How compound interest works
With simple interest, you earn a fixed amount on your original deposit and nothing more. With compound interest, the interest you earn is added to your balance, and future interest is calculated on that new, larger balance. Over time this creates a snowball effect: your money earns interest, then that interest earns interest, and so on.
The three levers that drive compound growth are the amount you start with, how much you add regularly, and how long you let it grow. Of these, time is usually the most powerful — which is why starting early, even with small amounts, often beats starting later with larger amounts.
The formula in plain English
For a single deposit, the classic formula is A = P × (1 + r ÷ n)^(n × t): the starting amount P grows by the annual rate r split into n compounding periods per year, repeated for t years. Monthly deposits are added on top, and each one compounds from the month it goes in.
This calculator works month by month. Each month it grows the balance by the rate that matches your chosen compounding (daily, monthly, quarterly or yearly), then adds your deposit at the end of the month. If you set a yearly increase, your monthly deposit rises by that percent after each full year.
Worked example: the snowball effect
Start with $10,000, add $300 a month, and earn 7% a year compounded monthly. After 20 years you have deposited $82,000 of your own money ($10,000 + $300 × 240 months). The balance reaches about $196,665, so $114,665 — more than you put in — came from growth.
- Raise the deposit 3% a year (roughly in line with pay raises) and the 20-year balance becomes $236,661.
- Compounding versus simple interest: the $10,000 alone, with no deposits, grows to $40,387 — $30,387 of interest. Simple interest at 7% would pay only $14,000 over the same 20 years.
- Compounding frequency: switching from monthly to daily adds only about $535 over 20 years. Rate, deposits and time matter far more.
Why starting early matters
Two savers each put $300 a month into an account earning 7%, compounded monthly. One starts at 25, the other at 35; both stop at 65.
- Start at 25 (40 years): deposits of $144,000 grow to about $787,444.
- Start at 35 (30 years): deposits of $108,000 grow to about $365,991.
The early starter puts in only $36,000 more but ends with more than twice as much, because those first ten years of deposits get the longest time to compound.
Reading your results
The chart plots your balance against what you deposited, and the gap between the two lines is growth. In the early years deposits make up most of the balance; later, growth takes over. In the main example, the balance after 10 years is about $72,022 — and it more than doubles again over the next ten, even though you deposit the same $300 a month the whole time. The doubling time shown with your results tells you roughly how long your money takes to double at the chosen rate, before counting new deposits.
Making compound interest work for you
- Start as early as you can — time is the biggest multiplier.
- Contribute consistently, even small amounts, and automate it if possible.
- Reinvest interest and dividends rather than withdrawing them.
- Keep fees low, since fees compound against you the same way returns compound for you.
- Pay off high-interest debt first — a credit card charging 20%+ compounds against you faster than most savings grow.
Common mistakes
- Assuming a steady return. Investments rise and fall; a constant 7% is a planning assumption, not a promise. Bank rates can change too.
- Forgetting inflation. $196,665 in 20 years will buy less than it does today. Use an inflation-adjusted (“real”) rate if you want results in today's dollars.
- Ignoring taxes. Interest in a regular savings or brokerage account is usually taxable each year, which slows growth. Tax-advantaged accounts like a 401(k) or IRA work differently.
- Confusing APR and APY. APY already includes compounding. If your bank quotes APY, enter it with yearly compounding to avoid counting the compounding twice.
When this calculator doesn't apply
It models a fixed rate with regular deposits. It isn't built for loans you are paying down, withdrawals in retirement, variable-rate products, or investments with uneven returns. For debt, compounding works in reverse — use a payoff calculator instead.
Related calculators
If you carry card balances, the credit card payoff calculator shows how compound interest works against you. The paycheck calculator helps you find a realistic monthly amount to save, and the mortgage payoff calculator lets you compare investing extra cash with paying down your home loan.