Compound interest calculator
Project compound growth with monthly contributions. Compare contribution timing and calculate the monthly amount needed for a savings target.
Inputs
Hypothetical example. Replace these inputs with your own.
Result
Calculation result
Example result
- Ending balance
- 2,384.65 USD
- Total contributions
- 2,200 USD
- Growth
- 184.65 USD
- Effective monthly rate
- 0.9489 %
- Required monthly contribution before rounding
- 306.8 USD
- Required monthly contribution rounded up
- 306.81 USD
- The annual rate is effective, constant and already net of any costs you choose to model. Months are equal compounding intervals; inflation, tax and exchange-rate changes are excluded.
- Contributions are added at the end of each month after that month's return.
- The goal solution uses the unrounded monthly amount. The separate amount rounded up to the next cent is checked against the same monthly recurrence.
| Month · month | Opening balance · USD | Contribution · USD | Growth · USD | Closing balance · USD |
|---|---|---|---|---|
| 0 | 1,000 | 0 | 0 | 1,000 |
| 1 | 1,000 | 100 | 9.48879293 | 1,109.48879293 |
| 2 | 1,109.48879293 | 100 | 10.52770942 | 1,220.01650235 |
| 3 | 1,220.01650235 | 100 | 11.57648397 | 1,331.59298632 |
| 4 | 1,331.59298632 | 100 | 12.63521012 | 1,444.22819644 |
| 5 | 1,444.22819644 | 100 | 13.70398231 | 1,557.93217875 |
| 6 | 1,557.93217875 | 100 | 14.78289585 | 1,672.7150746 |
| 7 | 1,672.7150746 | 100 | 15.87204698 | 1,788.58712158 |
| 8 | 1,788.58712158 | 100 | 16.97153284 | 1,905.55865442 |
| 9 | 1,905.55865442 | 100 | 18.0814515 | 2,023.64010592 |
| 10 | 2,023.64010592 | 100 | 19.20190194 | 2,142.84200786 |
| 11 | 2,142.84200786 | 100 | 20.3329841 | 2,263.17499196 |
| 12 | 2,263.17499196 | 100 | 21.47479887 | 2,384.64979084 |
Saved scenarios
Save up to 20 new-tool scenarios on this device, separately from the six existing tools. Prices and balances stay out of the URL. DCA, journal and correlation scenarios save settings and a digest, requiring the matching CSV again to calculate.
Saving requires browser storage and Web Locks.
Formula and worked example
Monthly rate = (1 + effective annual rate)^(1/12) − 1. End-of-month contributions are added after monthly growth; start-of-month contributions grow during that month. The target calculation solves the same recurrence and rounds the required contribution up to the next cent.
Hypothetical example: $1,000 initial balance, $100 monthly contributions, 12% effective annual growth and 12 months end at about $2,384.65. Contributions total $2,200; modeled growth is $184.65. A $5,000 target requires about $306.804305 monthly, rounded up to $306.81. Rates are constant and already net of any costs you choose to model.
These estimates use bounded decimal arithmetic. They are scenario calculations, not trading instructions or forecasts.
Methodology sources: Compound interest
Related tasks and reading
This website is developed by Zack White and Vincent Hale.