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Example data table
Example scenario: $10,000 starting balance, 6% nominal annual rate, monthly compounding, 3 years, $100 monthly contribution, end-of-period deposits, no tax, 2% inflation.
| Year | Balance | Total Contributions | Cumulative Interest |
|---|---|---|---|
| 1 | $11,850.33 | $11,200.00 | $650.33 |
| 2 | $13,814.79 | $12,400.00 | $1,414.79 |
| 3 | $15,900.42 | $13,600.00 | $2,300.42 |
Formula used
The periodic rate is calculated as: periodic rate = annual nominal rate ÷ periods per year.
For end-of-period contributions: ending balance = opening balance + net interest + contribution.
For beginning-of-period contributions: ending balance = opening balance + contribution + net interest, where the contribution is added before interest is computed.
Gross interest for a period is: gross interest = balance used for interest × periodic rate.
Net interest is: net interest = gross interest × (1 − tax rate), when gross interest is positive. Inflation-adjusted value is estimated as: real future value = future value ÷ (1 + inflation rate)years.
The effective annual rate is: EAR = (1 + periodic rate)periods per year − 1.
How to use this calculator
- Enter the starting principal you already have invested or saved.
- Add the annual nominal rate and choose how many times interest compounds each year.
- Set the investment length in years. Decimal years are allowed.
- Enter the contribution made every compounding period.
- Choose whether contributions happen at the beginning or end of each period.
- Add tax and inflation assumptions for more realistic projections.
- Press the calculate button to show the summary, graph, and full schedule above the form.
- Use the CSV or PDF buttons to save the report for planning, review, or sharing.
FAQs
1) What does periodic interest mean?
Periodic interest is the interest earned during one compounding interval, such as a month, quarter, or day. The calculator converts the annual nominal rate into a rate for each period.
2) Why does contribution timing matter?
Deposits made at the beginning of a period earn interest sooner than deposits made at the end. Over many periods, that timing difference can noticeably increase the final balance.
3) What is the difference between gross and net interest?
Gross interest is the full interest generated before tax. Net interest is the amount left after applying the interest tax rate. The calculator tracks both values separately.
4) Why is the inflation-adjusted value lower?
Inflation reduces future purchasing power. Even if your account grows in dollars, those dollars may buy less later. The real value estimate adjusts the ending balance using your inflation assumption.
5) What does effective annual rate show?
Effective annual rate shows the true yearly growth rate once compounding is included. It is often higher than the nominal annual rate when compounding happens more than once per year.
6) Can I use this for savings and investments?
Yes. It works for savings plans, recurring deposits, investment projections, and education examples. Just make sure the compounding frequency matches the way the balance actually earns interest.
7) Does this calculator support negative rates?
Yes. Negative annual rates are allowed within the validation range. That helps model low-yield or declining scenarios, although tax is only reduced from positive interest in this version.
8) Why does the schedule use many rows?
The schedule shows every compounding period so you can inspect how balance, contributions, and interest change step by step. That detail also makes the CSV and PDF exports more useful.