month instead, which would shift the result slightly.
Why compounding accelerates over time
Interest earned in early years starts earning its own interest in later years, so growth curves upward rather than in a straight line — the year-by-year table above makes this visible: the gap between contributions and total interest earned grows faster in later years than earlier ones, even though the monthly contribution never changes. This is also why starting early matters more than the exact monthly amount: money has more years to compound.
Frequently asked questions
What does "compound interest" actually mean?
It means interest is calculated on your growing balance, including interest you've already earned, rather than only on your original deposit. Over enough time, the interest-on-interest effect can add up to more than the contributions themselves.
Does this account for taxes or inflation?
No — the future balance shown is in today's dollars with no inflation adjustment, and doesn't subtract any taxes on interest or investment gains (which depend on the account type: a taxable account, a 401(k), or a Roth IRA are all taxed differently). Treat the result as a before-tax, non-inflation-adjusted projection.
Is a fixed interest rate realistic for investments?
Not for the stock market, which fluctuates year to year rather than growing at a smooth fixed rate — a fixed rate is a reasonable simplification for a rough long-term estimate (using something like a long-run historical average), but actual returns in any given year can be very different, and it's a realistic assumption for something like a high-yield savings account or CD with a stated rate.
Why does starting early matter so much?
Because compounding needs time to work: money contributed in year one earns interest for every remaining year of the term, while money contributed in the last year barely compounds at all. Try the calculator with the same monthly contribution but a longer time frame to see how much of the final balance comes from time rather than from the contribution amount itself.