Compound Interest Calculator with Monthly Contributions
Enter a starting balance and a monthly deposit to see what your money could grow to over up to 30 years, with a year-by-year chart.
※ The annual return you enter is an assumption. It does not guarantee any real investment result.
This result is for reference only. The annual return you enter is an assumption, and real investment returns vary with the market.
Balance by year
How to use
How this compound interest calculator works
This compound interest calculator shows what a starting balance plus a regular monthly deposit could grow to over time. Enter your starting amount, how much you add each month, the annual return you expect and the number of years (1 to 30). Press Calculate and you get the projected balance, the total you contributed, the interest earned and the time it takes your money to double, along with a year-by-year bar chart. Drag the year slider to see the balance at any point along the way. Everything is calculated in your browser, so nothing you type is sent anywhere.
The math behind the result
Compound interest means you earn interest on your interest. This tool compounds monthly: each month it takes your current balance, multiplies it by the annual rate divided by 12, adds that interest to the balance, and then adds your monthly contribution. Because the interest is added before the next month’s interest is calculated, the growth speeds up over time. In formula form, with i as the monthly rate (annual rate ÷ 12) and n as the number of months:
future value = start × (1 + i)n + deposit × [ (1 + i)n − 1 ] ÷ i
Here is a worked example. Start with $10,000, add $500 at the end of every month, and assume a 6% annual return for 20 years. The monthly rate is 0.5% and there are 240 months. The starting amount grows to about $33,102, and the deposits grow to about $231,020, for a total of roughly $264,122. You contributed $130,000 ($10,000 plus 240 × $500), so about $134,122, or 50.8% of the final balance, is interest. After 10 years the same plan is worth about $100,134 with 30.1% interest, and after 30 years about $562,483 with 66.2% interest. The longer the timeline, the more of your balance comes from growth instead of deposits.
The Rule of 72
The last line of the results shows how long it takes money to double, using the Rule of 72: divide 72 by the annual rate. At 6% that is 72 ÷ 6 = 12 years, and at 8% it is 9 years. It is a quick estimate that works best for rates between about 4% and 12%. If the rate is zero or negative, the calculator shows N/A because money that does not grow never doubles.
What the tax checkbox does
When you tick Subtract 22% tax on interest, the calculator removes 22% of each month’s interest before it is reinvested. For the example above, the final balance drops from $264,122 to $223,544, a difference of about $40,578. The 22% figure is an example of a federal income tax bracket for a middle income, not a rule: your real rate depends on your income and filing status, and state tax may come on top. Interest from savings accounts, certificates of deposit and most bonds is taxed as ordinary income, while qualified dividends and long-term capital gains are taxed at different rates, so treat the box as a rough measure of tax drag. If your money sits in a tax-advantaged account, such as a traditional 401(k) or IRA (taxes are deferred until you withdraw) or a Roth account (qualified withdrawals are tax-free), leave the box unchecked.
Start early or save more?
Time often beats size. Saving $300 a month at 7% for 30 years grows to about $365,991 on $108,000 of contributions. Saving double the amount, $600 a month, for only 15 years puts in the same $108,000 but ends near $190,177. Waiting 15 years costs you about $175,000 even though you deposited the same total. If you cannot raise your deposit right now, starting sooner is the next best lever.
Small changes in the rate add up
Keep the $10,000 start and $500 monthly deposit for 20 years. At 5% you end with about $232,643, at 6% about $264,122 and at 7% about $300,851. A single percentage point moves the result by roughly $31,000 to $37,000, which is why fees matter. If a fund charges 1% a year, enter your expected return after fees instead of before them.
Choosing a realistic return
The calculator assumes the same return every year, but real investments rise and fall. Savings accounts and certificates of deposit pay a rate the bank sets, which changes with interest rates. Broad stock-market funds have historically returned more over long periods but with large swings, and no result is guaranteed. A common habit is to run three cases, a cautious one, a middle one and an optimistic one, and plan around the cautious number. To see buying power in today’s dollars, subtract expected inflation from your rate; the Federal Reserve’s long-run inflation target is 2% a year.
APY or APR: which number do I enter?
This calculator uses an annual rate compounded monthly. Banks often advertise the APY (annual percentage yield), which already includes the effect of compounding. To convert, use: monthly rate = (1 + APY)1/12 − 1, then multiply by 12. For example, a 4.5% APY works out to about a 4.41% annual rate compounded monthly. For ordinary planning the small difference rarely changes the story, but converting makes the final dollars match your statement more closely.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original money and the interest it has already earned. In year one you earn interest only on your deposit, but in year two you earn interest on the deposit plus year one’s interest, and so on. Over many years this snowball effect makes balances grow faster and faster.
How is compound interest different from simple interest?
Simple interest is paid only on the original amount. $10,000 at 7% simple interest for 30 years earns $21,000 and ends at $31,000. The same $10,000 at 7% compounded monthly ends at about $81,165, more than two and a half times as much, because the interest keeps earning interest.
What rate of return should I enter?
Use a rate that matches where the money will actually be. For a savings account or certificate of deposit, enter the current rate the bank pays. For a diversified stock fund, many planners use a cautious figure well below the best historical years, and it is wise to subtract fees and inflation. Try several rates side by side rather than relying on one number.
Does the calculator include inflation, taxes and fees?
Not automatically. Inflation and fees are easy to include by lowering the rate you type in. Taxes are available through the checkbox, which subtracts 22% of each month’s interest as an example. The result is a projection from your assumptions, not a prediction, so use it for planning rather than as a promise.
When are the monthly deposits added?
At the end of each month, after that month’s interest has been calculated. That means each deposit starts earning interest the following month. If you deposit at the start of each month, your real balance will be slightly higher than shown.
Can I use this for 401(k) or IRA planning?
Yes, as a rough guide. Enter your yearly contributions divided by 12 as the monthly deposit, and add any employer match to that figure. Leave the tax box unchecked for a traditional 401(k), IRA or Roth account. Remember that contribution limits and withdrawal rules are set by the IRS and change over time, so check the current rules for your account.
Is my information saved or shared?
No. The calculation runs entirely in your browser and your numbers are never uploaded. The Copy link button only builds a web address that contains your inputs so you can bookmark or share the scenario yourself.