Build the scenario
Use a return assumption already net of the fees you want to reflect. The calculator does not fetch or invent a market rate.
Assumption-led investment planning
See what compounds—and what inflation takes back.
Project an initial investment and monthly contributions with explicit timing, compounding and inflation assumptions. The result is a scenario, not a promised return.
Beginning or endChoose contribution timing
1×, 4× or 12×Annual compounding frequency
Exact real returnNominal versus inflation
Local exportsCopy, CSV and PDF
Final projected value
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Monthly contributions are added after each month’s growth.
Two-point sensitivity check
The same cash flows at two percentage points below and above your entered rate. This is not a confidence interval.
| Period | Contributed | Gain / loss | Balance |
|---|
What the engine does
It converts the entered nominal annual rate and selected compounding frequency into an equivalent monthly rate, then applies each contribution at the timing you chose.
monthly rate = (1 + annual rate ÷ frequency)^(frequency ÷ 12) − 1
real annual return = (1 + effective return) ÷ (1 + inflation) − 1
Method sources
Investor.gov: Compound Interest Calculator
Supports the core planning inputs: initial investment, monthly contribution, time, estimated annual rate and compounding frequency.
Federal Reserve Bank of St. Louis: Fisher equation
Supports the distinction between nominal return, inflation and real return.
Know what is missing
- Fees and taxes are not deducted automatically.
- Volatility, missed contributions, default and liquidity risk are not simulated.
- Currency labels do not convert exchange rates.
- Results do not establish suitability or guarantee performance.
Questions worth asking before acting
Why is “gain on contributions” not CAGR?
Regular contributions enter at different dates. Dividing gain by total contributions is a simple scenario ratio, while CAGR assumes a single beginning value and no intermediate cash flows. The calculator therefore does not mislabel the cash-flow result as CAGR.
Why does contribution timing change the result?
A beginning-of-month contribution receives one more month of projected growth than an end-of-month contribution. Use the timing that matches when the provider credits your money.
Can I use a negative return?
Yes, down to just above −100%. This helps test loss scenarios. A negative projection is still smooth and does not represent real market volatility.
Where is my data stored?
Inputs remain in the page while it is open. They are not saved to local storage or sent to AI. CSV and PDF exports are created locally.