Use a range, not one magic number
A projection becomes much more useful when it includes a conservative case, a base case, and an optimistic case. A single number can make the future feel more certain than it really is.
Match the assumption to the goal and the portfolio
A short-term goal with a cautious portfolio should not use the same assumption as a long-term, equity-heavy portfolio. The question is not "what return sounds good?" It is "what assumption fits the kind of portfolio and risk I actually expect to hold?"
| Good habit | Why it helps |
|---|---|
| Test a lower-rate case | Shows whether the goal still works when markets disappoint. |
| Keep the base case grounded | Prevents planning around a number that only works in unusually strong conditions. |
| Think about inflation separately | A large nominal balance may still buy less than expected in the future. |
Do not confuse a smooth projection with a smooth market
Calculators often show a clean upward curve because they use a constant return. Real investing does not feel like that. The market path is uneven, and that can change how comfortable or disciplined a plan feels in practice.
What makes an assumption "reasonable"?
Reasonable does not mean perfect. It means the rate is honest enough that the projection still has value if the future turns out less exciting than you hope. If the goal only works at the optimistic assumption, the plan is probably too fragile.
Simple routine for choosing a rate
- Pick a conservative case you can emotionally live with if markets are disappointing.
- Pick a base case that reflects your actual long-term investing approach.
- Pick an optimistic case only as an upper range, not as the plan itself.
Label nominal, real, and net assumptions
| Assumption | What it represents | Planning use |
|---|---|---|
| Nominal | Growth before adjusting for inflation. | Compare with a future goal cost that also includes inflation. |
| Real | Growth after an inflation adjustment. | Express the result in approximate today's purchasing power. |
| Net | Return after specified fees and, if modeled, taxes. | Make the cost assumptions explicit; account and tax treatment vary. |
The site calculator accepts one annual-effective input and does not separately deduct inflation, fees, or taxes. Decide what the input represents, document it, and keep the comparison consistent.
Worked sensitivity example: the rate changes the story
Start with $10,000, add $400 at each month-end, and model 20 years. Contributions total $106,000. Only the constant annual-effective assumption changes.
| Scenario label | Assumption | Modeled balance |
|---|---|---|
| Lower case | 3% | $148,803 |
| Middle-lower case | 5% | $188,855 |
| Middle-higher case | 7% | $241,711 |
| Upper sensitivity case | 9% | $311,585 |
The 9% result is more than twice the 3% result, even though deposits are identical. That is exactly why a plan should not work only at the top assumption. The labels above are sensitivity cases, not suggested returns or probabilities.
Historical averages are inputs to research, not promises
- Match any historical series to the actual asset mix, currency, period, and reinvestment assumption.
- Prefer compounded or geometric returns for multi-period growth; an arithmetic average can overstate compound growth when returns vary.
- Account for fees and taxes that apply to the actual account rather than using a gross index result silently.
- Test a contribution change or delayed start instead of relying on return precision to close a planning gap.
What the smooth model leaves out
Real portfolios experience volatility, drawdowns, changing allocations, fees, taxes, behavioral decisions, and sequence risk. The order of returns matters especially when contributions or withdrawals occur. A constant equivalent rate is useful for comparing assumptions but cannot reproduce those paths.
Common assumption mistakes
- Using a strong historical period without labeling the period or portfolio.
- Mixing a nominal return with a goal stated in today's dollars.
- Ignoring fees while comparing the result with a net-of-fee goal.
- Adding false decimal precision to an uncertain long-term input.
- Calling the highest scenario the “expected” result without evidence.
Return-assumption questions
There is no universal rate. Use a range tied to the actual portfolio, horizon, fees, taxes, and goal, and make sure the plan does not depend only on the highest case.
Label the model clearly. A nominal return produces nominal future dollars; a real-return model expresses growth after inflation. Do not subtract inflation twice or compare nominal balances with today's prices without adjustment.
The order of gains and losses matters when money is added or withdrawn. A constant-rate calculator omits volatility and sequence risk, so use it for sensitivity analysis rather than a forecast.
Sources and further reading
Reference for the risk difference between saving and investing.
Investor.gov: compound interest calculatorUseful comparison point for growth projections using different rate assumptions.
Bottom line
Define what the rate means, use several cases, and let contributions or time do more of the planning work than an optimistic assumption. Run the range in the investment calculator and read the compounding model before interpreting the curve.