What This Calculator Estimates
This compound interest calculator estimates how a starting balance can grow when you add a monthly contribution and apply a monthly rate derived from the annual return you enter. It is useful for savings goals, investment projections, and side-by-side scenario testing.
Formula / Method Used
The calculator loops through each month using this pattern:
- Monthly rate = Annual rate / 12
- New balance each month = (Current balance + Monthly contribution) x (1 + Monthly rate)
- Total invested = Starting amount + (Monthly contribution x Number of months)
- Growth = Ending balance - Total invested
Worked Example
With a starting amount of $5,000, a monthly contribution of $200, an annual rate of 6%, and a time period of 10 years, the estimated ending balance is about $42,036.73. That includes about $13,036.73 in growth on $29,000 contributed.
What the Result Means
The main result shows the projected ending balance. The detail line separates investment growth from the amount you personally contributed, which helps you see how much of the ending balance came from compounding rather than deposits.
Common Mistakes
- Entering an expected annual return as if it were already a monthly rate.
- Assuming the same return every month in real markets.
- Ignoring taxes, account fees, or contribution limits.
- Comparing results without matching the contribution schedule.
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Frequently Asked Questions
What does this compound interest calculator estimate?
It estimates the ending balance of an account after monthly contributions and monthly compounding over the time period you enter.
Does this include monthly contributions?
Yes. This page adds the monthly contribution each month before applying the monthly interest rate.
Why is compound interest different from simple interest?
Compound interest earns returns on prior growth as well as the original principal, while simple interest is calculated only on the principal.
Can I use this for savings or investing?
Yes. It works well for rough planning of savings, brokerage, or similar balances as long as you understand the return assumption is simplified.
Why can the real account value differ from this estimate?
Real balances can differ because rates change, contributions are irregular, taxes and fees apply, or returns do not compound evenly every month.
General Disclaimer
This calculator provides educational growth estimates only and is not investment, tax, or financial advice. Actual account performance can be materially different from a steady-rate projection.
Last updated: May 22, 2026