How to use the compound interest calculator
- Enter your initial deposit — the amount you are starting with.
- Add a monthly contribution if you plan to save regularly (enter 0 if not).
- Enter the annual interest rate or expected rate of return.
- Choose how many years the money will grow and how often interest compounds.
- Review your future balance, total interest earned and the year-by-year table.
The compound interest formula
For a single deposit with no additional contributions, compound interest follows this formula:
- A = final amount
- P = initial principal
- r = annual interest rate (as a decimal)
- n = number of times interest compounds per year
- t = number of years
With regular monthly deposits, each contribution also starts earning interest. This calculator adds contributions at the end of each month and applies the rate that matches your chosen compounding frequency.
Example: $10,000 at 7% for 10 years
If you invest $10,000 at 7% compounded monthly and leave it alone for 10 years, it grows to about $20,097 — the money roughly doubles. Add $200 a month and the balance reaches about $54,714, of which $34,000 is your own deposits and about $20,714 is interest.
How compounding frequency matters
| Compounding | $10,000 at 7% after 10 years |
|---|---|
| Annually | $19,672 |
| Monthly | $20,097 |
| Daily | $20,136 |
More frequent compounding helps, but the rate, the time horizon and how much you contribute matter far more.
The Rule of 72
To estimate how long it takes money to double, divide 72 by the annual interest rate. At 7%, money doubles in about 72 ÷ 7 ≈ 10.3 years; at 10%, about 7.2 years. It's a handy mental shortcut that matches the example above.
Tips to make compounding work for you
- Start early. Time is the most powerful factor — ten extra years can double your result.
- Contribute consistently, even small amounts, and increase contributions as your income grows.
- Reinvest earnings such as dividends instead of withdrawing them.
- Mind fees and taxes, which reduce your effective return. Tax-advantaged accounts can help.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and on the interest it has already earned. Over time this creates exponential growth, often described as "interest on interest."
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus accumulated interest, so it grows faster over time.
How often is interest compounded?
It depends on the account. Savings accounts often compound daily or monthly, many bonds compound semiannually, and investment returns are usually quoted as an annual rate. Choose the option that matches your account.
What rate of return should I use?
For a savings account, use its APY. For long-term stock investments, many people use a conservative 5–7% after inflation, but returns are not guaranteed and can vary widely from year to year.
What is APY?
Annual percentage yield (APY) is the effective yearly rate after compounding. A 7% rate compounded monthly gives an APY of about 7.23%. The calculator shows the APY for your inputs.
Results are estimates for educational purposes and are not financial advice. Actual loan terms, rates, taxes and fees vary by lender and location — confirm figures with a qualified professional before making decisions.
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