Compound Interest Calculator
Calculate how your money grows with compound interest, with optional monthly contributions and a year-by-year growth breakdown.
Use calculatorQuestions & answers
Try an example — choose a starting point, then adjust the inputs.
| Year | Deposits | Interest | Balance |
|---|
How to Use the Compound Interest Calculator
Enter your starting amount, an optional monthly contribution, the expected annual interest rate, the number of years, and how often interest compounds. The calculator shows your projected final balance, how much you deposited, how much you earned in interest, and a year-by-year growth table.
How monthly contributions are applied
Deposits are added at the end of each month, after that month's growth. The selected compounding frequency is converted to an equivalent monthly growth rate: i = (1 + r/n)n/12 − 1. This is a mathematical estimate, not a model of a particular bank's daily balance or deposit rules.
For m months, the balance is P(1+i)m + C × ((1+i)m − 1)/i, where C is the monthly contribution. At 0% interest, it is simply P + C × m. Use whole years from 1 to 100 and nonnegative amounts. Taxes, fees, inflation, withdrawals and changing returns are not included.
For background, see Investor.gov's explanation of compounding. The example rate below is hypothetical, not a predicted return.
The Compound Interest Formula
A = final amount
P = principal (starting amount)
r = annual interest rate (as a decimal)
n = compounding periods per year
t = number of years
When you add regular contributions, each deposit also begins compounding, which is why consistent investing is so powerful over long periods.
Example: The Power of Compounding
Suppose you invest $10,000, add $200 every month, and earn an average of 7% per year compounded monthly:
- After 10 years: about $54,714 ($34,000 deposited)
- After 20 years: about $144,573 ($58,000 deposited)
- After 30 years: about $325,159 ($82,000 deposited)
Notice how growth accelerates — the last decade adds far more than the first. That is compounding at work.
Compounding Frequency Comparison
On a $10,000 investment at 7% for 20 years (no contributions):
| Frequency | Final Amount |
|---|---|
| Annually | $38,697 |
| Quarterly | $40,064 |
| Monthly | $40,387 |
| Daily | $40,547 |
Disclaimer: Projections are hypothetical and for education only. Investment returns vary and are not guaranteed. Not financial advice.