rebalioFree tools / Compound interest
Compound interest calculator.
See your money grow.
A compound interest calculator for the bigger picture. Explore how a lump sum, monthly contributions and time could shape your portfolio.
Free to use · No sign-up · Calculated in your browser
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Understand the numbers
How compounding
adds up.
Compounding means earning returns on earlier returns as well as your original money. Time and regular contributions both influence the result.
Learn about regular investing →How does this compound interest calculator work?
Start with a lump sum, add a monthly contribution, choose a time horizon in years, and enter an annual return. The calculator applies growth each month and then adds your contribution at the end of that month. The curve shows your projected balance alongside the total money you put in.
How is the annual return applied?
The annual return is the growth rate you enter for a full year, including compounding. We convert it to an equivalent monthly rate: (1 + annual return / 100)^(1/12) − 1. For example, $1,000 at 8% becomes $1,080 after one year without contributions. Investment returns fluctuate, so this is an illustration rather than a forecast.
Can I calculate growth with only a lump sum or monthly contributions?
Yes. Set the monthly contribution to zero to see a lump sum compound on its own, or set the initial lump sum to zero to start with monthly contributions. With a zero return, your final balance is simply your lump sum plus all monthly contributions.
Are taxes, fees and inflation included?
No. The figures are nominal amounts in USD and exclude taxes, fees and inflation. Contributions stay constant throughout the horizon. Negative annual rates are supported to illustrate losses. Actual investment results will depend on the assets you hold and the path of their returns.
The compound interest formula
For a lump sum P, monthly contribution C, monthly rate r and n months, the future value is P × (1 + r)n + C × ((1 + r)n − 1) / r. When r is zero, use P + C × n. This assumes every contribution arrives at the end of the month.