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Compound Interest Calculator

See how your money grows with compound interest and regular monthly contributions, year by year.

Runs locally — files never leave your device

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How to use Compound Interest Calculator

  1. 1Enter your initial amount and how much you plan to add every month (enter 0 for a one-off deposit).
  2. 2Enter the annual interest rate or expected return and the number of years.
  3. 3Choose how often interest is compounded: yearly, quarterly, monthly or daily.
  4. 4Read the final balance, your total contributions and the interest earned, then review the yearly table or download it as CSV.

Calculate compound interest on savings and investments

Compound interest is often called the most powerful force in personal finance: money earns interest, and then that interest earns interest too. This compound interest calculator shows exactly how a deposit grows over time, with optional monthly contributions and a choice of compounding frequency. You get the final balance, the split between what you paid in and what you earned, and a year-by-year table you can download. Everything is calculated in your browser.

Features

  • Initial deposit and monthly contributions — model a lump sum, a regular savings plan, or both.
  • Compounding frequency — yearly, quarterly, monthly or daily.
  • Final balance, total contributions and total interest, plus a bar that shows how much of the final balance is interest.
  • Year-by-year growth table with contributions, accumulated interest and balance.
  • CSV export for Excel, Google Sheets or Numbers.
  • Any currency — enter amounts in your own currency.

Example

Invest 10,000 at 7% a year, compounded monthly, and add 100 every month for 10 years. You pay in 22,000 in total (10,000 plus 120 contributions of 100). The balance grows to about 37,400, so roughly 15,400 is interest — more than the original deposit. Extend the period to 30 years and interest becomes the largest part of the balance by far.

Why time matters most

Compound growth accelerates. In the first years, interest is small because the balance is small. Later, each year's interest is calculated on a much bigger balance:

  1. Start early. Ten extra years can matter more than doubling your monthly contribution.
  2. Keep contributing. Regular deposits give compounding more money to work on.
  3. Avoid withdrawals. Taking money out resets part of the growth.

A quick way to estimate growth is the rule of 72: divide 72 by the annual rate to get the approximate number of years it takes to double your money. At 6% that's about 12 years; at 9%, about 8 years.

Nominal vs. effective rate

Banks often quote a nominal annual rate along with a compounding frequency. The effective annual rate (also called APY or AER) shows what you actually earn in a year: (1 + r/n)^n − 1. A 5% nominal rate compounded monthly gives an effective rate of about 5.12%. When comparing savings accounts, compare effective rates.

Limits of the calculation

The calculator assumes a constant rate for the whole period. Savings rates change and investment returns fluctuate, sometimes sharply. Results don't include taxes, account fees or inflation. Use it to compare scenarios and understand the effect of time and contributions, not as a guarantee.

To see the opposite side of interest — what borrowing costs — try the Loan Calculator. For quick percentage questions, use the Percentage Calculator.

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. Each period the interest is added to the balance, so the next period's interest is calculated on a larger amount. Over long periods this creates exponential growth.
What formula does the calculator use?
For a single deposit the formula is A = P × (1 + r/n)^(n×t), where P is the principal, r the annual rate, n the number of compounding periods per year and t the number of years. With monthly contributions, the calculator simulates each month: the balance grows by the equivalent monthly rate and the contribution is added at the end of the month.
Does compounding frequency make a big difference?
It makes some difference, but less than people expect. At 5% a year, yearly compounding gives an effective rate of 5.00%, monthly 5.12% and daily 5.13%. The interest rate, the time invested and the amount you contribute matter far more.
Are contributions added at the start or end of each month?
At the end of each month, which is the usual convention for regular savings plans. Contributions made at the start of the month would earn slightly more interest.
Does it account for inflation, taxes or fees?
No. The result is a nominal balance before tax, fees and inflation. To estimate growth in today's money, subtract the expected inflation rate from the interest rate (for example, use 4% instead of 7% if you expect 3% inflation).
Can I use it for stock market returns?
You can enter an expected average annual return, but real investments don't grow at a fixed rate. Returns vary from year to year and can be negative, so treat the result as an illustration, not a forecast.

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