Compound Interest Calculator
See your projected balance, total contributions, and estimated interest with regular monthly saving.
Quick answer
Enter a starting amount, monthly contribution, annual rate and time. The calculator compounds the opening balance at the selected frequency and adds monthly contributions at the end of each month.
Formula used
Opening balance growth uses A = P(1 + r/n)^(nt). Monthly deposits are then accumulated month by month so each contribution earns interest only after it is added.
Worked example
Starting with $5,000, adding $200 monthly for 10 years at a constant 5% annual rate produces a result above the $29,000 contributed because earlier deposits have longer to compound.
How to interpret the result
This is an illustration, not a forecast. It assumes a constant rate and excludes taxes, fees, inflation and market losses.
Sources and methodology
Investor.gov compound interest calculator · Consumer Financial Protection Bureau: saving. Sources explain general concepts; ToolGemini calculations use only the values you enter and the formula shown above.
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Frequently asked questions
What does compounding frequency change?
More frequent compounding credits interest more often. The difference is usually modest at ordinary savings rates.
Are monthly contributions added at the start or end of the month?
This calculator adds them at the end of each month, a conservative assumption for regular saving.
Can I use this for investments?
You can model a constant hypothetical return, but actual investment returns vary and can be negative.