Calculator
Compound Interest Calculator
Use this free compound interest calculator to see how a starting amount plus regular monthly contributions grows over time once compounding takes over. You'll get your final balance, the total you put in, and the interest earned.
Enter Your Values
20 years
Your projected balance
Balance after 20 years
$144,573
Total Contributed
$58,000
Total Interest Earned
$86,573
Estimates only. Assumes a constant rate of return and doesn't include taxes or fees.
Year-by-year projection
Interest is what your balance earned during that year. Total Interest adds it up since the start.| Year | Contributions | Interest | Total Interest | Ending Balance |
|---|---|---|---|---|
| Start | — | — | — | $10,000 |
| 1 | $2,400 | $801 | $801 | $13,201 |
| 2 | $2,400 | $1,033 | $1,834 | $16,634 |
| 3 | $2,400 | $1,281 | $3,115 | $20,315 |
| 4 | $2,400 | $1,547 | $4,662 | $24,262 |
| 5 | $2,400 | $1,832 | $6,495 | $28,495 |
| 6 | $2,400 | $2,138 | $8,633 | $33,033 |
| 7 | $2,400 | $2,466 | $11,100 | $37,900 |
| 8 | $2,400 | $2,818 | $13,918 | $43,118 |
| 9 | $2,400 | $3,196 | $17,114 | $48,714 |
| 10 | $2,400 | $3,600 | $20,714 | $54,714 |
| 11 | $2,400 | $4,034 | $24,747 | $61,147 |
| 12 | $2,400 | $4,499 | $29,246 | $68,046 |
| 13 | $2,400 | $4,998 | $34,244 | $75,444 |
| 14 | $2,400 | $5,532 | $39,776 | $83,376 |
| 15 | $2,400 | $6,106 | $45,882 | $91,882 |
| 16 | $2,400 | $6,721 | $52,603 | $101,003 |
| 17 | $2,400 | $7,380 | $59,983 | $110,783 |
| 18 | $2,400 | $8,087 | $68,070 | $121,270 |
| 19 | $2,400 | $8,845 | $76,915 | $132,515 |
| 20 | $2,400 | $9,658 | $86,573 | $144,573 |
Highlighted row: the first year your interest earned is greater than what you contributed that year.
How compound interest works
Compounding means your money earns returns, and then those returns start earning returns too. A dollar that grows 7% in a year becomes $1.07 — and the next year, that 7% applies to $1.07, not the original dollar. Left alone long enough, that small difference turns into most of your final balance.
The compound interest formula with monthly contributions
Your starting amount grows on its own, and each monthly contribution grows from the month it's added. Written out: balance = P × (1 + r/12)^(12t) + PMT × [(1 + r/12)^(12t) − 1] ÷ (r/12), where P is your starting amount, PMT is your monthly contribution, r is the annual rate as a decimal, and t is the number of years. You don't have to do this by hand, since the calculator steps through every month, but the exponent shows why time matters so much.
With the default inputs, $10,000 to start, $200 a month, a 7% rate, and 20 years, the balance reaches about $144,573. You put in $58,000 of that ($10,000 plus $200 for each of 240 months), so roughly $86,573 is interest.
Compounding frequency, and why it matters less than people expect
Interest can compound daily, monthly, quarterly, semi-annually, or annually. More frequent compounding means slightly more growth, but the effect is modest at typical savings and investment rates — the gap widens more at higher rates and over longer stretches. Monthly is the standard convention for most savings and brokerage accounts, which is why it's the default here.
Why time matters more than you'd expect
Contributing $200 a month at 7% from a zero balance grows to about $104,185 in 20 years, but about $243,994 in 30. Your contributions only rise from $48,000 to $72,000, yet the result more than doubles because the extra ten years give the earlier money more time to compound. A quick rule of thumb: divide 72 by your rate to estimate how many years it takes your money to double.
