What is a compound interest calculator?
A compound interest calculator estimates how an initial amount and regular savings can grow over time at an assumed interest rate. Compounding means that interest is added to the balance and can itself earn interest later. Enter an initial investment, a monthly contribution, the annual rate, an investment period and the compounding frequency to see an estimated future balance.
This calculator runs in your browser, so the figures you enter stay on your device. It uses the same currency unit throughout: an initial investment and contribution entered in dollars produce results in dollars, while another currency works the same way.
How to use the calculator
- Enter the amount you are starting with, or enter zero if you are beginning with regular contributions only.
- Add the amount you expect to contribute at the end of every month.
- Enter an estimated annual interest rate and choose how often interest compounds.
- Choose the whole number of years you want to project.
- Review the future value, total amount contributed, estimated interest earned and year-by-year balance.
Try several conservative scenarios instead of relying on a single optimistic rate. You can compare the effect of contributing more each month, investing for longer or earning a different rate. The schedule is especially useful for seeing that the balance does not necessarily rise by the same amount every year.
How the projection is calculated
The calculator converts the stated annual rate and selected compounding frequency into an equivalent monthly growth rate. It applies that growth to the current balance each month, then adds the regular contribution at the end of the month. At each year end, it records the resulting balance for the schedule.
With no monthly contribution, the calculation follows ordinary compound growth. With contributions, later deposits have less time to earn returns than the initial amount, so timing matters. The selected frequency affects the rate used in the projection: compounding more frequently at the same stated rate produces a slightly different outcome. A zero rate simply adds the starting amount and monthly contributions.
Important limits of a compound interest estimate
An assumed interest rate is not guaranteed. Investment returns can rise or fall, and savings accounts, bonds, funds and other products each have different rules, risks, rates and fees. Taxes, inflation, account fees, withdrawals and changing contributions are not included here. Use the result for education and planning, not as investment advice or a promise of future performance. Check the terms of a specific account or consult a qualified professional before making a financial decision.