Compound Interest Calculator
See what a starting balance plus monthly contributions grows to at any rate and horizon, how much of the result is your money versus interest, and a year-by-year table.
Enter what you start with, what you add each month, a return, and a horizon. The calculator compounds monthly and separates what you contributed from what the money earned on its own.
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Sends your inputs and result to the AI you choose.
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Quick answers
Common cases, computed by this calculator with the assumptions listed under the method. Select one to load it above.
- How much is $500 a month for 20 years at 7%?
- $253,768 Final balance Open in the calculator
- What does $10,000 grow to in 10 years at 5%?
- $16,289 Final balance Open in the calculator
- How much is $200 a month for 30 years at 8%?
- $281,710 Final balance Open in the calculator
- How much is $1,000 a month for 15 years at 6%?
- $286,912 Final balance Open in the calculator
The formula
The input is an effective annual return. Monthly rate = (1 + annual return/100)^(1/12) − 1. Each month the balance grows at that rate, then the contribution is added. This preserves the annual return: $10,000 at 7% with no deposits becomes $10,700 after one year.
What the estimate includes
Contributions arrive at month end and the rate is constant. Enter a nominal return for future dollars or an inflation-adjusted return for today’s purchasing power. Taxes, fees and investment volatility are not modeled. For an account quoting APY, enter that APY directly.
Worked example
At zero return, $5,000 plus $300 monthly for 30 years becomes $113,000. At 7% effective annual return, with no starting balance, $300 monthly for 30 years grows to about $351,000. This is a scenario, not a return forecast.
Calculation notes updated .