Compound Interest Calculator
Enter what you start with, how much you add each month, the yearly return and how many years you will wait. The calculator shows your final balance, how much came from your deposits and how much was growth.
Compounding means you earn interest on your interest. The longer the money stays, the faster it grows.
Please note: Estimate only. This is not financial advice. Investment returns are not guaranteed and can be negative.
How it works
Each month the balance grows by the monthly share of the yearly rate. Your deposit is then added, or it is added first if you choose “start of month”. The next month you earn on the bigger balance. That repeating step is compounding.
How often interest compounds changes the result slightly. Daily compounding earns a little more than yearly compounding at the same rate. The “effective yearly rate” in the result shows what you really earn in a year after compounding.
The return you enter is an assumption. Real savings accounts pay a known rate, but stocks and funds go up and down. A steady yearly rate is a way to compare plans, not a promise.
Inflation lowers what money can buy. If you want to see growth in today’s buying power, enter a return that is lowered by the inflation rate, or use the retirement calculator.
each month: balance = balance × (1 + rate ÷ n)^(n ÷ 12) + deposit
n = times interest compounds per year
effective yearly rate = (1 + rate ÷ n)^n − 1Worked example: Saving for 20 years
Start with $10,000 and add $200 a month at a 7% yearly return, compounded monthly. After 20 years the balance is $144,572.72.
You put in $58,000 ($10,000 plus 240 deposits of $200). The other $86,572.72 is growth.
A single $10,000 deposit at 7% compounded monthly with no deposits becomes $20,096.61 after 10 years, a little more than double.
Frequently asked questions
What return should I use?
It depends on where the money is. Insured savings accounts pay a posted rate. For stock funds, people often use a long-run estimate lower than past highs, and test a few values. Try a range, such as 4%, 6% and 8%.
Does it matter how often interest compounds?
A little. More frequent compounding gives a slightly higher result at the same rate. The gap between monthly and daily is small. The rate you earn and the time you wait matter far more.
What is the difference between start and end of month deposits?
Deposits at the start of the month earn one extra month of growth. Over many years this adds up to a small but real difference.
Does this include taxes or fees?
No. Taxes on gains and fund fees reduce your result. If your fees are 1% a year, subtract 1 from the return you enter.
What is the rule of 72?
It is a shortcut. Divide 72 by the yearly rate to guess how many years it takes to double. At 6% it is about 12 years. The calculator gives the exact answer.
Related tools
- Retirement CalculatorSee if your savings can fund the income you want in retirement, in today’s money. Shows the gap and the extra monthly saving that would close it.
- Inflation CalculatorSee what a dollar amount from any year since 1913 is worth today, using official U.S. consumer price data. Includes the average yearly inflation rate.
- Loan CalculatorCalculate the monthly payment and total interest on any loan. Add extra payments to see how much interest and time you save. Free, no sign-up.
Content last reviewed 2026-10-13.