Compound Interest Calculator
See how a one-off principal plus regular contributions grow over time when interest compounds month-on-month.
Calculation runs in your browser — your inputs never leave the device.
Your inputs
Takes ~ 1 minuteYour result
Fill in the fields above and click Calculate — your personalised plan appears here in a moment.
How compound interest works
Compound interest pays interest on previously earned interest. Each compounding period the balance grows by the rate × current balance, and the next period earns interest on that larger balance. Over decades the snowball effect dominates: the same monthly contribution invested at 6% for 30 years grows roughly four times larger than at simple interest of the same nominal rate.
Reading the result
The chart below shows the balance, total contributions, and interest earned. The gap between the contribution line and the balance line is the interest the calculator has paid you.
Plain-language notes
Use this section if the finance words on the page are new to you. The calculator is meant to support a decision, not to reward perfect terminology.
- Compound interest: growth earned on earlier growth. Time matters because the later years can do more work.
What to compare
Compare at least two scenarios before trusting the first answer. A useful result should tell you what changes if income, costs, rates, or timing move.