Compound Interest Calculator

The Compound Interest Calculator helps you estimate how much your savings or investments could be worth in the future. The key difference between simple and compound interest is that compound interest earns interest not just on your original deposit but also on all the interest that has already accumulated. This is often called "interest on interest," and it is one of the most important principles in personal finance.

This tool is particularly useful if you are saving for retirement, planning an investment strategy, or simply trying to understand how your money grows over time. By adjusting the numbers — your starting amount, how much you add regularly, the interest rate, and the time horizon — you can compare different scenarios and see why starting early makes such a dramatic difference. It removes the guesswork and shows you the actual math behind your financial future.

1. Initial Investment (Principal): Enter the amount of money you are starting with. This is your first deposit or lump-sum investment.
2. Regular Addition (Optional): If you plan to contribute money on a regular basis (such as monthly), enter that amount here. Consistent additions significantly speed up growth over time.
3. Interest Rate (%): Enter the annual return rate you expect. For savings accounts, this will typically be low. For a diversified stock market portfolio, historical averages tend to be higher.
4. Years to Grow: Enter how many years you plan to leave the money invested.
5. Compound Frequency: Choose how often interest is added to your principal — annually, monthly, daily, and so on. More frequent compounding results in slightly higher returns over time.
6. Calculate: Click the button to see your projected total, total interest earned, and a full breakdown of your growth over time.

The standard compound interest formula, when no regular contributions are made, is:

• A: The future value of the investment, including all accumulated interest.
• P: The principal — your initial deposit or investment amount.
• r: The annual interest rate expressed as a decimal (e.g., 8% becomes 0.08).
• n: The number of times interest compounds per year.
• t: The total number of years the money stays invested.

When you add regular contributions, the calculation becomes more involved and uses the Future Value of a Series formula. Our calculator handles all of this automatically in the background, so you get accurate results without needing to work through the math yourself.

Example 1: Starting Early

You invest $1,000 at an annual rate of 8%, compounded once per year, and leave it untouched for 30 years. By the end of that period, your balance grows to $10,062.66. You earned over $9,000 in interest from a single $1,000 investment — simply by giving it time.

Example 2: The Power of Monthly Contributions

You start with $0 but commit to investing $500 every month at a 7% annual return, compounded monthly, for 20 years. Over those 20 years, you personally put in $120,000. But your final balance comes to $260,463.04 — meaning the growth on top of your contributions is an additional $140,463.04.

Example 3: Short-Term Savings Goal

You want $10,000 for a car purchase in 3 years. You start with $2,000 and can save $200 a month at a 4% annual rate compounded monthly. After 3 years your projected balance is $9,864, which tells you that to hit your exact $10,000 target, you would need to save just slightly more each month.

Read our full guide to learn more: Understanding Compound Interest: The Key to Wealth