Compound Interest Calculator (General Version)

Compound interest calculator for savings and investment growth

This compound interest calculator is built for one decision: estimating how much your savings or investment balance could be worth in the future when interest earns interest over time. That is the core idea of compounding. Instead of interest being added only to your original starting amount, each period’s interest is added to the balance, and future interest is calculated on that larger balance.

Use it when you want a quick, practical estimate of an ending balance based on inputs you can reasonably know. Most people can provide a starting amount, an annual interest rate (even if it is an estimate), and a time period in years. If you are also contributing regularly, you can add a monthly contribution. The calculator then breaks the result into total contributions versus total interest earned so you can see whether your outcome is being driven more by deposits or by growth.

This page is intentionally not a loan repayment tool. Loans and credit products introduce payment schedules, interest charges on a declining balance, fees, and amortisation rules that are a different problem. Here, the assumption is growth: your balance is increasing over time because you are saving money and the balance is earning compound interest. If you only want a simple ending value estimate, the default inputs are enough. If you care about how the rate is applied, you can set a compounding frequency (monthly, quarterly, annually, or daily) and whether your contribution happens at the beginning or end of each month.

Assumptions and how to use this calculator

• The annual interest rate is treated as a nominal rate with the selected compounding frequency. If you only have an advertised annual rate, choose monthly compounding as a practical default.
• Contributions are assumed to be monthly and consistent (the same amount each month). If your contributions vary, use an average monthly estimate for a usable projection.
• If you enter a non-integer number of years, partial years are treated as fractional compounding, and only full monthly contributions are applied. This avoids inventing contribution timing for a partial month.
• Taxes, investment fees, account charges, and withdrawal penalties are not included. Real outcomes can be lower if these apply.
• The result is a projection, not a guarantee. Small changes in rate or time can materially change the ending balance because compounding is exponential.

Common questions

What is the difference between simple interest and compound interest?

Simple interest calculates interest only on the original starting amount. Compound interest adds each period’s interest to the balance, so future interest is calculated on both the original amount and accumulated interest. Over longer periods, compounding usually produces a much larger ending balance, especially when contributions continue over time.

What does “compounding frequency” change in practice?

Compounding frequency describes how often interest is added to the balance. More frequent compounding (like monthly or daily) generally increases the ending balance compared to annual compounding, even with the same annual rate. The difference is often modest over short time frames, but it becomes more visible as the time period grows.

Should I include monthly contributions, or just the starting amount?

Include monthly contributions if you are actively saving or investing each month, because contributions can dominate the outcome. A smaller starting amount with steady contributions often outgrows a larger starting amount with no contributions over long periods. If you do not contribute regularly, leave the monthly contribution as zero and the calculator will still work.

Why is the interest earned number sometimes smaller than I expected?

First, check the time period: compounding needs time. Second, check the annual rate and whether it is realistic for your account type. Third, remember this calculator does not assume increasing contributions, bonuses, or market jumps. Finally, if most of your ending balance is made up of recent contributions, those newer contributions have not had much time to earn interest.

How can I make the estimate more accurate for my situation?

Use an annual rate that reflects your actual product after typical fees and taxes, and match the compounding frequency to how your account credits interest (many savings accounts are monthly; some products quote an effective annual rate). If your contributions are not constant, calculate an average monthly amount or run multiple scenarios. For long horizons, even small differences in the rate matter, so it can be useful to test a conservative rate and an optimistic rate to bracket reality.