Compound Interest Calculator
See exactly how your money grows over time. Add a starting amount and a monthly contribution — the projected balance, total contributions, and compound growth update instantly. The formula is shown, and the chart separates what you put in from what compounding earned you. No sign-up.
- Total you contribute$0
- Compound interest earned$0
Quick answer
Starting with $10,000 and adding $500 a month at 7% for 25 years ends at $462,290. You would have paid in $160,000 of that — the other $302,290 is growth, meaning roughly two thirds of the final balance is money you never contributed. Change the inputs above and this answer updates with your own numbers.
Contributions vs. compound growth
Grey = what you put in. Green = total value. The widening gap is compounding.
What compound interest actually is
Simple interest pays you on your original deposit and nothing else. Compound interest pays you on your deposit and on all the interest it has already earned, so each period starts from a slightly larger base than the last.
That single difference is why the growth line on the chart above is a curve rather than a straight line, and why the shape of that curve is so unintuitive. For years it looks disappointingly flat. Then it doesn't.
The last decade does most of the work
This is the part almost nobody internalises until they see the numbers laid out. Same plan throughout — $10,000 to start, $500 a month, 7% a year — with only the time changing:
| Years | Final balance | You contributed | Growth | Growth as a share |
|---|---|---|---|---|
| 5 | $49,973 | $40,000 | $9,973 | 20% |
| 10 | $106,639 | $70,000 | $36,639 | 34% |
| 15 | $186,971 | $100,000 | $86,971 | 47% |
| 20 | $300,851 | $130,000 | $170,851 | 57% |
| 25 | $462,290 | $160,000 | $302,290 | 65% |
| 30 | $691,150 | $190,000 | $501,150 | 73% |
| 35 | $1,015,589 | $220,000 | $795,589 | 78% |
Compare years 15 to 25. The balance rises by $275,319, while you contribute only $60,000 more. The remaining $215,319 comes from money that was already there.
Then compare the first five years with the last five of the 35-year run. The first five add $49,973. The five years from 30 to 35 add $324,439 — more than six times as much, from identical contributions.
This is the honest argument for starting early, and it is also why quitting early is so costly. The years that produce the most are the ones at the end, and you only reach them by holding on through the flat part at the beginning.
What the rate does
Same $10,000, same $500 a month, same 25 years:
| Annual return | Final balance |
|---|---|
| 4% | $284,202 |
| 6% | $391,147 |
| 7% | $462,290 |
| 8% | $548,915 |
| 10% | $783,986 |
Between 4% and 10% the outcome nearly triples, on identical contributions. That sensitivity cuts both ways: it is why costs matter — a fee is a permanent deduction from this rate — and why any projection should be treated as a range rather than a promise. Nobody knows which row the next 25 years will resemble.
Three things that quietly distort the result
- Inflation. The figures above are nominal. At 3% inflation, $462,290 in 25 years buys roughly what $220,000 buys today. To think in today's money, enter a real return — your expected return minus inflation — instead.
- Fees. A 1% annual fee is not 1% of your profit, it is 1% off the compounding rate every year. Over 25 years, moving 7% to 6% costs $71,143 in this example — on the same contributions.
- Tax. Growth inside a tax-advantaged account compounds untouched; growth in a taxable account may be reduced each year, which lowers the effective rate. The calculator models the gross figure.
Does compounding frequency matter?
Less than people expect. Moving from annual to monthly compounding at the same nominal rate produces a modest improvement, and moving from monthly to daily produces a very small one. The reason is that the extra benefit of compounding more often shrinks quickly as the interval gets shorter.
The rate, the amount and above all the time dominate. If you are choosing between two accounts, the headline rate and the fees will decide the outcome long before the compounding interval does.
The compound interest formula
- FV — the future value, what you end up with
- P — your starting balance
- PMT — the contribution made each compounding period
- i — the rate per period: annual rate divided by the number of periods per year
- N — the total number of periods: years times periods per year
The first term grows what you started with; the second grows everything you add along the way. Note that N sits in an exponent while P and PMT are only multiplied — which is the mathematical reason time outperforms amount over long horizons.
See the formula with your own numbers
These update live from the calculator inputs above.
Example: $500 a month for 25 years
$10,000 to begin, $500 a month, 7% a year, compounded monthly, for 25 years.
- Final balance: $462,290.
- You contributed $160,000 — the initial $10,000 plus $150,000 of monthly deposits.
- Growth: $302,290, which is 65% of the final balance.
Two thirds of the result was never your money going in. Extend the same plan by ten years and the balance passes $1,015,589 while your contributions rise only to $220,000 — the growth share reaching 78%.
Frequently asked questions
How does compound interest actually work?
Each period you earn a return on your balance, and that return joins the balance, so the next period earns a return on a larger figure. Over one year the difference against simple interest is trivial; over decades it dominates. In the default example, $160,000 of contributions become $462,290 — the extra $302,290 is entirely growth on growth.
How much difference does starting early make?
Enormous, because the most productive years are the last ones. In this example the first five years add $49,973 and the five years from 30 to 35 add $324,439 — from identical contributions. Starting earlier does not just give you more years, it gives you the years that produce the most.
Does compounding frequency matter?
Much less than the rate, the amount or the time. Moving from annual to monthly compounding at the same nominal rate gives a modest improvement, and monthly to daily gives a very small one. When comparing accounts, the headline rate and the fees will decide the outcome long before the compounding interval does.
Should I use a real or nominal return?
Use a nominal return if you want the future dollar figure, and a real return — nominal minus inflation — if you want the answer in today's purchasing power. It matters more than it sounds: at 3% inflation, $462,290 in 25 years buys roughly what $220,000 buys now.
How much do fees really cost?
A 1% annual fee is a permanent 1% off your compounding rate, not 1% of your gains. In this example, 7% instead of 6% is worth $71,143 over 25 years on identical contributions — which is why fund costs deserve more attention than they usually get.
What return should I assume?
There is no right answer, and the calculator deliberately lets you change it because the assumption matters so much: between 4% and 10% the outcome nearly triples. Run your plan at a cautious rate as well as an optimistic one, and treat the gap as the honest range rather than picking a single number and believing it.
Method & sources
- Calculation: Standard future-value formula (shown above) with contributions added each compounding period. Assumes a constant rate and ignores taxes and fees.
- Reviewed: · Assumptions reviewed quarterly.
Educational estimate, not financial advice. Investment returns are not guaranteed.
Run your own numbers
Enter your starting balance, monthly contribution, rate and time horizon, and see how much of the final figure is your money and how much is growth.
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