CalcDime

Compound Interest Calculator

How a starting balance and regular monthly deposits grow when interest compounds — and how much of the final amount is interest rather than money you put in.

Reviewed November 2026How we calculate

Shows the result in today’s dollars
Balance after 20 years$0
Total deposited$0
Interest earned$0
Effective annual yield0%
Money doubles roughly every—

Growth by year

Year-by-year table
YearDepositedInterest earnedBalance

How the growth is calculated

The calculator works month by month. Your starting balance and each month’s deposit earn interest at the effective monthly rate implied by the annual rate and compounding frequency. Deposits are added at the end of each month.

FV = P(1 + i)ᵐ + C × ((1 + i)ᵐ − 1) ÷ i, where i = (1 + r/k)^(k/12) − 1

P is the starting balance, C the monthly deposit, r the annual rate, k the compounding periods per year and m the number of months. With inflation set, the final balance is divided by (1 + inflation)years to show its purchasing power today.

Time does most of the work

With the default numbers — $10,000 plus $300 a month at 6% — you deposit $82,000 over 20 years and end with about $171,000. Wait ten more years and the deposits rise to $118,000, but the balance more than doubles, to about $362,000. Starting earlier beats almost any other change you can make to the inputs.

Where to use it

For a high-yield savings account or CD, enter the APY and leave compounding on monthly or daily. For retirement accounts, the 401(k) and Roth IRA calculators apply the 2026 contribution limits, and the retirement savings calculator checks whether your savings can cover the income you want.

Common questions

What is compound interest?

Interest earned on both your deposits and the interest already added to the account. In early years most growth comes from contributions; over longer periods the interest on past interest becomes the larger share — the chart shows where that crossover happens for your numbers.

Does compounding frequency matter much?

Less than people expect. At 5%, daily compounding yields an effective 5.13% a year, monthly 5.12% and annual 5.00%. The rate itself and the number of years matter far more.

What return should I use?

For savings accounts and CDs, use the APY the bank quotes. For a stock-heavy portfolio, many long-term plans assume 5% to 7% a year, but actual returns vary widely from year to year and are not guaranteed.

What is the rule of 72?

Divide 72 by the annual return to estimate how many years it takes money to double. At 6%, about 12 years; at 9%, about 8. It is a shortcut that works best for rates between about 4% and 12%.

Sources

Results are estimates for educational use and are not financial, tax or legal advice. Confirm figures with your lender, plan administrator or a tax professional before you act. Spotted an error? Tell us.