How Compound Interest Works (With a Simple Example)
Updated July 2026 · Educational guide
Compound interest means you earn returns not only on your original savings, but also on interest that has already been added to the balance. Over many years, that “interest on interest” effect can matter more than any single year’s rate.
Simple interest vs compound interest
With simple interest, growth is calculated only on the starting principal. With compound interest, each period’s earnings join the principal, so the next period starts from a larger base. The longer the timeline, the wider the gap between the two approaches.
A worked example
Suppose you start with $10,000, add $500 every month, and earn 8% per year with monthly compounding for 20 years. Your total cash contributions are $10,000 + ($500 × 240) = $130,000. The projected ending balance is much higher because growth applies to both contributions and prior earnings. Exact results depend on timing and rate assumptions—use the calculator to stress-test them.
Open the Compound Interest Calculator →
What actually moves the result
- Time: Extra years usually beat chasing a slightly higher assumed return.
- Monthly contributions: Early on, saving more often helps more than optimistic rates.
- Compounding frequency: Monthly compounding typically beats annual compounding at the same stated rate.
- Fees and taxes: Real accounts may grow slower than a clean simulation.
How to use this on FinanceSliders
Set a conservative rate for cash/bonds and a separate, more optimistic case for long-term investing. Compare both. Export a PDF if you want to revisit the scenario later. Remember: the tool is educational, not a forecast of market returns.