Compound Interest Calculator
Enter a starting balance, a monthly contribution, the yearly rate and how long you will invest. The result shows the final balance, how much you put in and how much came from interest.
Enter your numbers to see the result.
The compound interest formula
With no extra deposits, the final amount is A = P × (1 + r/n)nt, where P is the starting balance, r is the annual rate as a decimal, n is how many times a year interest is added, and t is the number of years. Putting 10,000 at 7% for 10 years with yearly compounding gives 19,671.51. Your interest earned interest of its own, which is the whole point.
With monthly deposits, this calculator steps through each month, adds interest at the periodic rate, then adds your contribution at the end of the month. That gives a realistic figure for a savings plan instead of a single textbook formula.
Why time matters more than rate
Try the default numbers, then change the years to 20 and leave everything else alone. The balance does far more than double. Growth in the later years comes mostly from interest on earlier interest, so starting a few years sooner often beats finding a slightly higher rate.
The rule of 72
To estimate how long money takes to double, divide 72 by the annual rate. At 6% it takes about 12 years, at 9% about 8 years. It is a mental shortcut, and the calculator gives you the exact figure.
Things the result does not include
Returns are never guaranteed, and real investments rise and fall. Inflation, fees and taxes also reduce what you keep. Use this as a planning tool for comparing scenarios, not as a forecast. To see how percentages behave, try the percentage calculator.
[csp_calc type=”compound”]
What the compound interest calculator shows
Compound interest means you earn interest on your interest. This compound interest calculator projects how a starting amount grows when you also add a fixed sum every month. Enter the starting amount, the monthly contribution, the annual rate, the number of years and how often interest is compounded. It shows the future value, the total you put in and the interest earned.
The formula
A = P × (1 + r ÷ n)n × t
P is the starting amount, r is the annual rate as a decimal, n is the number of times interest is compounded per year and t is the number of years. For monthly contributions, this calculator converts the rate to an equivalent monthly rate and adds each contribution at the end of every month.
Worked example
Start with $10,000, add $200 a month at 7% a year, compounded monthly, for 10 years. You put in $10,000 + ($200 × 120) = $34,000. The balance grows to about $54,714, so about $20,714 is interest. Without the monthly contributions, the $10,000 alone would grow to about $20,097.
How compounding frequency matters
| $10,000 at 7% for 10 years | Value |
|---|---|
| Compounded yearly | $19,672 |
| Compounded monthly | $20,097 |
| Compounded daily | $20,136 |
| Simple interest (no compounding) | $17,000 |
More frequent compounding helps, but the effect is small compared with the rate, the time and the amount you add. Time matters most: the longer the money grows, the larger the share of the balance that comes from interest.
Limits of this estimate
Real investments do not earn a steady rate, and fees, taxes and inflation reduce what you keep. Use this as a planning tool to compare scenarios, not as a forecast. For savings accounts, check the rate the bank actually pays and how often it compounds.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original money and the interest already earned, so growth speeds up over time.
Does more frequent compounding help a lot?
It helps a little. Moving from yearly to monthly compounding adds a small amount, while the rate and the number of years have a much bigger effect.
What is the rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes for money to double. At 8% that is about 9 years.
Are the results guaranteed?
No. The calculator assumes a constant rate. Real investment returns vary and fees, taxes and inflation reduce what you actually keep.