Live scenario calculator
Educational estimates only. Results depend entirely on the inputs and assumptions you choose.
01 — FormulaThe math underneath
Future value = starting balance compounded monthly + monthly contributions compounded monthly
Compound interest is slow at first, then loud. The point is not that any return is guaranteed; the point is to see how time does more of the heavy lifting the longer you stay in the game.
02 — Reality checkWhat this does not include
- Assumes a steady return, which real markets do not provide.
- Does not include taxes, inflation, fees, or withdrawals.
- Small changes in return can create huge long-term differences.
How to use it
Run several scenarios: conservative, middle, and optimistic. The wide gap between those outputs is often more useful than any single answer.
03 — ExampleWhy time changes the shape of the result
Using the default example — $5,000 to start, $500 per month, 25 years, and a steady 7% annual return — the calculator produces roughly $433,663. About $155,000 comes from money contributed; the rest is modeled growth. That split is the point: compounding becomes more visible after the habit has had years to work.
Reduce the rate to see how much of the result depended on optimism.
Cut five or ten years and watch time remove more than most people expect.
Increase the monthly amount to see what is controlled by behavior rather than markets.
04 — InterpretationWhat the ending value does and does not say
The calculator assumes the same return every month. Real markets arrive unevenly, and the order of gains and losses can matter when money is withdrawn. Use the output as a planning estimate, then rerun it with lower rates and higher fees.
FAQCommon compound-interest questions
Is a 7% annual return guaranteed?
No. It is a common planning assumption, not a promise. Different assets, fees, taxes, and market periods can produce very different results.
Are contributions assumed at the beginning or end of each month?
This calculator models monthly contributions at the end of each period, which is the more conservative standard convention.
Why does an extra decade matter so much?
Because later growth is earned not only on contributions but also on prior modeled growth. The longer the timeline, the larger that second layer can become.
05 — Scenario tableHow $100 per month changes with time
These values use end-of-month contributions and smooth returns. They are not forecasts.
| Return | 10 years | 20 years | 30 years |
|---|---|---|---|
| 5% | $15,528 | $41,103 | $83,226 |
| 7% | $17,308 | $52,093 | $121,997 |
| 10% | $20,484 | $75,937 | $226,049 |
For a deeper explanation of contribution timing, inflation, fees, and return assumptions, read What Is Compound Interest?
Keep going
This calculator is for education only. It is not financial, investment, tax, legal, or retirement advice.