Explore compounding, fees, and inflation
Use growth illustrations to examine assumptions, costs, and purchasing power.
Lesson 10.4 · 8 min read + practice
See what compounding actually does
Compounding means a period’s result becomes part of the amount exposed to the next period’s result. With a hypothetical $2,000 earning 4% annually and reinvested earnings, the first year ends at $2,080. The next year’s 4% is $83.20, producing $2,163.20. No contributions, fees, or taxes are included in that small example.
The arithmetic describes an assumed path. It does not establish that an investment will earn 4%, or that positive returns happen every year. With market investments, reinvestment can combine with gains or losses. A smooth calculator curve is a simplification that helps compare inputs, not a view of the future.
Understand the course calculator’s convention
The investing calculator uses a constant effective annual gross return assumption, subtracts the entered annual fee in percentage points, and converts that net annual rate to an equivalent monthly rate. It grows the balance each month and adds the contribution at month-end. The fee convention is deliberately simplified; actual fees may use different bases, timing, and additional charges.
For a 5% gross return assumption and a 0.5% annual fee, the model uses 4.5% net annually. Its monthly equivalent is (1.045 raised to the power of 1/12) minus 1. It does not simply use 4.5% divided by twelve. Do not enter an already-after-fee return and deduct the same fee again.
Reproduce one complete illustration
Set starting investment to $1,000, monthly contribution to $200, horizon to twenty years, gross annual return to 5%, annual fee to 0.5%, and inflation to 2%. These are fictional assumptions, not recommended returns. Total contributions are $1,000 + ($200 × 240 months) = $49,000.
Under those exact assumptions, the calculator gives a nominal ending balance of $79,243.67. The difference from contributed dollars is $30,243.67. Nominal means expressed in the dollars of the future date. This result assumes every contribution occurs, returns follow the same rate throughout, and earnings remain invested. It excludes taxes, withdrawals, and costs beyond the simplified fee input.
Change one input to understand fees
Keep every input fixed except the annual fee, which rises from 0.5% to 1.5%. The model’s net annual return becomes 3.5%, and the nominal ending value becomes $70,943.03. The difference from the original illustration is $8,300.64. That difference includes both the modelled cost effect and the growth no longer earned on money absorbed by costs.
This does not prove that the lower-fee option in any real comparison is automatically better. Different products may have different exposures and services. Compare equivalent arrangements, identify what you receive, and ask for total costs. The useful lesson is that a recurring percentage deserves attention even when it looks small on one year’s statement.
Translate dollars into purchasing power
Inflation describes changes in prices. To express the first illustration in today’s purchasing-power terms, the calculator divides $79,243.67 by 1.02 raised to the twentieth power. Using its unrounded internal balance, it displays $53,328.72 in today’s dollars. This is a different unit of comparison, not another account balance that you could withdraw.
The 2% assumption is not a forecast of your personal cost of living. Housing, care, education, and other goals may change at different rates. Compare a goal stated in today’s dollars with a result stated in today’s dollars, or convert both consistently. Mixing a future balance with an unadjusted current price can overstate what the balance will buy.
Test alternatives and uneven returns
Return assumptions matter. With the same starting amount, contributions, fee, horizon, and inflation, gross returns of 3% and 7% produce nominal illustrations of $63,645.14 and $99,448.83. These are sensitivity cases, not lower and upper bounds or probabilities. Actual outcomes can fall outside them.
An average can also mislead. A $1,000 holding that rises 20% and then falls 20% ends at $960, before other changes. The percentages apply to different starting balances. With deposits or withdrawals, the timing of gains and losses also affects the outcome. The calculator’s constant-rate illustration does not model that sequence risk.
Your practice
Reproduce the base case and the higher-fee case, then increase the monthly contribution to $250 while restoring the original fee. The nominal illustration becomes $98,451.66, with $61,000 contributed. Explain which change you can control and which remains uncertain.
Save the assumptions alongside the results. Complete the exercise by identifying the contribution timing, the fee simplification, one excluded cost or tax, and the difference between nominal and real purchasing-power values. A number without those assumptions is an incomplete conclusion.
Check your understanding
Try the same checks learners use in the course.
Sources
- OSC: How does compound interest help your money grow? · Reviewed Oct 14, 2026
- OSC: How inflation affects your investments · Reviewed Oct 14, 2026
- OSC: How to get started with investing · Reviewed Oct 14, 2026
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