How to use the compound interest calculator
Enter the starting balance, monthly deposit, nominal annual percentage rate, years and compounding frequency. The model deposits contributions after each month’s interest. Compare the ending balance with total contributions to distinguish modeled interest from money you deposited. Changing compounding frequency changes the effective rate; the projection does not promise an investment return.
Prepare the values for this calculation
Calculate compound interest with monthly contributions, compare contribution and interest totals, and view the balance by year. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- Starting balance ($): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Nominal annual rate (%): enter a value at least -99.99 and no more than 1000. Use the unit named in the label or selected above. Do not leave this required field blank.
- Number of years: enter a value at least 1 and no more than 100 and a whole number. Use the unit named in the label or selected above. Do not leave this required field blank.
- Compounding frequency: choose Annually, Semiannually, Quarterly, Monthly, Daily (365). Match this selection to the data you enter.
- Monthly contribution ($): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Projected future value first. With the unchanged example inputs, it is $63,367.82. Compare the additional metrics with the primary result; each has its own label and unit.
Compare and keep a useful result
For a controlled comparison, keep every other example input unchanged and change Starting balance ($) from 10000 to 11000. The resulting Projected future value is $65,377.48. This illustrates the equation’s response to that input, rather than a forecast of real events.
Read the chart’s axis labels and data table. A scenario curve changes one stated input while holding the others fixed; it does not add external observations. Use Download graph to save an available chart as a PNG.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Compound Interest Calculator formula
Monthly equivalent rate = (1 + annual rate/compounds per year)^(compounds per year/12) − 1. Each month: balance = balance × (1 + monthly rate) + contribution.
Compounding and contribution timing
The nominal annual interest rate and compounding frequency determine an effective monthly rate. Monthly contributions are deposited after that month’s interest is applied. This makes contribution timing explicit: beginning-of-month deposits would produce a different balance.
Total contributed includes the starting balance and every monthly deposit. Modeled interest is the projected balance minus total contributed. The balance chart and yearly table make that distinction visible.
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| Starting balance ($) | minimum 0 | 10000 |
| Nominal annual rate (%) | minimum -99.99; maximum 1000 | 7 |
| Number of years | minimum 1; maximum 100; whole number | 10 |
| Compounding frequency | Select an option | Monthly |
| Monthly contribution ($) | minimum 0 | 250 |
Worked example
Using the example inputs above, the projected future value is $63,367.82.
- Projected future value
- $63,367.82
- Total contributed
- $40,000.00
- Modeled interest
- $23,367.82
Results and assumptions
Contributions are added at the end of each month. This is a constant-rate projection before taxes, fees and inflation; returns are not guaranteed.
Frequently asked questions
How do I use this compound interest with monthly contributions?
Enter starting balance ($), nominal annual rate (%), number of years, compounding frequency. Use the remaining fields for the assumptions shown in the input table.
What formula does the compound interest calculator use?
Monthly equivalent rate = (1 + annual rate/compounds per year)^(compounds per year/12) − 1. Each month: balance = balance × (1 + monthly rate) + contribution.
Sources and method references
Fix an input or unexpected result
Starting balance ($) is required. Enter starting balance ($) using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.