Compound Interest Calculator
Enter a starting amount, an optional monthly contribution, an annual return, and a time horizon to see how compounding grows the balance — with a year-by-year breakdown. Interest compounds monthly.
| Year | Balance |
|---|---|
| 1 | $13,821 |
| 2 | $17,918 |
| 3 | $22,312 |
| 4 | $27,023 |
| 5 | $32,074 |
| 6 | $37,491 |
| 7 | $43,300 |
| 8 | $49,528 |
| 9 | $56,206 |
| 10 | $63,368 |
| 11 | $71,047 |
| 12 | $79,281 |
| 13 | $88,110 |
| 14 | $97,578 |
| 15 | $107,730 |
| 16 | $118,616 |
| 17 | $130,289 |
| 18 | $142,806 |
| 19 | $156,227 |
| 20 | $170,619 |
Frequently asked questions
How is the calculation done?
The balance is compounded monthly: each month the balance grows by one-twelfth of the annual rate, then the monthly contribution is added. This matches how most savings and brokerage projections are modeled.
What annual return should I assume?
That depends on the asset: high-yield savings currently pay roughly 4–5%, and the long-run average for a broad US stock index has been about 7% after inflation (roughly 10% nominal). Projections are estimates, not guarantees.
Why is compound interest so powerful over long periods?
Each period's interest itself earns interest in every later period, so growth accelerates. Doubling your time horizon far more than doubles the interest earned.