Planning around uncertainty
Nobody knows their actual future rate of return in advance. The rate variance option in advanced settings runs the same projection at a rate somewhat below and above your estimate, adding two dashed lines to the chart so you can see a rough range of outcomes rather than a single, falsely precise number.
How this calculator works
The calculator converts whatever compounding frequency you choose into an equivalent monthly rate, then steps through every month, adding your contribution (or subtracting a withdrawal, if you've set one) either before or after that month's growth depending on the timing setting. The chart splits the outcome into the money you actually put in versus the total balance — the growing gap between them is interest working on your behalf. The year-by-year table below the calculator shows the same thing one year at a time: contributions, interest earned, and the ending balance. It assumes a constant rate and leaves out taxes and fees.
This is a general-purpose growth calculator, not specific to dividend stocks: it works just as well for a savings account, an index fund, or any investment with a fairly steady long-term average return. If your money is in dividend payers, reinvested dividends compound in the same way, and the DRIP Calculator compares reinvesting with taking the cash. The FIRE Calculator uses the same monthly approach to estimate how long it could take to build a portfolio that lives off dividends, and the Dividend Calculator shows the income a portfolio pays today.
Frequently asked questions
How do you calculate compound interest with monthly contributions?+
Grow the balance by one-twelfth of the annual rate each month, then add your contribution, and repeat for every month. In formula form: balance = P × (1 + r/12)^(12t) + PMT × [(1 + r/12)^(12t) − 1] ÷ (r/12), where P is your starting amount, PMT is the monthly contribution, r is the annual rate as a decimal, and t is the number of years.
How much will $200 a month grow to?+
It depends on the rate and the time. At a 7% rate with no starting balance, $200 a month grows to about $104,185 in 20 years and about $243,994 in 30 years. Only $48,000 and $72,000 of those totals is money you actually put in.
What is the difference between compound and simple interest?+
Simple interest is paid only on the original amount, while compound interest is also paid on the interest already earned. $10,000 at 7% for 20 years earns $14,000 with simple interest, for a total of $24,000, but about $30,387 when compounded monthly, for a total of $40,387. Turn on the comparison in advanced options to see this side by side with your own numbers.
Does compounding frequency actually matter much?+
Less than people expect, especially at typical rates. Moving from annual to daily compounding on a 7% rate raises the effective annual rate from 7% to about 7.25% — a real but modest difference. It matters more at higher rates and over longer periods. Monthly is the most common convention for savings and investment accounts, which is why it's the default here.
How often does this calculator compound, and does it use APY?+
By default it compounds monthly, converting whatever compounding frequency you select into an equivalent monthly rate internally so contributions can still be added every month. A 7% rate compounded monthly equals an effective annual rate of about 7.23%. If your rate is quoted as an APY, it already includes compounding, so entering it here will slightly overstate growth — select "Annually" as the frequency in that case.
Why is my result different from another compound interest calculator?+
The most common reason is contribution timing: whether deposits are added at the start or end of each period. This calculator defaults to end-of-month, matching most bank and brokerage statements, but you can switch to start-of-month in advanced options to match a calculator that uses that convention instead. Compounding frequency and whether fees are included can also cause small differences.
Can I model regular withdrawals instead of contributions?+
Yes — add a monthly withdrawal amount in advanced options. The calculator subtracts it each month alongside any contribution, and if your balance would run out before your selected number of years, it tells you roughly when.
Does this calculator include taxes, fees, or inflation?+
Not by default, and it assumes a constant rate of return, while real returns vary from year to year. The advanced options let you add an inflation rate to see your final balance in today's purchasing power, and a rate variance to see a rough best/worst case range — but taxes and fees still aren't modeled, so treat every result as an estimate rather than a forecast.
What does the year-by-year table show?+
For each year it lists what you contributed, any withdrawals, the interest your balance earned that year, the running total of interest since the start, and the ending balance. The highlighted row is the first year your interest earned exceeds what you contributed that year — the point where compounding starts doing more work than you are. You can download the whole table as a CSV.
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