Compound Interest Calculator
See how your savings or investments can grow with compound interest. Enter a starting amount, a monthly contribution, a rate and a time frame to see your future balance and how much of it comes from interest.
Assumes a fixed rate of return. Investment returns vary and are not guaranteed, and this does not include taxes, fees or inflation.
How your money grows
Each bar shows your balance at the end of a year, split into what you put in and what interest added.
| Year | Total put in | Interest | Balance |
|---|
The compound interest formula
Compound interest means you earn interest on your original money and on the interest it has already earned. For a single deposit, the formula is:
Regular contributions work the same way: each deposit starts compounding from the month it is added. The calculator runs month by month, converting your rate and compounding choice into an equivalent monthly growth rate, adding interest to the balance and then adding that month's contribution.
A handy shortcut is the rule of 72. Divide 72 by your yearly return to estimate how many years it takes money to double. At 7%, that is about 10 years; at 10%, about 7 years.
Worked example: $10,000 plus $200 a month
Say you invest $10,000 today and add $200 at the end of every month for 10 years, earning 7% a year compounded monthly.
- Starting amount$10,000.00
- Contributions: $200 × 120 months$24,000.00
- Total put in$34,000.00
- Interest earned$20,713.58
- Balance after 10 years$54,713.58
Time is the biggest lever. Keep the same plan for 20 years and the balance reaches about $144,573 on $58,000 of deposits. At 30 years it is about $325,159 on $82,000, and interest makes up three-quarters of the total.
Does compounding frequency matter?
Less than most people think. Here is $10,000 at 7% for 10 years with no extra deposits:
| Compounding | Balance after 10 years |
|---|---|
| Yearly | $19,671.51 |
| Quarterly | $20,015.97 |
| Monthly | $20,096.61 |
| Daily | $20,136.18 |
Going from yearly to daily compounding adds about $465 over 10 years. Your rate, how long you stay invested and how much you add each month make a much bigger difference. When comparing savings accounts, look at the APY, which already includes the effect of compounding.
Ways to make compounding work harder
- Start early. A few extra years at the beginning can matter more than larger deposits later.
- Automate contributions. A fixed monthly transfer keeps the plan going without effort. See what monthly investing adds up to with the investment calculator.
- Increase deposits with raises. Directing part of each pay raise to savings grows the balance faster.
- Get paid more on safe money. With yields near their highest in years, compare a Treasury bill, an I bond or a CD for cash you will not need soon.
- Keep costs low. Investment fees reduce your effective return every year, and that loss compounds too.
- Pay off high-interest debt first. A credit card at 22% costs far more than most investments earn. The Debt Payoff Calculator can help you plan it.
Frequently asked questions
How is compound interest calculated?
For a single deposit, the future value is A = P × (1 + r/n)^(n×t), where P is the starting amount, r is the yearly rate, n is the number of times interest compounds each year and t is the number of years. Regular contributions each grow the same way from the month they are added.
How much will $10,000 grow in 10 years?
At a 7% yearly return compounded monthly, $10,000 grows to about $20,097 in 10 years with no additional deposits. Adding $200 a month brings the total to about $54,714.
Does compounding frequency matter?
A little. At 7% for 10 years, $10,000 grows to $19,672 with yearly compounding and $20,097 with monthly compounding. The rate, the time invested and how much you add matter far more.
What is the rule of 72?
Divide 72 by the yearly rate of return to estimate how many years it takes for money to double. At 7%, money doubles in roughly 72 ÷ 7, or about 10 years.
What return should I use?
For savings accounts and CDs, use the current APY your bank offers. For stock investments, returns vary from year to year and are not guaranteed, so many people test a conservative rate, such as 5% to 7%, and a lower one to see a range of outcomes.
This calculator is for educational purposes only and is not investment advice. It assumes a constant rate of return, which real investments do not provide. Results do not include taxes, fees or inflation. Past performance does not guarantee future results.