Compound interest calculator
Enter a starting amount, an annual rate, how often interest compounds and how many years, with an optional monthly contribution, to see what your money grows to.
Runs in your browser; nothing you type leaves your device
Final balance
- Total contributions
- 0
- Interest earned
- 0
- Effective annual rate
- 0
Year by year
| Year | Contributions | Interest | Balance |
|---|
A constant rate for the whole period, with no fees, taxes or inflation. Contributions are made monthly, at the end of each month unless you choose the start, and grow at the monthly equivalent of the compounding rate. Terms up to 100 years in whole months. This is an illustration, not a forecast.
How to calculate compound interest
- Enter the starting amount, the annual interest rate and the number of years.
- Choose how often the interest compounds, and add a monthly contribution if you will save regularly.
- Read the final balance, the total you put in and the interest earned, and check the year-by-year table.
How compound interest grows money
Simple interest is paid only on the amount you started with. Compound interest is paid on the starting amount and on the interest already added, so the balance grows faster each period. Over a few years the difference is small; over decades it is large, which is why starting early matters more than almost anything else in saving.
Without regular contributions the result follows A = P(1 + r/n)^(nt). The calculator also handles a monthly contribution by moving forward one month at a time: the balance grows by the monthly equivalent of the compounding rate, and then the contribution is added. With monthly compounding and contributions at the end of the month this matches the textbook future value of an annuity. The total contributions figure counts the starting amount plus every contribution, and the interest earned is the final balance minus that total.
The year-by-year table lists the contributions made in each year, the interest earned in that year and the balance at its end, so you can see the interest part overtake the contribution part. If the term is not a whole number of years, the last row covers the part year. The effective annual rate shows what the nominal rate and compounding frequency really amount to over a year.
Tips
- Borrowing instead of saving? The loan calculator shows the payment and the interest you will pay.
- Check a rate change or a share of a total with the percentage calculator.
- Spell the final balance out for a document with Number to words.
Questions
What is the compound interest formula?
Without contributions, the balance is A = P × (1 + r/n)^(n × t), where P is the starting amount, r the annual rate as a decimal, n the number of times interest compounds each year and t the number of years. With continuous compounding it is A = P × e^(r × t).
How are monthly contributions handled?
The calculator works month by month. Each month the balance grows by the monthly equivalent of your compounding rate, then your contribution is added at the end of the month, or at the start if you choose that. With monthly compounding this is the standard future value of an annuity.
What difference does the compounding frequency make?
The more often interest is added, the more the balance grows, but the gain flattens out quickly. At 5% for ten years, 10,000 grows to 16,288.95 with yearly compounding, 16,470.09 with monthly and 16,486.65 with daily compounding.
What is the effective annual rate?
It is the yearly growth you actually get once compounding is counted. A nominal 5% compounded monthly is an effective 5.12% a year.
Does it include tax, fees or inflation?
No. Real returns are lower after fees and tax, and the buying power of the final balance is lower after inflation. Use a lower rate to see a cautious case.
Is what I type uploaded?
No. Everything is calculated in your browser and nothing leaves your device.