Compound Interest Calculator: See How Your Money Can Grow
Enter a starting amount, monthly contributions, a rate and a time frame to see your future balance, how much of it is interest and how compounding frequency, time and inflation change the result.
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Key takeaways
- Compound interest means you earn interest on your interest. Over long periods, that second layer becomes the largest part of the balance.
- $10,000 plus $300 a month at 6% for 20 years grows to about $171,700: $82,000 of contributions and roughly $89,700 of interest.
- Time matters more than the rate: $300 a month at 6% grows to about $597,000 over 40 years but about $301,000 over 30.
- Compounding frequency matters far less than people think. Daily instead of annual compounding adds only a fraction of a percentage point a year.
Compound interest is the reason a modest monthly habit can turn into a large sum. The calculator below shows how a starting balance and regular contributions grow at a constant rate, how much of the result is your own money and how much is earnings, and how the balance builds year by year. It works for savings accounts and CDs with a fixed rate as well as for long-term investment scenarios, where the rate is an assumption rather than a promise.
Compound interest calculator
Before taxes and fees and without inflation. A constant rate is assumed for the whole period.
What compound interest is
With simple interest, you earn interest only on the money you put in. With compound interest, the interest you earn is added to the balance and starts earning interest itself. In the first years the difference is small. Over decades it becomes dominant: in the 20-year example above, about $34,700 of the $89,700 in interest is interest earned on earlier interest, money that simple interest would never produce.
The compound interest formula
For a single deposit, the future value is:
A = P × (1 + r ÷ n)n × t
where P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. For regular monthly contributions, the calculator converts the rate into an equivalent monthly rate, i = (1 + r ÷ n)n ÷ 12 − 1, and adds each contribution at the end of the month:
A = P × (1 + i)m + C × ((1 + i)m − 1) ÷ i
with C the monthly contribution and m the number of months.
Example: $10,000 plus $300 a month at 6%
| After | Your contributions | Balance at 6% |
|---|---|---|
| 10 years | $46,000 | $67,358 |
| 20 years | $82,000 | $171,714 |
| 30 years | $118,000 | $361,580 |
In the first decade, interest adds about $21,000 to $46,000 of contributions. In the third decade alone, the balance grows by almost $190,000, while you contribute only $36,000 of it. That acceleration is compounding at work.
Why starting early matters
Consider two savers who each invest $300 a month at 6% until 65. One starts at 25, the other at 35.
| Start | Years invested | Total contributed | Balance at 65 |
|---|---|---|---|
| Age 25 | 40 | $144,000 | $597,447 |
| Age 35 | 30 | $108,000 | $301,355 |
The early starter contributes $36,000 more but ends with nearly twice as much. The ten extra years at the start are worth more than any later catch-up, because the earliest dollars compound the longest. Our guide to investing for retirement covers the accounts that let this growth happen tax-deferred or tax-free.
How compounding frequency affects the result
Here is $10,000 at a 6% rate for 10 years with no additional deposits:
| Compounding | Effective annual yield (APY) | Balance after 10 years |
|---|---|---|
| Annually | 6.000% | $17,908 |
| Quarterly | 6.136% | $18,140 |
| Monthly | 6.168% | $18,194 |
| Daily | 6.183% | $18,220 |
Daily compounding beats annual compounding by about $312 over ten years on $10,000. That is why banks advertise the APY, which already includes compounding: when you compare savings accounts or CDs, compare APYs and ignore the compounding schedule. How savings account interest works explains APY in detail.
The rule of 72
A quick way to estimate growth without a calculator: divide 72 by the annual rate to get the number of years it takes money to double. At 6%, that is 72 ÷ 6 = 12 years (the exact figure is 11.9). At 4%, money doubles in about 18 years; at 9%, in about 8. The rule works in reverse for debt: at a 24% credit card APR, an unpaid balance doubles in about three years.
Choosing a realistic rate
- Savings accounts and CDs: use the APY you are offered. The FDIC national average for savings was 0.37% on September 21, 2026, while leading high-yield savings accounts paid around 4%. Savings rates are variable; CD rates are fixed for the term.
- Investments: stocks have historically grown faster than savings over long periods, but with sharp declines along the way and no guaranteed rate. Many planners use conservative assumptions such as 5% to 7% for diversified long-term portfolios. Treat the result as a scenario, not a forecast.
- Fees: subtract them. A fund with a 1% expense ratio turns a 7% return into 6%, which over 30 years costs a large share of the final balance. Low-cost index funds, covered in best ETFs to buy, keep more of the compounding for you.
Inflation and taxes
The calculator shows nominal dollars. Inflation reduces what those dollars buy, so for long horizons it helps to use a real (inflation-adjusted) rate: subtract expected inflation from the rate. Taxes also slow compounding when interest and dividends are taxed every year. Tax-advantaged accounts such as a 401(k), traditional IRA or Roth IRA let the full amount compound; in a taxable account, interest is taxed as ordinary income each year, as explained in is savings interest taxable.
Compound interest works against you on debt
The same math that builds savings makes unpaid debt expensive. Credit card interest compounds daily on most cards, and minimum payments barely cover it. A $5,000 balance at 22% paid at the minimum can take close to 20 years to clear. The credit card interest calculator shows your payoff time and total cost; paying off high-rate debt is often the best "investment" available.
Frequently asked questions
How much will $10,000 grow in 10 years?
At 6% compounded monthly with no further deposits, about $18,194. At 4%, about $14,908 with monthly compounding; at 8%, about $22,196.
Is compound interest calculated daily or monthly?
It depends on the account. Many savings accounts compound daily and credit interest monthly. For comparing accounts, the APY already reflects the compounding schedule.
What is the difference between APR and APY?
APR is the annual rate without compounding; APY includes the effect of compounding over a year. For savings, a higher APY means more interest. For loans, lenders quote APR.
Do stocks earn compound interest?
Stocks do not pay interest, but reinvested dividends and rising share prices compound in a similar way. Unlike a savings account, the return varies from year to year and can be negative.
How can I make compound interest work faster?
Start early, contribute regularly, reinvest earnings, keep fees low and use tax-advantaged accounts so taxes do not interrupt the growth.
