Compound Interest Calculator
Project investment growth from compound interest and monthly contributions.
Project investment growth
Projected value
Balance after 10 years
- Total contributions
- $70,000
- Estimated interest
- $36,639
Projection only. Returns are not guaranteed and actual results will vary.
Your entries and results stay in this browser. This tool does not send, save, or add them to the page address.
How to use the compound interest calculator
Enter the amount already invested, the amount you plan to add each month, an assumed annual return, and the number of years. Choose whether contributions occur at the beginning or end of each month.
The results separate the money contributed from the projected interest earned. This distinction helps show how much of the ending balance comes from saving and how much comes from compounding.
Understanding the projection
This calculator converts the annual rate to a monthly rate and compounds the balance once per month. It assumes the rate and contribution remain unchanged for the entire period. Beginning-of-month contributions receive that month’s growth; end-of-month contributions begin earning in the following month.
Actual investments rarely produce the same return every month. Fees, taxes, inflation, market losses, withdrawals, and changes to your contributions are not included. Use the estimate to explore scenarios, not to predict a specific outcome.
Comparing savings scenarios
Change one assumption at a time. You can compare a higher monthly contribution at the same rate, then compare a longer time period without changing the contribution. This makes the effect of saving more distinct from the effect of waiting longer.
An assumed return should reflect the kind of account or investment being considered and its risks. A higher assumed rate produces a larger projection but does not make that return more likely.
Frequently asked questions
How does this compound interest calculator work?
The calculator applies one-twelfth of the annual rate each month and adds your chosen monthly contribution at either the beginning or end of each month. It repeats that process for the full investment period.
Why does contribution timing change the result?
A beginning-of-month contribution earns one additional month of growth compared with an equal end-of-month contribution. The difference becomes larger as the rate, contribution, or investment period increases.
Does the result account for investment fees or taxes?
No. The projection does not include management fees, trading costs, taxes, inflation, changing contribution amounts, or changing rates of return. Each of those factors can materially affect an actual result.
Is the estimated return guaranteed?
No. The calculation is a mathematical projection based on a constant rate, not a forecast or guarantee. Real investments can gain or lose value, and their returns vary over time.