Compound interest calculator with monthly contributions
Separate your own deposits from estimated growth, and compare lower and higher return assumptions.
No account needed · Financial inputs are calculated in your browser · Example values shown
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Understand the assumptions
These are estimates from the values you enter. Review the assumptions in your report and check the actual terms before making a financial commitment.
How this calculator works
Project a starting balance plus recurring monthly deposits. The results separate money contributed from modeled growth and show how different return assumptions change the outcome.
Worked example: $100 a month for ten years
Start with $1,000, contribute $100 at the end of every month and assume 6% nominal annual growth compounded monthly. After ten years the modeled balance is $18,207.33: $13,000 contributed and $5,207.33 in growth.
How compounding frequency is handled
For nominal annual rate r and m compounding periods per year, the equivalent monthly rate is i = (1 + r / m)m / 12 − 1. With n months and monthly contribution C, the future balance is P × (1 + i)n + C × ((1 + i)n − 1) / i. At zero growth, the balance is P + C × n.
Is the return guaranteed?
No. The calculation smooths a constant rate across the entire period. It excludes changing market returns, fees, taxes and inflation. The lower and higher scenarios show sensitivity to assumptions; they are not a probability range.
Work backwards from a savings target or estimate inflation’s effect on purchasing power.
Understand your result
Why an Investment Calculator Is a Scenario, Not a Forecast
Learn what constant-return projections include, what they omit and how to compare assumptions without mistaking them for promises.