Compound interest calculator
See how your money grows over time with monthly contributions and an evolution chart.
Final amount
$24,600.15
Total invested$19,000.00
Total interest$5,600.15
Return on invested29.5%
Accumulated total Total invested
Estimate with a constant rate; taxes and inflation are not included.
How the calculation works
Compound interest is "interest on interest": each period, the return is calculated on the full accumulated balance, not just the initial amount — which is why growth is exponential.
The formula with monthly contributions is:
Amount = P·(1+i)^n + PMT·[((1+i)^n − 1) / i]
P= initial amountPMT= monthly contributioni= rate per period (decimal)n= number of periods
Examples
- $1,000 initial + $300/mo at 0.8%/mo for 60 months ≈ $24,300.
Frequently asked questions
Simple vs compound interest?
Simple interest always applies to the initial amount. Compound interest applies to the accumulated balance, producing exponential growth.
How to convert a yearly rate to monthly?
Use the equivalence: i_monthly = (1 + i_yearly)^(1/12) − 1. Dividing by 12 is not accurate.
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Updated on June 17, 2026 · by Rafael Rossi · Methodology & sources